Marchetti B Karsili T N V Kelly O Kapetanopoulos P ampAshfold M N R (2015) Near ultraviolet photochemistry of 2-bromo-and 2-iodothiophene Revealing photoinduced ring opening in the gasphase Journal of Chemical Physics 142(22) [224303]httpsdoiorg10106314921315

Publishers PDF also known as Version of record

Link to published version (if available)10106314921315

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Near ultraviolet photochemistry of 2-bromo- and 2-iodothiophene Revealingphotoinduced ring opening in the gas phaseBarbara Marchetti Tolga N V Karsili Orla Kelly Panos Kapetanopoulos and Michael N R Ashfold Citation The Journal of Chemical Physics 142 224303 (2015) doi 10106314921315 View online httpdxdoiorg10106314921315 View Table of Contents httpscitationaiporgcontentaipjournaljcp14222ver=pdfcov Published by the AIP Publishing Articles you may be interested in Photodissociation dynamics of C3H5I in the near-ultraviolet region J Chem Phys 141 104316 (2014) 10106314894393 Experimental and theoretical study of the photodissociation reaction of thiophenol at 243 nm Intramolecularorbital alignment of the phenylthiyl radical J Chem Phys 126 034306 (2007) 10106312424939 Probing mechanistic photochemistry of glyoxal in the gas phase by ab initio calculations of potential-energysurfaces and adiabatic and nonadiabatic rates J Chem Phys 124 054324 (2006) 10106312165179 Photochemistry of pyrrole Time-dependent quantum wave-packet description of the dynamics at the π 1 σ -S 0 conical intersections J Chem Phys 123 144307 (2005) 10106312049250 Time-dependent quantum wave-packet description of the π 1 σ photochemistry of phenol J Chem Phys 122 224315 (2005) 10106311906218

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THE JOURNAL OF CHEMICAL PHYSICS 142 224303 (2015)

Near ultraviolet photochemistry of 2-bromo- and 2-iodothiopheneRevealing photoinduced ring opening in the gas phase

Barbara Marchetti1 Tolga N V Karsili1 Orla Kelly2 Panos Kapetanopoulos2and Michael N R Ashfold1a)1School of Chemistry University of Bristol Cantockrsquos Close Bristol BS8 1TS United Kingdom2Photek Ltd 26 Castleham Road St Leonards-on-Sea East Sussex TN38 9NS United Kingdom

(Received 24 March 2015 accepted 7 May 2015 published online 9 June 2015)

Velocity map imaging methods with a new and improved ion optics design have been used toexplore the near ultraviolet photodissociation dynamics of gas phase 2-bromo- and 2-iodothiophenemolecules In both cases the ground (X) and spin-orbit excited (X) (where X = Br I) atomproducts formed at the longest excitation wavelengths are found to recoil with fast anisotropicvelocity distributions consistent with prompt CndashX bond fission following excitation via a transitionwhose dipole moment is aligned parallel to the breaking bond Upon tuning to shorter wavelengthsthis fast component fades and is progressively replaced by a slower isotropic recoil distributionComplementary electronic structure calculations provide a plausible explanation for this switchin fragmentation behaviourmdashnamely the opening of a rival CndashS bond extension pathway to aregion of conical intersection with the ground state potential energy surface The resulting groundstate molecules are formed with more than sufficient internal energy to sample the configurationspace associated with several parent isomers and to dissociate to yield X atom products in tan-dem with both cyclic and ring-opened partner fragments C 2015 Author(s) All article contentexcept where otherwise noted is licensed under a Creative Commons Attribution 30 UnportedLicense [httpdxdoiorg10106314921315]

I INTRODUCTION

Near ultraviolet (UV) photoexcitation of an isolated gasphase molecule at energies above its lowest dissociationlimit often results in bond fission This may occur followingradiationless transfer to the ground state potential energysurface (PES) and appropriate intramolecular vibrationalredistribution (IVR) or may occur on an excited state PESthat is repulsive in the relevant bond coordinate Excited statedissociations can be viewed in two classes the dissociativestate may be populated directly in the photoexcitation processor indirectly following radiationless transfer from an opticallyldquobrightrdquo state reached in the initial absorption event In bothcases there is growing recognition that the excited state PESon which the eventual bond extension occurs is likely to ariseas a result of electron promotion to a σlowast orbital (ie to anorbital that is antibonding in the coordinate of interest)12

This growing recognition accounts for the frequent referencein the recent literature to nσlowast andor πσlowast excited statesie dissociative excited states formed by electron promotionfrom an occupied non-bonding (n) or bonding (π) orbital to aσlowast orbital

The present study employs nanosecond pulsed laserpump (photolysis)ndashprobe (resonance enhanced multiphotonionisation (REMPI)) velocity map imaging (VMI) methods toinvestigate the bromine and iodine atoms formed followingexcitation of 2-bromo- and 2-iodothiophene molecules in the

a)Author to whom correspondence should be addressed Electronic mailmikeashfoldbristolacuk Tel +44 (0)117 928 8312

gas phase at many near UV wavelengths The experimentsexploit a new ion optics assembly that offers superiorvelocity mapping (and slicing) capabilities to that we haveused previously Aspects of the design implementation andevaluation of this ion optics assembly are presented in Sec IIand in more detail in the supplementary material3

Analogy with previous experimental studies of bareand substituted halobenzenes4ndash8 encourages the expectationthat near UV excitation of the title halothiophenes shouldresult in eventual CX (X = Br I) bond fission on one ormore (nπ)σlowast excited state PESs A previous one colour ionimaging study of Br atoms resulting from 267 nm photolysisof 2-bromothiophene lends support to this expectation9

as do evident similarities in the resonance Raman (RR)spectra measured following excitation of 2-iodothiopheneand iodobenzene (in cyclohexane solution)10 However theobservation of nominal antisymmetric CndashSndashC stretchingvibrations in the RR spectra of 2-iodothiophene10 (and of barethiophene11) hints at some contribution from an alternativeCndashS bond extension (ring-opening) pathway This findingserved to reinforce speculations offered to account for variousof the products observed in earlier experimental studiesof thiophene photolysis under both bulk12 and molecularbeam1314 conditions and accords with conclusions fromseveral recent theoretical studies of the ultrafast deactivationof gas phase thiophene molecules15ndash18

Finding direct evidence for the operation of such path-ways experimentally is a challenge as any primary ring-opened products will almost inevitably be formed withsufficiently high levels of vibrational excitation to defy

0021-96062015142(22)22430311 142 224303-1 copy Author(s) 2015

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224303-2 Marchetti et al J Chem Phys 142 224303 (2015)

definitive spectroscopic characterisation and in many caseswill decay further to smaller secondary products One wayof circumventing these difficulties is to study such photo-induced ring-opening reactions in a weakly interacting solventas illustrated by our recent ultrafast pumpndash(broadband infrared(IR) absorption) probe studies of the 267 nm photolysisof thiophenone in solution in acetonitrile19 The additionalC==O group (cf bare thiophene) in this system was crucialserving as the ldquomessengerrdquo that allowed us to track the photo-induced evolution from parent (the photoexcitation of whichresults in a characteristic carbonyl stretch bleach feature atsim1650 cmminus1) to product (which shows a ketene asymmetricstretch band that narrows and blue shifts to an asymptoticwavenumber of sim2250 cmminus1 with increasing pump-probe timedelays) The time dependence of the ketene absorption reflectsthe formation rearrangement and subsequent vibrationalrelaxation of the ring-opened product through interaction withthe solvent Such vibrational quenching is of course notavailable in the analogous gas-phase study

Here we show that the X atoms in the title halothiophenescan also serve as messengers and from the wavelengthdependence of the X atom product velocities (as revealedby ion imaging) provide clear evidence for photo-inducedring expansion at shorter UV excitation wavelengths Com-plementary electronic structure calculations serve to reinforcethis interpretation and to highlight the key role of a (nπ)σlowastPES in driving the CndashS bond extension required to accessa region of conical intersection (CI) with and promoteradiationless transfer to the ground state PES and (potentially)to fully ring opened products This work adds to the growingliterature highlighting the importance of (nπ)σlowast excited statesin facilitating both emergent and complete ring opening inheterocyclic molecules20ndash24

II EXPERIMENTAL

2-Bromo- and 2-iodothiophene (purity sim98) wereobtained from Sigma Aldrich and used as supplied Theindividual samples (both liquids at room temperature) werepacked in a stainless steel inline filter positioned upstream ofa pulsed valve which was resistively heated (to sim30 C) toboost its vapour pressure This vapour pressure was seededin helium (at stagnation pressures sim600 and sim400 millibarsrespectively) and expanded though the pulsed valve (GeneralValve Series No 9 Parker) into the VMI spectrometerin the form of a skimmed pulsed molecular beam Thevacuum chamber has been detailed previously as have theNd-YAG pumped frequency doubled pulsed dye lasers usedto generate the requisite photolysis and 2 + 1 REMPI probewavelengths25 The probe wavelengths used in the two colourstudies reported here were 26663 nm and 26670 nm forground and spin-orbit (SO) excited Br atoms26 (henceforthwritten simply as Br and Brlowast) and 30368 nm and 30402 nm forI and Ilowast atoms27 All reported images recorded at wavelengthsλ gt 250 nm are from ldquoone-colourrdquo experiments where thesame wavelength is used both to effect photodissociation andprobe the resulting halogen atom products whereas thoserecorded at λ lt 250 nm are from traditional ldquotwo-colourrdquoexperiments wherein the two pulses are arranged to counter-

propagate through the interaction region with a time delayδt sim 20-45 ns between the photolysis and REMPI laser pulsesThe probe laser bandwidth is greater than the BrI atomDoppler width in all cases and thus was simply fixed at theline centre of the respective transition

Ions formed in the interaction region were accelerated witha new purpose designed ion optics assembly detailed belowand in the supplementary material3 through a field free time-of-flight (TOF) region towards a position sensitive detector(double microchannel plates (MCPs)) coupled to a phosp-hor screen and a CCD camera (Allied Vision TechnologiesModel GC1385) The images were acquired and processed us-ing event counting (LaVision Davis 62) and could be collectedas traditional ldquopancakedrdquo images or by appropriate adjust-ment of the electrode voltages by time gating the front MCP(Photek GM-MCP-2) so as to take a central slice out of theextended ion cloud of interest at the detector The pancaked im-ages were transformed and analysed as described previously28

The trigger pulses for the pulsed valve the flashlamps andQ-switches of both Nd-YAG lasers the gating of the frontMCP and the camera were supplied by a BNC Model 555 delaygenerator operating under Labview control

A The new ion optics assembly

The ion optics assembly developed in this work wasdesigned built and assembled following extensive SIMION(v 81)29 simulations As with several other reported spectrom-eter designs30ndash36 the ion optics assembly described in Ref 25provided a convenient starting point The result of this analysisis the four electrode stack shown in Fig 1 which is designedto support a linear mapping of image radius (r) to ion velocity(v) right down to r = 0 a larger interaction volume capableof supporting optimal velocity mapping than in our previousdesign and an up to 5-fold improvement in the (modelled)

FIG 1 Cross-sectional view of the new VMI optics illustrating the electrodegeometries and the lines of equipotential returned by the SIMION modellingStainless steel components and insulating components are shaded grey andyellow respectively

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224303-3 Marchetti et al J Chem Phys 142 224303 (2015)

limiting velocity (ie ∆rr or ∆vv) resolution The overallassembly is longer (eg the distance between the plane ofionisation and the ground (G) electrode is 82 mm (cf 70 mmin the previous design)) The electrodes and support rods aremade of stainless steel and the latter are covered with aninsulating sleeve which is in turn covered with a thin stainlesssteel sleeve that is electrically connected to the adjacent lowerpotential electrodemdashthereby ensuring that at no stage do theions of interest have direct line of sight to any insulatingpart

There are an infinite number of possible geometryconfigurations that could in principle be modelled in the questfor optimum performance Thus it was necessary to adopta systematic approach to optimising the ion optics designwithin the constraints imposed by the physical size of theexisting vacuum chamber the diameter of the annular cryo-shield surrounding the interaction region and the requirementthat the laser propagation axis must lie between the repeller(R) and extractor (E) electrodes The ion optics geometry wasmodified iteratively using a routine written in Lua and a seriesof 2-D simulations run to determine the velocity mappingcapability of each configuration Initial simulations concen-trated on understanding the effects of changing aspects of thegeometry (eg aperture diameters and electrode spacings)These preliminary calculations encouraged more thoroughinvestigation of three design features (i) the shapedepth of theR electrode (ii) the shapecurvature of the E electrode and (iii)the extent to which the electrode assembly design shields theacceleration region from stray fields Our exploration of eachof these factors is now described Further details the results ofsupporting SIMION simulations and illustrative test data forphotolysis of IBr are included in the supplementary material3

(i) Our earlier design introduced a top hat shaped Relectrode in place of the traditional annular plate25 Such adesign in combination with a conical E electrode yielded fieldlines that are suitably parallel to one another and perpendicularto the extraction axis The present modelling reveals thatfurther improvements in velocity mapping capability can beachieved by increasing the recess depth d of this R electrodefurther as illustrated in Fig 2(a) In practice the additionaldepth that can be accommodated in the existing vacuumchamber is limited by the location of the laser beam axis (andthe entrance and exit windows) relative to the bulkhead thatsupports the skimmer separating the source and interactionregions but the simulations suggest a sim2-fold improvement inthe minimum ∆rr should be achieved (with both the previousand new ion optics designs) upon increasing d to sim14 mm

(ii) The earlier design25 also introduced a thicker Eelectrode with a conical aperture as a route to reducingchromatic and spherical aberrations and shielding the ions ofinterest from external stray fields The larger volume createdby using appropriately shaped R and E electrodes results ina weaker field gradient in the interaction region This ldquosoftfocussingrdquo configuration offers several potential benefits Thecloud of ions formed in the interaction region has more timeto expand before entering the later acceleration and TOFregionsmdashwith consequent benefits for slice imaging (see thesupplementary material3) Further the ions and their resultingtrajectories are less sensitive to their point of creation in

FIG 2 Illustrations of the ∆rr resolution returned by SIMION modellingof the new (and previous (Ref 25)) velocity mapping ion optics for (a)different depths d of the R electrode and (b) different aperture diametersfor the E electrode with a spherical entrance face For each new geometrythe simulations employ the best operational voltages (holding VR fixed at+25 kV) and assume the ejection of 35Cl+ ions with 15 eV kinetic energyperpendicular to the time-of-flight axis from a volume with ∆x = 05 mm inthe TOF direction and ∆y = 2 mm in the laser propagation direction

the interaction region (cf when using standard flat annularelectrodes) The velocity mapping condition is thus muchless sensitive to small changes in laser alignment or inthe electrode voltages and the volume within which goodvelocity mapping can be achieved is substantially increasedIntroducing a sphericalcurved surface in place of the previousconical aperture further improves the field definition withinthe interaction volume resulting in a sim20 improvement in∆rr (for the same entrance and exit aperture diameters) AsFig 2(b) shows this can be further improved by increasingthe aperture of the E electrode which in the new designcomprises an entrance face defined by a sphere of radius291 mm (centred on the TOF axis) and entrance and exitapertures of respectively 56 mm (as before) and 44 mm Asnoted previously37 use of a spherical entrance surface alsoensures that the desired linear relationship between imageradius (r) and ion velocity (v) extends all the way to r = 0

(iii) Additional large aperture ldquostabilisingrdquo electrodeshave been introduced between the E and L electrodes andbetween the L and G electrodes (all electrically linked viaa resistor chain) as shown in Fig 1 These provide further

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224303-4 Marchetti et al J Chem Phys 142 224303 (2015)

improvements in field definition in the later accelerationregions and provide a valuable counter against perturbingfields from the earthed support rods and any other externalinfluences

Further comparative simulations and sample Br andor Iatom images from the near UV photodissociation of jet-cooledIBr molecules recorded with the new ion optics assembly areincluded in the supplementary material3

B Ab initio calculations

Using the Molpro computational package38 the groundstate minimum energy geometries of 2-iodothiophene and 2-bromothiophene were optimised using Moslashller-Plesset secondorder perturbation theory (MP2) and Dunningrsquos contractedcc-pVDZ basis set (for the H C S and Br atoms)39 Forthe I atom a pseudopotential was added to the double-ζ(cc-pVDZ-pp) basis function along with a 46 electronrelativistic effective core potential40 Rigid body potentialenergy curves (PECs) were then calculated as a functionof the CndashX bond length RCndashX with the remainder of thenuclear framework fixed at the MP2 equilibrium geometry Cssymmetry was maintained at each point along this scan Spinorbit free (SOF) PECs were calculated using a state-averaged(SA) complete active space self-consistent field (CASSCF)reference wavefunction involving five 1Aprime four 1Aprimeprime five3Aprime and four 3Aprimeprime states The corresponding state energieswere calculated using the complete active space with secondorder perturbation theory (CASPT2) based on the above SA-CASSCF reference wavefunction with an imaginary levelshift of 05 Eh to aid convergence and circumvent the effectsof intruder states The active space for these calculationscomprised 12 electrons in the following nine orbitals threeoccupied π orbitals two unoccupied πlowast orbitals the px and pylone pairs centred on the X atom and the σ and σlowast orbitalslocalised around the CndashX bond Spin-orbit coupled states werethen calculated by evaluating the spin-orbit Hamiltonian (HSO)in a basis of ψelec these states are henceforth labelled using theextended irreducible representation including both spin-orbitfree and spin parts In order to account for dynamic correlationof electrons the obtained SOF CASPT2 energies were thenused in the diagonalisation of the spin-orbit coupling matrix

Gaussian 0941 was used to calculate ldquorelaxedrdquo PECsalong the C(2)ndashS ring opening coordinate (RCndashS) Thesecalculations started from the MP2 optimised ground stategeometry (above) RCndashS was then extended in 01 Aring incrementsto RCndashS = 37 Aring and at each value the remainder of thenuclear framework was optimised at the MP2LANL2DZ level(with Cs symmetry constraints) The excited states were thencalculated at these various relaxed ground state geometriesalong RCndashS using SOF CASPT2 These CASPT2 calculationsutilised the same state averaging active space basis set andimaginary level shift as the previously described calculationsalong RCndashX

Following C(2)ndashS bond extension various ground stateminima and transition state (TS) geometries linking potentialisomers were calculated using Density Functional The-ory (DFT) along with the Becke 3-parameter Lee-Yang-Parr (B3LYP) functional The default 6-311G(dp) basis

set embedded within Gaussian 09 was used for the 2-bromothiophene calculations whereas for 2-iodothiophenethe functions of the equivalent 6-311G(dp) basis set weregenerated artificially (and are listed in the supplementarymaterial3) The biradicals formed upon ring-opening in 2-bromo- and 2-iodothiophene were identified as local minimaand could thus be optimised in the normal way usingDFTB3LYP6-311G(dp) and a singlet spin multiplicityGiven the evident differences in the Br product recoilvelocity distributions from 267 nm photolysis of 2- and3-bromothiophene9 we also attempted to characterise theequivalent ring-opened biradical on the ground state PES of3-bromothiophene These calculations were unsuccessfulmdashminimising instead to a lower energy cyclopropene adductThus the lowest root of triplet spin multiplicity was used tooptimise this critical point in the case of 3-bromothiopheneand a linear interpolation in internal coordinate (LIIC) wasthen scanned (using the above level of DFT) between thering opened biradical and the cyclopropene adduct in order toestablish the energy profile along this coordinate

III RESULTS AND DISCUSSION

Near UV absorption spectra of the two title halothio-phenes are shown in Figure 3 along with an illustrativemolecular structure Both spectra show an intense band centredatsim230-240 nm and a tail stretching to longer wavelengths thatis much more evident in the case of 2-iodothiophene The 2-iodothiophene absorption profile agrees well with that reportedpreviously (in cyclohexane solution)10 but additionally showsa sequence of diffuse resonances separated by sim630 cmminus1

on the long wavelength side of the intense feature Analogywith bare thiophene (and the present ab initio calculationsvide infra) suggests that this feature is associated with oneor more πlowast larr π excitations and that these resonances arelikely attributable to symmetric (or antisymmetric) CndashSndashCstretch motions The long wavelength tail can be viewed asthe analogue of the σlowast larr nπ ldquoA-bandrdquo absorptions of otheralkyl (and aryl) halides The greater prominence and extent ofthis tail in the spectrum of 2-iodothiophene accords with thefindings of our recent studies of 4-bromo- and 4-iodophenol

FIG 3 Room temperature near-UV absorption spectra of 2-bromo- and2-iodothiophene recorded using the vapour above a small sample of the neatliquid in an atmosphere of air

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224303-5 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 4 Images of Br(2P32) products resulting from photolysis of jet-cooled2-bromothiophene at λ= (a) 2666 nm (b) 2606 nm (c) 2542 nm (d)2498 nm and (e) 2449 nm with the ε vector of the photolysis laser radiationaligned vertically in the plane of the images (as shown in panel (a)) TheTKER spectrum derived from analysis of each data set is shown to the rightof each image with the vertical arrow showing the TKERmax value assumingD0(RminusBr)= 29 000 cmminus1

nominally spin-forbidden singlet-triplet transitions appearwith much greater transition strength in the latter8

A Ion imaging data

The left hand panels in Figs 4ndash7 show raw images ofrespectively the Br and Brlowast fragments resulting from photol-ysis of 2-bromothiophene and the I and Ilowast fragments fromphotolysis of 2-iodothiophene recorded at several differentnear UV wavelengths For consistency only pancaked imagesare reported in the main paper All images of a given specieswere recorded using the same extraction voltages (ie therespective radii are reliable indicators of the relative velocity)and in all cases the electric (ε) vector of the photolysis laserradiation was vertical (ie parallel to the front face of thedetector) as shown on the first image in each figure Thecorresponding total kinetic energy release (TKER) spectra(shown to the right of each image) were derived using theappropriate Jacobian together with the assumption that theunobserved fragment partnering the BrI atom has chemicalformula C4SH3 and mass m = 8313 u

These data show a number of trends which we review byfirst focussing on 2-bromothiophene The Br image recordedat longest photolysis wavelengthmdasha one colour experimentat 2666 nmmdashshows a fast anisotropic recoil velocitydistribution peaking at TKER sim6000 cmminus1 with a full width

FIG 5 Images of Br(2P12) products resulting from photolysis of jet-cooled2-bromothiophene at λ= (a) 2667 nm (b) 2625 nm (c) 2542 nm and (d)2448 nm with the ε vector of the photolysis laser radiation aligned verticallyin the plane of the images (as shown in panel (a)) The TKER spectrumderived from analysis of each image is shown on the right with the verticalarrow indicating the TKERmax value assuming D0(RminusBr)= 29 000 cmminus1

half maximum (FWHM) of sim3000 cmminus1 and an associatedanisotropy parameter β sim 2 ie the limiting value for promptaxial recoil following excitation via a transition dipole moment(TDM) for which is aligned parallel to the breaking bondmdashasreported (following photolysis at 267 nm) by Zhang et al9

Moving to shorter wavelengths the anisotropic fast ringfades in relative intensity and is progressively replaced bya smaller isotropic featuremdashexemplified by the data recordedat 2449 nm (Fig 4(e)) The TKER spectrum derived from thisimage peaks close to zero

The Brlowast fragment images from photolysis of 2-bro-mothiophene (and the TKER spectra derived from them) reveala similar trend The images recorded at longer wavelengthsie at 2667 and 2625 nm (Figs 5(a) and 5(b)) are dominatedby an anisotropic feature peaking at TKER sim3500 cmminus1with a FWHM of sim3000 cmminus1 The β parameter is velocitydependent sim15 around the peak of the distribution anddeclining at lower TKER The weak Brlowast image taken atλ = 2448 nm in contrast is essentially isotropic and peaks atlow TKER

I and Ilowast images from 2-iodothiophene were recordedover a greater range of photolysis wavelengths and againshow similar trends A weak one-colour I atom signal wasdiscernible following excitation at 30368 nm As Fig 6(a)shows these I atoms display a narrow (sim2000 cmminus1 (FWHM))anisotropic (β sim 17) translational energy distribution peakingat TKER sim7000 cmminus1 which initially expands upon scanningto shorter wavelengths but then shrinks (in radius) becomesmore isotropic fades (in relative intensity) and is supple-mented by a slower isotropic component as the photolysiswavelength is reduced Nonetheless as Fig 6(f) shows a

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1372224331 On Tue 09 Jun 2015 131203

224303-6 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 6 Images of I(2P32) products resulting from photolysis of jet-cooled 2-iodothiophene at λ= (a) 3037 nm (b) 2779 nm (c) 2668 nm (d) 2547 nm(e) 2437 nm and (f) 2202 nm with the ε vector of the photolysis laserradiation aligned vertically in the plane of the images (as shown in panel(a)) The TKER spectrum derived from analysis of each data set is shownon the right with the vertical arrow showing the TKERmax value assumingD0(Rminus I)= 24 500 cmminus1

faster (albeit more anisotropic) component peaking at TKERsim7000 cmminus1 is still discernible at λ = 2202 nm

As in the cases of iodobenzene6 various fluorinatediodobenzenes7 and 4-iodophenol8 the Ilowast images measuredfollowing excitation of 2-iodothiophene at long wavelengthshow hints of product vibrational structure as illustrated inFig 7(a) (λ = 2774 nm) Following past precedent we assignthe most obvious peak in the TKER spectrum taken at longestwavelength (indicated by the arrow in Fig 7(a)) to formationof Ilowast atoms together with radical fragments in their ground(ie zero-point v = 0) vibrational level Such an attributionallows estimation of the CndashI bond strength via the usual energyconservation relationship

D0(R minus I) = Ephot minus A(I) minus TKERmax

where R represents the (α-)thienyl (sometimes also referredto as the thiophenyl) radical Ephot is the photolysis photonenergy A(I) is the spin-orbit energy of the Ilowast atom (7603 cmminus1)TKERmax is the TKER associated with this fastest featureand we neglect the small amount of internal excitation in

FIG 7 Images of I(2P12) products resulting from photolysis of jet-cooled 2-iodothiophene at λ= (a) 2774 nm (b) 2670 nm (c) 2547 nm (d) 2437 nmand (e) 2202 nm with the ε vector of the photolysis laser radiation alignedvertically in the plane of the images (as shown in panel (a)) The TKERspectrum derived from analysis of each data set is shown on the rightwith the vertical arrow showing the TKERmax value assuming D0(Rminus I)= 24 500 cmminus1

the jet-cooled parent molecules this analysis yields D0(R minus I)= 24 500 plusmn 500 cmminus1mdasha value that is sensibly consistent witha typical CndashI bond strength in other aromatic systems6ndash8 Abinitio calculations for the ground state thienyl radical provideclues as to the assignment of the vibrational peak observedin Fig 7(a) A unique assignment is not possible giventhe limited energy resolution but the observed wavenumberseparation (sim1500 cmminus1 from the peak assigned to v = 0products) is sensibly consistent with one quantum of the in-plane symmetric (and antisymmetric) CndashC stretching modesmdashsee the supplementary material3 We note that the formeris prominent in the previously reported RR spectra of2-iodothiophene (albeit when exciting at slightly shorterwavelengths)10

The short vertical arrows at the high TKER end of thespectra shown in Figs 6 and 7 indicate the maximum TKERspredicted for the I (or Ilowast) + R products arising at eachphotolysis wavelength given D0(R minus I) = 24 500 cmminus1 Thesedata emphasise that the mean TKER ⟨TKER⟩ associatedwith each channel is rather insensitive to changes in Ephotimplying progressively greater partitioning of the availableenergy (ie of Ephot minus D0(R minus I)) into internal excitation ofthe thienyl radical productmdasha conclusion that again is ingood accord with those reached from the earlier resonanceRaman studies10

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1372224331 On Tue 09 Jun 2015 131203

224303-7 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 8 Diabatic SO resolved PECs along RCndashX for the ground and lower lying excited states of (a) 2-bromothiophene and (b) 2-iodothiophene

None of the TKER distributions derived from the Br or Brlowast

images from photolysis of 2-bromothiophene show any hintsof product vibrational structure (as was the case also in theimaging study of 4-bromophenol photolysis8) Nonethelessif we assume similar dissociation dynamics to that revealedby 2-iodothiophene and that the TKERmax value for productsformed from 2-bromothiophene photolysis at the longestUV wavelengths similarly lies on the fast edge of the highTKER feature (as shown by the short vertical arrow in bothFigs 4(a) and 5(a)) we arrive at a CndashBr bond strength D0(R minus Br) sim 29 000 cmminus1 (given A(Br) = 3685 cmminus1) that isagain sensibly consistent with values reported for bro-mobenzene (3428 kJ molminus1 28 650 cmminus1 Ref 42) andfor 4-bromophenol (28 700 cmminus1 Ref 8) but substantiallylarger than an earlier suggested CndashBr bond strength in 2-bromothiophene43 As the other panels in Figs 4 and 5 showtuning to shorter photolysis wavelengths again results in arelative increase in the internal energy partitioned into thethienyl partner

B Ab initio calculations of CndashBrCndashI bond fissionand CndashS ring opening pathways in 2-bromothiopheneand 2-iodothiophene

Figures 8(a) and 8(b) present SO resolved PECs alongRCndashX for the ground and lower energy excited states of respec-tively 2-bromothiophene and 2-iodothiophene In both casesthe molecule has been constrained to planar geometries andthe PECs have been approximately ldquodiabatisedrdquo in terms of(nπ)σlowast and ππlowast states by inspecting the energies symmetriesand wavefunction coefficients The adiabatic SO resolvedpotentials from which these are derived are shown in thesupplementary material3 As in the case of the iodobenzenes67

and halophenols8 the lowest repulsive potentials all arise fromσCndashX

lowast larr nπ excitations these potentials correlate with theground (X2Aprime) state thienyl + XXlowast dissociation limits Thespin-orbit split analogues involving the first (A2Aprimeprime) excitedstate of the thienyl radical lie higher in energy while the(diabatically) bound interloper PECs arise from excitationsto ring centred πlowast orbitals The calculated vertical excitationenergies suggest that many of these excited states could besampled within the wavelength ranges explored in the presentstudy but analogy with the previous VMI studies of the XXlowast

products from photolysis of iodobenzenes and 4-iodo- and4-bromophenol suggests that the σlowast larr nπ TDMs returnedby the present calculations are likely to be of limited valuein deciding which are the optically ldquobrightrdquo states (illustrativeTDMs are listed in the supplementary material3) Rather asin those cases6ndash8 the observation that the faster X and Xlowast

products formed at long excitation wavelengths show nearlimiting parallel recoil anisotropy suggests that the populatedstates are those that gain intensity by vibronic interactionwith the higher lying intense CndashX bond centred σlowast larr σtransition

The predicted energy separations between the variousasymptotic limits merit comment The present calculationsreproduce the BrBrlowast and IIlowast spin-orbit splittings well butpredict a larger separation between the A and X statesof the thienyl radical (sim125 eV) than that derived fromphotodetachment studies of the thiophenide anion (sim06 eVRef 44) This is not surprising given that the PECs are derivedvia rigid body scans and thus preclude any structural relaxationof the ring Nonetheless the present calculations confirm theearlier suggestion that (nπ)σlowast excited states (ie states thatare dissociative with respect to CndashX bond extension and whichcorrelate to X or Xlowast products) are dominant contributors tothe long wavelength tail of the respective absorption spectraand locate the minimum of the first (spin-allowed) πlowast larr πtransition at sim53 eV (sim59 eV) for X = I (Br) in reasonableaccord with the observed absorption maxima (Fig 3)

Cuts through the first few SOF potentials of 2-bro-mothiophene and 2-iodothiophene along the RCndashS stretch(ring-opening) coordinate are shown in Figs 9(a) and 9(c)respectively The corresponding adiabatic SOF PECs alongRCndashX are shown alongside in Figs 9(b) and 9(d) The S0 stateis stable with respect to ring opening in both halothiophenesand its PEC thus rises steadily with increasing RCndashS Incontrast the PECs for the 11Aprimeprime and 21Aprimeprime states (and fortheir triplet analogues all of which arise via σCndashS

lowast larr (nπ)excitations) show obvious instability with respect to extendingRCndashS Notwithstanding the limitations of these SOF PECs(all of which are calculated at the MP2 relaxed ground stategeometries) they clearly point to the availability of exoergicpathways from the bright 1ππlowast (21Aprime) state via one or more(nπ)σCndashS

lowast PECs to extended CndashS bond lengths and regionsof conical intersection with the S0 state potential

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224303-8 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 9 Approximately diabatised SO free PECs along the RCndashS (ring-opening) coordinate and RCndashX stretch for the first few singlet (solid lines) and triplet(dashed lines) states of 2-bromothiophene ((a) and (b) respectively) and 2-iodothiophene ((c) and (d) respectively)

Figure 10 summarises some of the various possible fatesof 2-halothiophene molecules following non-adiabatic transferto the S0 PES The primary ring-opened structure is a biradical(structure B) that can exist as essentially degenerate cis andtrans isomersmdashboth of which are unstable with respect tothe original ground state halothiophene B can also isomeriseto various ring-opened isomers (including structures D-G)via transition states three of which that involve migrating(at most) one atom from the biradical structure (TS1 TS2and TS3) have been optimised As Fig 10 also shows arange of Br + radical (H I J K as well as R) dissociationlimits are calculated to lie below the energy of a 250 nmphoton (illustrated by the horizontal red line) The variousstructures and product pairs along with their energies (incmminus1 defined relative to that of the ground state minimum)are detailed below the figure along with the correspondingquantities for 2-iodothiophene (in parentheses) The calculatedRndashBr and RndashI bond strengths are seen to match well withthe experimental values derived in Sec III A Readersshould also note that the dashed lines at the far right ofFig 10 are simply intended to correlate various isomers toparticular dissociation products we have not checked for thepresence (or otherwise) of any barrier in the various fragmen-tation paths

C A plausible explanation for the observedwavelength dependent fragmentation dynamics

The X and Xlowast images measured at the longer UVwavelengths are very reminiscent of those reported previouslyfollowing photolysis of for example iodobenzenes67 and 4-iodo- and 4-bromophenol8 The X and Xlowast products recoilwith close to the maximum speed permitted by energyconservation and with close to limiting parallel anisotropyThe velocities of the faster XXlowast products increase littleupon tuning to shorter excitation wavelengths implying apreferential partitioning of the additional photon energy intointernal (vibrational) excitation of the thienyl radical partnerAs in the previously studied aryl iodides and bromides suchbehaviour is fully consistent with population of one or more(nπ)σlowast excited states and prompt CndashX bond fission Theprogressive increase in product vibrational excitation can beunderstood on Franck-Condon grounds given the changes inequilibrium ring geometry upon σlowast larr (nπ) excitation andsubsequent evolution to the radical product As noted abovethe near limiting parallel recoil anisotropy of the X and Xlowast

products can be understood if (as has been shown in detail inthe case of methyl iodide45) the participating (nπ)σlowast state(s)gains transition probability by vibronic interaction with a

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224303-9 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 10 Diagram showing the calculated relative energies of 2-bromothiophene (A) the biradical intermediate B formed upon CndashS bondfission TSs leading to various ring-opened isomers (D-G) and selectedBr + radical (H I J K) product limits The dashed lines at the far right of thefigure simply correlate given structures to particular dissociation productswe have not checked for the presence (or otherwise) of any barrier in thevarious fragmentation paths The various structures along with their energies(in cmminus1 defined relative to the ground state minimum) are detailed below to-gether with the corresponding quantities for 2-iodothiophene (in parentheses)The horizontal dashed line indicates the total energy reached upon absorptionof a 250 nm photon

higher lying CndashX bond localised σσlowast state and the subsequentbond fission occurs on a time scale that is fast compared to themolecular rotational period

The primary novelty of the present study however is theclear switch in energy disposal upon reducing the excitationwavelength This switch is revealed by the appearance (andgrowing dominance) of a slow isotropic component withinthe X and Xlowast imagesmdashmore obviously in the case of2-bromothiophenemdashand roughly coincides with the start ofthe obvious increase in parent absorption that we associatewith πlowast larr π absorption (Fig 3) Careful inspection of Figs 6and 7 also shows that the ldquofastrdquo component in the I and Ilowast

images slows and becomes less anisotropic as the photonenergy is increased further

The PES for this ππlowast state (the 21Aprime state in Fig 9)shows CIs (at geometries not far from the vertical FranckCondon region) with other excited states the potential energiesof which decline with increases in RCndashX andor RCndashS Thusit is reasonable to propose that excitation at these shorterwavelengths populates the (diabatically bound) 21Aprime statewhich can then decay by coupling to states that are dissociativewith respect to CndashX andor CndashS bond extension Which ofthese decay processes dominates (ie the CndashX bond fissionor the channel initiated by CndashS bond extension) will besensitively dependent upon the respective minimum energypathways from the 21Aprime state and the details of the couplingmodes that facilitate the population transfer

The almost total absence of ldquofastrdquo BrBrlowast products in theimages recorded following photolysis of 2-bromothiopheneat λ sim 245 nm suggests that (i) the partial absorption crosssection to the 21Aprime state far exceeds that to the underlying(nπ)σlowast state(s)mdashconsistent with the relative weakness of thelong wavelength tail in the UV absorption spectrum of 2-bromothiophenemdashand (ii) CndashS bond extension constitutes themajor decay pathway from the 21Aprime state and the subsequentnuclear motion samples the predicted region of CI with theS0 PES at extended CndashS bond lengths Radiationless transferat this CI yields highly vibrationally excited S0 moleculesThese have more than enough internal energy to decay notonly by RndashBr bond fission (yielding thienyl cofragments)but also to ring-open isomerise and decay to yield slowBr fragments in tandem with a range of C4H3S radicalproducts Figure 10 suggests similar energetics for CndashBr bondfission leading to ring closed (R) and some of the morestable ring opened radical products (eg H and I) Entropicconsiderations might favour the latter products but any seriousdiscussion of the likely product yields would require betterknowledge of the topology of the CI via which population isfunnelled to the biradical intermediate on the S0 PES and ofthe rate with which the latter can overcome the barrier to thestable ring-opened products featured on the right hand side ofFig 10

The data for 2-iodothiophene are less clear cut but againthe relative yield of slow I atoms increases upon tuning toshorter wavelengthsmdashconsistent with initial excitation to thecorresponding 21Aprime state and radiationless transfer to (andunimolecular decay from) highly vibrationally excited S0molecules formed by coupling through the predicted CI atextended RCndashS The ldquofastrdquo IIlowast feature also persists at shorterwavelengths however This could be explained by one orother or both of two mechanisms The first recognises the(relatively) greater partial absorption cross sections of theσCndashI

lowast larr nπ excitations in 2-iodothiophene (Fig 3) while thesecond assumes that following excitation to the 21Aprime state thethreshold energies for predissociation enabled by CndashI and CndashSbond extension are more similar than in 2-bromothiopheneThe evident reduction in the TKER and recoil anisotropyof the ldquofastrdquo I signal at shorter wavelengths hints that CndashXbond fission is a relatively more important decay channelfrom the 21Aprime state of 2-iodothiophene than in the case of2-bromothiophene

Finally we contrast the fast anisotropic Br and Brlowast

product velocity distributions formed in the 267 nm photolysis

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1372224331 On Tue 09 Jun 2015 131203

224303-10 Marchetti et al J Chem Phys 142 224303 (2015)

of 2-bromothiophene with the slow isotropic distributionsobserved when exciting 3-bromothiophene at this samewavelength9 The latter are reminiscent of that found whenexciting 2-bromothiophene at much shorter wavelengths(below sim245 nm) As noted previously all of our attemptsto characterise the analogue of biradical B on the S0 PES of3-bromothiophene minimised to a lower energy cyclopropeneadduct (an enantiomer of G) which will have ample internalenergy to decay further (eg to H + X products) Thepublished photolysis data for 3-bromothiophene may thusbe explicable if the corresponding σCndashS

lowast larr (nπ) PESs aresomewhat red-shifted cf 2-bromothiophene and provide anefficient route for funnelling photoexcited molecules to regionsof CI with the S0 PES and thence to CndashBr bond fission onthis latter PES Photolysis studies of 3-halothiophenes at arange of (longer) UV excitation wavelengths could be usefulin confirming (or refuting) this proposal

IV CONCLUSIONS

The near UV photofragmentation dynamics of jet-cooledgas phase 2-bromo- and 2-iodothiophene molecules havebeen investigated at many different excitation wavelengthsby measuring the velocity distributions of the XXlowast productsusing a VMI spectrometer equipped with a new purposedesigned ion optics assembly The new ion optics design isnot much more complicated than the traditional flat annularelectrode stack and offers a number of potentially significantadvantages stable velocity mapping from a larger interactionvolume protection from stray fields improved limiting veloc-ity resolution and natural compatibility with slice imagingThe present study addressing the photodissociation of a 9-atomic system does not provide an appropriate vehicle forillustrating the predicted velocity resolution As discussedin the supplementary material3 the ultimate test of theresolving power of the new ion optics assembly should involvephotodissociation studies of fully quantum state selectedparent molecules such as those reported by the Janssen groupfor methyl iodide46

The present imaging study reveals a marked change inphotofragmentation behaviour upon tuning to shorter excita-tion wavelengths particularly in the case of 2-bromothiopheneLong wavelength excitation results in prompt CndashX bondfission the resulting XXlowast products are formed withkinetic energies close to the maximum allowed by energyconservation and with near limiting parallel recoil anisotropyCompanion ab initio calculations show that these productsarise following excitation to one or more (nπ)σlowast states andserve to highlight the photophysical similarities with otheraryl iodides (and bromides) when excited at longer UVwavelengths4ndash8

At shorter wavelengths the anisotropic fast ring asso-ciated with these products fades in relative intensity and isprogressively replaced by a smaller isotropic feature thatsignifies the formation of much slower XXlowast atom productsThe appearance of this new channel roughly coincides withthe onset of more intense parent absorption to the 21Aprime excitedstate Ab initio theory suggests that this ππlowast excited statecan predissociate by interaction with neighbouring (nπ)σlowast

states the potentials for which decline in energy along boththe RCndashX and RCndashS stretch coordinates The 2-bromothiophenedata can be understood if the latter offers the lower energydecay route from the optically bright 21Aprime state to a CIwith the ground state PES at extended CndashS bond lengthsThe present DFTB3LYP6-311G(dp) calculations suggestthat the resulting biradicals have more than sufficient energyto sample the configuration space associated with severaldifferent isomeric forms of the starting molecule and to formboth ring closed and ring opened C4H3S radical productsfollowing subsequent CndashBr bond fission on the S0 PES

The data for 2-iodothiophene are less clear cut butagain the relative yield of slow I atoms increases markedlyupon tuning to shorter wavelengthsmdashconsistent with initialexcitation to the corresponding 21Aprime state and radiationlesstransfer to (and unimolecular decay from) highly vibrationallyexcited S0 molecules formed by coupling through the predictedCI at extended RCndashS The ldquofastrdquo IIlowast feature also persists atshorter wavelengths however This could be explained byone or other or both of two mechanisms The first recognisesthe (relatively) greater partial absorption cross sections ofthe σCndashI

lowast larr nπ excitations in 2-iodothiophene while thesecond assumes that following excitation to the 21Aprime state thethresholds for predissociation enabled by CndashI and CndashS bondextension are closer in energy than in 2-bromothiophene

It is interesting to compare the present findings withthose from recent condensed phase studies of thiophenonein acetonitrile solution wherein the photoexcited parentmolecules were similarly deduced to convert to highlyvibrationally excited levels of the ground (S0) state bycoupling via a CI at extended CndashS bond lengths19 Ringopening was confirmed in that case by the observation ofketene products but the time evolution of the parent bleachsignals indicated that the majority (sim60) of these highlyvibrationally excited S0 molecules relax and re-thermalise asthe ring closed thiophenone Such vibrational relaxation is notavailable in the present isolated molecule gas phase studies soeventual dissociation is assured on energetic grounds but therelative probabilities for forming ring opened or ring closedfragmentation products remains an open question

ACKNOWLEDGMENTS

The authors are grateful to EPSRC (Programme GrantNos EPG00224X and EPL005913) to the TSB (KnowledgeTransfer Partnership KTP008481 that stimulated the Bristol-Photek collaboration) to the Marie Curie Initial TrainingNetwork ICONIC (Contract Agreement No 238671) andto Rebecca Ingle Keith Rosser Dr Stuart Greaves DrJames Smith Dr Andreas Wenge Dr Eckart Wrede and DrDimitrios Zaouris for their many and varied contributions tothis work Data accessibility The underlying data for thispaper has been placed in the University of Bristolrsquos researchdata repository and can be accessed using the following DOI105523brisqwlb0o0e9tbd1bqojd8gxrnjj

1A L Sobolewski W Domcke C Dedonder-Lardeux and C Jouvet PhysChem Chem Phys 4 1093 (2002)

2M N R Ashfold G A King D Murdock M G D Nix T A A Oliverand A G Sage Phys Chem Chem Phys 12 1218 (2010)

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224303-3 Marchetti et al J Chem Phys 142 224303 (2015)

limiting velocity (ie ∆rr or ∆vv) resolution The overallassembly is longer (eg the distance between the plane ofionisation and the ground (G) electrode is 82 mm (cf 70 mmin the previous design)) The electrodes and support rods aremade of stainless steel and the latter are covered with aninsulating sleeve which is in turn covered with a thin stainlesssteel sleeve that is electrically connected to the adjacent lowerpotential electrodemdashthereby ensuring that at no stage do theions of interest have direct line of sight to any insulatingpart

There are an infinite number of possible geometryconfigurations that could in principle be modelled in the questfor optimum performance Thus it was necessary to adopta systematic approach to optimising the ion optics designwithin the constraints imposed by the physical size of theexisting vacuum chamber the diameter of the annular cryo-shield surrounding the interaction region and the requirementthat the laser propagation axis must lie between the repeller(R) and extractor (E) electrodes The ion optics geometry wasmodified iteratively using a routine written in Lua and a seriesof 2-D simulations run to determine the velocity mappingcapability of each configuration Initial simulations concen-trated on understanding the effects of changing aspects of thegeometry (eg aperture diameters and electrode spacings)These preliminary calculations encouraged more thoroughinvestigation of three design features (i) the shapedepth of theR electrode (ii) the shapecurvature of the E electrode and (iii)the extent to which the electrode assembly design shields theacceleration region from stray fields Our exploration of eachof these factors is now described Further details the results ofsupporting SIMION simulations and illustrative test data forphotolysis of IBr are included in the supplementary material3

(i) Our earlier design introduced a top hat shaped Relectrode in place of the traditional annular plate25 Such adesign in combination with a conical E electrode yielded fieldlines that are suitably parallel to one another and perpendicularto the extraction axis The present modelling reveals thatfurther improvements in velocity mapping capability can beachieved by increasing the recess depth d of this R electrodefurther as illustrated in Fig 2(a) In practice the additionaldepth that can be accommodated in the existing vacuumchamber is limited by the location of the laser beam axis (andthe entrance and exit windows) relative to the bulkhead thatsupports the skimmer separating the source and interactionregions but the simulations suggest a sim2-fold improvement inthe minimum ∆rr should be achieved (with both the previousand new ion optics designs) upon increasing d to sim14 mm

(ii) The earlier design25 also introduced a thicker Eelectrode with a conical aperture as a route to reducingchromatic and spherical aberrations and shielding the ions ofinterest from external stray fields The larger volume createdby using appropriately shaped R and E electrodes results ina weaker field gradient in the interaction region This ldquosoftfocussingrdquo configuration offers several potential benefits Thecloud of ions formed in the interaction region has more timeto expand before entering the later acceleration and TOFregionsmdashwith consequent benefits for slice imaging (see thesupplementary material3) Further the ions and their resultingtrajectories are less sensitive to their point of creation in

FIG 2 Illustrations of the ∆rr resolution returned by SIMION modellingof the new (and previous (Ref 25)) velocity mapping ion optics for (a)different depths d of the R electrode and (b) different aperture diametersfor the E electrode with a spherical entrance face For each new geometrythe simulations employ the best operational voltages (holding VR fixed at+25 kV) and assume the ejection of 35Cl+ ions with 15 eV kinetic energyperpendicular to the time-of-flight axis from a volume with ∆x = 05 mm inthe TOF direction and ∆y = 2 mm in the laser propagation direction

the interaction region (cf when using standard flat annularelectrodes) The velocity mapping condition is thus muchless sensitive to small changes in laser alignment or inthe electrode voltages and the volume within which goodvelocity mapping can be achieved is substantially increasedIntroducing a sphericalcurved surface in place of the previousconical aperture further improves the field definition withinthe interaction volume resulting in a sim20 improvement in∆rr (for the same entrance and exit aperture diameters) AsFig 2(b) shows this can be further improved by increasingthe aperture of the E electrode which in the new designcomprises an entrance face defined by a sphere of radius291 mm (centred on the TOF axis) and entrance and exitapertures of respectively 56 mm (as before) and 44 mm Asnoted previously37 use of a spherical entrance surface alsoensures that the desired linear relationship between imageradius (r) and ion velocity (v) extends all the way to r = 0

(iii) Additional large aperture ldquostabilisingrdquo electrodeshave been introduced between the E and L electrodes andbetween the L and G electrodes (all electrically linked viaa resistor chain) as shown in Fig 1 These provide further

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224303-4 Marchetti et al J Chem Phys 142 224303 (2015)

improvements in field definition in the later accelerationregions and provide a valuable counter against perturbingfields from the earthed support rods and any other externalinfluences

Further comparative simulations and sample Br andor Iatom images from the near UV photodissociation of jet-cooledIBr molecules recorded with the new ion optics assembly areincluded in the supplementary material3

B Ab initio calculations

Using the Molpro computational package38 the groundstate minimum energy geometries of 2-iodothiophene and 2-bromothiophene were optimised using Moslashller-Plesset secondorder perturbation theory (MP2) and Dunningrsquos contractedcc-pVDZ basis set (for the H C S and Br atoms)39 Forthe I atom a pseudopotential was added to the double-ζ(cc-pVDZ-pp) basis function along with a 46 electronrelativistic effective core potential40 Rigid body potentialenergy curves (PECs) were then calculated as a functionof the CndashX bond length RCndashX with the remainder of thenuclear framework fixed at the MP2 equilibrium geometry Cssymmetry was maintained at each point along this scan Spinorbit free (SOF) PECs were calculated using a state-averaged(SA) complete active space self-consistent field (CASSCF)reference wavefunction involving five 1Aprime four 1Aprimeprime five3Aprime and four 3Aprimeprime states The corresponding state energieswere calculated using the complete active space with secondorder perturbation theory (CASPT2) based on the above SA-CASSCF reference wavefunction with an imaginary levelshift of 05 Eh to aid convergence and circumvent the effectsof intruder states The active space for these calculationscomprised 12 electrons in the following nine orbitals threeoccupied π orbitals two unoccupied πlowast orbitals the px and pylone pairs centred on the X atom and the σ and σlowast orbitalslocalised around the CndashX bond Spin-orbit coupled states werethen calculated by evaluating the spin-orbit Hamiltonian (HSO)in a basis of ψelec these states are henceforth labelled using theextended irreducible representation including both spin-orbitfree and spin parts In order to account for dynamic correlationof electrons the obtained SOF CASPT2 energies were thenused in the diagonalisation of the spin-orbit coupling matrix

Gaussian 0941 was used to calculate ldquorelaxedrdquo PECsalong the C(2)ndashS ring opening coordinate (RCndashS) Thesecalculations started from the MP2 optimised ground stategeometry (above) RCndashS was then extended in 01 Aring incrementsto RCndashS = 37 Aring and at each value the remainder of thenuclear framework was optimised at the MP2LANL2DZ level(with Cs symmetry constraints) The excited states were thencalculated at these various relaxed ground state geometriesalong RCndashS using SOF CASPT2 These CASPT2 calculationsutilised the same state averaging active space basis set andimaginary level shift as the previously described calculationsalong RCndashX

Following C(2)ndashS bond extension various ground stateminima and transition state (TS) geometries linking potentialisomers were calculated using Density Functional The-ory (DFT) along with the Becke 3-parameter Lee-Yang-Parr (B3LYP) functional The default 6-311G(dp) basis

set embedded within Gaussian 09 was used for the 2-bromothiophene calculations whereas for 2-iodothiophenethe functions of the equivalent 6-311G(dp) basis set weregenerated artificially (and are listed in the supplementarymaterial3) The biradicals formed upon ring-opening in 2-bromo- and 2-iodothiophene were identified as local minimaand could thus be optimised in the normal way usingDFTB3LYP6-311G(dp) and a singlet spin multiplicityGiven the evident differences in the Br product recoilvelocity distributions from 267 nm photolysis of 2- and3-bromothiophene9 we also attempted to characterise theequivalent ring-opened biradical on the ground state PES of3-bromothiophene These calculations were unsuccessfulmdashminimising instead to a lower energy cyclopropene adductThus the lowest root of triplet spin multiplicity was used tooptimise this critical point in the case of 3-bromothiopheneand a linear interpolation in internal coordinate (LIIC) wasthen scanned (using the above level of DFT) between thering opened biradical and the cyclopropene adduct in order toestablish the energy profile along this coordinate

III RESULTS AND DISCUSSION

Near UV absorption spectra of the two title halothio-phenes are shown in Figure 3 along with an illustrativemolecular structure Both spectra show an intense band centredatsim230-240 nm and a tail stretching to longer wavelengths thatis much more evident in the case of 2-iodothiophene The 2-iodothiophene absorption profile agrees well with that reportedpreviously (in cyclohexane solution)10 but additionally showsa sequence of diffuse resonances separated by sim630 cmminus1

on the long wavelength side of the intense feature Analogywith bare thiophene (and the present ab initio calculationsvide infra) suggests that this feature is associated with oneor more πlowast larr π excitations and that these resonances arelikely attributable to symmetric (or antisymmetric) CndashSndashCstretch motions The long wavelength tail can be viewed asthe analogue of the σlowast larr nπ ldquoA-bandrdquo absorptions of otheralkyl (and aryl) halides The greater prominence and extent ofthis tail in the spectrum of 2-iodothiophene accords with thefindings of our recent studies of 4-bromo- and 4-iodophenol

FIG 3 Room temperature near-UV absorption spectra of 2-bromo- and2-iodothiophene recorded using the vapour above a small sample of the neatliquid in an atmosphere of air

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1372224331 On Tue 09 Jun 2015 131203

224303-5 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 4 Images of Br(2P32) products resulting from photolysis of jet-cooled2-bromothiophene at λ= (a) 2666 nm (b) 2606 nm (c) 2542 nm (d)2498 nm and (e) 2449 nm with the ε vector of the photolysis laser radiationaligned vertically in the plane of the images (as shown in panel (a)) TheTKER spectrum derived from analysis of each data set is shown to the rightof each image with the vertical arrow showing the TKERmax value assumingD0(RminusBr)= 29 000 cmminus1

nominally spin-forbidden singlet-triplet transitions appearwith much greater transition strength in the latter8

A Ion imaging data

The left hand panels in Figs 4ndash7 show raw images ofrespectively the Br and Brlowast fragments resulting from photol-ysis of 2-bromothiophene and the I and Ilowast fragments fromphotolysis of 2-iodothiophene recorded at several differentnear UV wavelengths For consistency only pancaked imagesare reported in the main paper All images of a given specieswere recorded using the same extraction voltages (ie therespective radii are reliable indicators of the relative velocity)and in all cases the electric (ε) vector of the photolysis laserradiation was vertical (ie parallel to the front face of thedetector) as shown on the first image in each figure Thecorresponding total kinetic energy release (TKER) spectra(shown to the right of each image) were derived using theappropriate Jacobian together with the assumption that theunobserved fragment partnering the BrI atom has chemicalformula C4SH3 and mass m = 8313 u

These data show a number of trends which we review byfirst focussing on 2-bromothiophene The Br image recordedat longest photolysis wavelengthmdasha one colour experimentat 2666 nmmdashshows a fast anisotropic recoil velocitydistribution peaking at TKER sim6000 cmminus1 with a full width

FIG 5 Images of Br(2P12) products resulting from photolysis of jet-cooled2-bromothiophene at λ= (a) 2667 nm (b) 2625 nm (c) 2542 nm and (d)2448 nm with the ε vector of the photolysis laser radiation aligned verticallyin the plane of the images (as shown in panel (a)) The TKER spectrumderived from analysis of each image is shown on the right with the verticalarrow indicating the TKERmax value assuming D0(RminusBr)= 29 000 cmminus1

half maximum (FWHM) of sim3000 cmminus1 and an associatedanisotropy parameter β sim 2 ie the limiting value for promptaxial recoil following excitation via a transition dipole moment(TDM) for which is aligned parallel to the breaking bondmdashasreported (following photolysis at 267 nm) by Zhang et al9

Moving to shorter wavelengths the anisotropic fast ringfades in relative intensity and is progressively replaced bya smaller isotropic featuremdashexemplified by the data recordedat 2449 nm (Fig 4(e)) The TKER spectrum derived from thisimage peaks close to zero

The Brlowast fragment images from photolysis of 2-bro-mothiophene (and the TKER spectra derived from them) reveala similar trend The images recorded at longer wavelengthsie at 2667 and 2625 nm (Figs 5(a) and 5(b)) are dominatedby an anisotropic feature peaking at TKER sim3500 cmminus1with a FWHM of sim3000 cmminus1 The β parameter is velocitydependent sim15 around the peak of the distribution anddeclining at lower TKER The weak Brlowast image taken atλ = 2448 nm in contrast is essentially isotropic and peaks atlow TKER

I and Ilowast images from 2-iodothiophene were recordedover a greater range of photolysis wavelengths and againshow similar trends A weak one-colour I atom signal wasdiscernible following excitation at 30368 nm As Fig 6(a)shows these I atoms display a narrow (sim2000 cmminus1 (FWHM))anisotropic (β sim 17) translational energy distribution peakingat TKER sim7000 cmminus1 which initially expands upon scanningto shorter wavelengths but then shrinks (in radius) becomesmore isotropic fades (in relative intensity) and is supple-mented by a slower isotropic component as the photolysiswavelength is reduced Nonetheless as Fig 6(f) shows a

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1372224331 On Tue 09 Jun 2015 131203

224303-6 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 6 Images of I(2P32) products resulting from photolysis of jet-cooled 2-iodothiophene at λ= (a) 3037 nm (b) 2779 nm (c) 2668 nm (d) 2547 nm(e) 2437 nm and (f) 2202 nm with the ε vector of the photolysis laserradiation aligned vertically in the plane of the images (as shown in panel(a)) The TKER spectrum derived from analysis of each data set is shownon the right with the vertical arrow showing the TKERmax value assumingD0(Rminus I)= 24 500 cmminus1

faster (albeit more anisotropic) component peaking at TKERsim7000 cmminus1 is still discernible at λ = 2202 nm

As in the cases of iodobenzene6 various fluorinatediodobenzenes7 and 4-iodophenol8 the Ilowast images measuredfollowing excitation of 2-iodothiophene at long wavelengthshow hints of product vibrational structure as illustrated inFig 7(a) (λ = 2774 nm) Following past precedent we assignthe most obvious peak in the TKER spectrum taken at longestwavelength (indicated by the arrow in Fig 7(a)) to formationof Ilowast atoms together with radical fragments in their ground(ie zero-point v = 0) vibrational level Such an attributionallows estimation of the CndashI bond strength via the usual energyconservation relationship

D0(R minus I) = Ephot minus A(I) minus TKERmax

where R represents the (α-)thienyl (sometimes also referredto as the thiophenyl) radical Ephot is the photolysis photonenergy A(I) is the spin-orbit energy of the Ilowast atom (7603 cmminus1)TKERmax is the TKER associated with this fastest featureand we neglect the small amount of internal excitation in

FIG 7 Images of I(2P12) products resulting from photolysis of jet-cooled 2-iodothiophene at λ= (a) 2774 nm (b) 2670 nm (c) 2547 nm (d) 2437 nmand (e) 2202 nm with the ε vector of the photolysis laser radiation alignedvertically in the plane of the images (as shown in panel (a)) The TKERspectrum derived from analysis of each data set is shown on the rightwith the vertical arrow showing the TKERmax value assuming D0(Rminus I)= 24 500 cmminus1

the jet-cooled parent molecules this analysis yields D0(R minus I)= 24 500 plusmn 500 cmminus1mdasha value that is sensibly consistent witha typical CndashI bond strength in other aromatic systems6ndash8 Abinitio calculations for the ground state thienyl radical provideclues as to the assignment of the vibrational peak observedin Fig 7(a) A unique assignment is not possible giventhe limited energy resolution but the observed wavenumberseparation (sim1500 cmminus1 from the peak assigned to v = 0products) is sensibly consistent with one quantum of the in-plane symmetric (and antisymmetric) CndashC stretching modesmdashsee the supplementary material3 We note that the formeris prominent in the previously reported RR spectra of2-iodothiophene (albeit when exciting at slightly shorterwavelengths)10

The short vertical arrows at the high TKER end of thespectra shown in Figs 6 and 7 indicate the maximum TKERspredicted for the I (or Ilowast) + R products arising at eachphotolysis wavelength given D0(R minus I) = 24 500 cmminus1 Thesedata emphasise that the mean TKER ⟨TKER⟩ associatedwith each channel is rather insensitive to changes in Ephotimplying progressively greater partitioning of the availableenergy (ie of Ephot minus D0(R minus I)) into internal excitation ofthe thienyl radical productmdasha conclusion that again is ingood accord with those reached from the earlier resonanceRaman studies10

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224303-7 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 8 Diabatic SO resolved PECs along RCndashX for the ground and lower lying excited states of (a) 2-bromothiophene and (b) 2-iodothiophene

None of the TKER distributions derived from the Br or Brlowast

images from photolysis of 2-bromothiophene show any hintsof product vibrational structure (as was the case also in theimaging study of 4-bromophenol photolysis8) Nonethelessif we assume similar dissociation dynamics to that revealedby 2-iodothiophene and that the TKERmax value for productsformed from 2-bromothiophene photolysis at the longestUV wavelengths similarly lies on the fast edge of the highTKER feature (as shown by the short vertical arrow in bothFigs 4(a) and 5(a)) we arrive at a CndashBr bond strength D0(R minus Br) sim 29 000 cmminus1 (given A(Br) = 3685 cmminus1) that isagain sensibly consistent with values reported for bro-mobenzene (3428 kJ molminus1 28 650 cmminus1 Ref 42) andfor 4-bromophenol (28 700 cmminus1 Ref 8) but substantiallylarger than an earlier suggested CndashBr bond strength in 2-bromothiophene43 As the other panels in Figs 4 and 5 showtuning to shorter photolysis wavelengths again results in arelative increase in the internal energy partitioned into thethienyl partner

B Ab initio calculations of CndashBrCndashI bond fissionand CndashS ring opening pathways in 2-bromothiopheneand 2-iodothiophene

Figures 8(a) and 8(b) present SO resolved PECs alongRCndashX for the ground and lower energy excited states of respec-tively 2-bromothiophene and 2-iodothiophene In both casesthe molecule has been constrained to planar geometries andthe PECs have been approximately ldquodiabatisedrdquo in terms of(nπ)σlowast and ππlowast states by inspecting the energies symmetriesand wavefunction coefficients The adiabatic SO resolvedpotentials from which these are derived are shown in thesupplementary material3 As in the case of the iodobenzenes67

and halophenols8 the lowest repulsive potentials all arise fromσCndashX

lowast larr nπ excitations these potentials correlate with theground (X2Aprime) state thienyl + XXlowast dissociation limits Thespin-orbit split analogues involving the first (A2Aprimeprime) excitedstate of the thienyl radical lie higher in energy while the(diabatically) bound interloper PECs arise from excitationsto ring centred πlowast orbitals The calculated vertical excitationenergies suggest that many of these excited states could besampled within the wavelength ranges explored in the presentstudy but analogy with the previous VMI studies of the XXlowast

products from photolysis of iodobenzenes and 4-iodo- and4-bromophenol suggests that the σlowast larr nπ TDMs returnedby the present calculations are likely to be of limited valuein deciding which are the optically ldquobrightrdquo states (illustrativeTDMs are listed in the supplementary material3) Rather asin those cases6ndash8 the observation that the faster X and Xlowast

products formed at long excitation wavelengths show nearlimiting parallel recoil anisotropy suggests that the populatedstates are those that gain intensity by vibronic interactionwith the higher lying intense CndashX bond centred σlowast larr σtransition

The predicted energy separations between the variousasymptotic limits merit comment The present calculationsreproduce the BrBrlowast and IIlowast spin-orbit splittings well butpredict a larger separation between the A and X statesof the thienyl radical (sim125 eV) than that derived fromphotodetachment studies of the thiophenide anion (sim06 eVRef 44) This is not surprising given that the PECs are derivedvia rigid body scans and thus preclude any structural relaxationof the ring Nonetheless the present calculations confirm theearlier suggestion that (nπ)σlowast excited states (ie states thatare dissociative with respect to CndashX bond extension and whichcorrelate to X or Xlowast products) are dominant contributors tothe long wavelength tail of the respective absorption spectraand locate the minimum of the first (spin-allowed) πlowast larr πtransition at sim53 eV (sim59 eV) for X = I (Br) in reasonableaccord with the observed absorption maxima (Fig 3)

Cuts through the first few SOF potentials of 2-bro-mothiophene and 2-iodothiophene along the RCndashS stretch(ring-opening) coordinate are shown in Figs 9(a) and 9(c)respectively The corresponding adiabatic SOF PECs alongRCndashX are shown alongside in Figs 9(b) and 9(d) The S0 stateis stable with respect to ring opening in both halothiophenesand its PEC thus rises steadily with increasing RCndashS Incontrast the PECs for the 11Aprimeprime and 21Aprimeprime states (and fortheir triplet analogues all of which arise via σCndashS

lowast larr (nπ)excitations) show obvious instability with respect to extendingRCndashS Notwithstanding the limitations of these SOF PECs(all of which are calculated at the MP2 relaxed ground stategeometries) they clearly point to the availability of exoergicpathways from the bright 1ππlowast (21Aprime) state via one or more(nπ)σCndashS

lowast PECs to extended CndashS bond lengths and regionsof conical intersection with the S0 state potential

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224303-8 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 9 Approximately diabatised SO free PECs along the RCndashS (ring-opening) coordinate and RCndashX stretch for the first few singlet (solid lines) and triplet(dashed lines) states of 2-bromothiophene ((a) and (b) respectively) and 2-iodothiophene ((c) and (d) respectively)

Figure 10 summarises some of the various possible fatesof 2-halothiophene molecules following non-adiabatic transferto the S0 PES The primary ring-opened structure is a biradical(structure B) that can exist as essentially degenerate cis andtrans isomersmdashboth of which are unstable with respect tothe original ground state halothiophene B can also isomeriseto various ring-opened isomers (including structures D-G)via transition states three of which that involve migrating(at most) one atom from the biradical structure (TS1 TS2and TS3) have been optimised As Fig 10 also shows arange of Br + radical (H I J K as well as R) dissociationlimits are calculated to lie below the energy of a 250 nmphoton (illustrated by the horizontal red line) The variousstructures and product pairs along with their energies (incmminus1 defined relative to that of the ground state minimum)are detailed below the figure along with the correspondingquantities for 2-iodothiophene (in parentheses) The calculatedRndashBr and RndashI bond strengths are seen to match well withthe experimental values derived in Sec III A Readersshould also note that the dashed lines at the far right ofFig 10 are simply intended to correlate various isomers toparticular dissociation products we have not checked for thepresence (or otherwise) of any barrier in the various fragmen-tation paths

C A plausible explanation for the observedwavelength dependent fragmentation dynamics

The X and Xlowast images measured at the longer UVwavelengths are very reminiscent of those reported previouslyfollowing photolysis of for example iodobenzenes67 and 4-iodo- and 4-bromophenol8 The X and Xlowast products recoilwith close to the maximum speed permitted by energyconservation and with close to limiting parallel anisotropyThe velocities of the faster XXlowast products increase littleupon tuning to shorter excitation wavelengths implying apreferential partitioning of the additional photon energy intointernal (vibrational) excitation of the thienyl radical partnerAs in the previously studied aryl iodides and bromides suchbehaviour is fully consistent with population of one or more(nπ)σlowast excited states and prompt CndashX bond fission Theprogressive increase in product vibrational excitation can beunderstood on Franck-Condon grounds given the changes inequilibrium ring geometry upon σlowast larr (nπ) excitation andsubsequent evolution to the radical product As noted abovethe near limiting parallel recoil anisotropy of the X and Xlowast

products can be understood if (as has been shown in detail inthe case of methyl iodide45) the participating (nπ)σlowast state(s)gains transition probability by vibronic interaction with a

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224303-9 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 10 Diagram showing the calculated relative energies of 2-bromothiophene (A) the biradical intermediate B formed upon CndashS bondfission TSs leading to various ring-opened isomers (D-G) and selectedBr + radical (H I J K) product limits The dashed lines at the far right of thefigure simply correlate given structures to particular dissociation productswe have not checked for the presence (or otherwise) of any barrier in thevarious fragmentation paths The various structures along with their energies(in cmminus1 defined relative to the ground state minimum) are detailed below to-gether with the corresponding quantities for 2-iodothiophene (in parentheses)The horizontal dashed line indicates the total energy reached upon absorptionof a 250 nm photon

higher lying CndashX bond localised σσlowast state and the subsequentbond fission occurs on a time scale that is fast compared to themolecular rotational period

The primary novelty of the present study however is theclear switch in energy disposal upon reducing the excitationwavelength This switch is revealed by the appearance (andgrowing dominance) of a slow isotropic component withinthe X and Xlowast imagesmdashmore obviously in the case of2-bromothiophenemdashand roughly coincides with the start ofthe obvious increase in parent absorption that we associatewith πlowast larr π absorption (Fig 3) Careful inspection of Figs 6and 7 also shows that the ldquofastrdquo component in the I and Ilowast

images slows and becomes less anisotropic as the photonenergy is increased further

The PES for this ππlowast state (the 21Aprime state in Fig 9)shows CIs (at geometries not far from the vertical FranckCondon region) with other excited states the potential energiesof which decline with increases in RCndashX andor RCndashS Thusit is reasonable to propose that excitation at these shorterwavelengths populates the (diabatically bound) 21Aprime statewhich can then decay by coupling to states that are dissociativewith respect to CndashX andor CndashS bond extension Which ofthese decay processes dominates (ie the CndashX bond fissionor the channel initiated by CndashS bond extension) will besensitively dependent upon the respective minimum energypathways from the 21Aprime state and the details of the couplingmodes that facilitate the population transfer

The almost total absence of ldquofastrdquo BrBrlowast products in theimages recorded following photolysis of 2-bromothiopheneat λ sim 245 nm suggests that (i) the partial absorption crosssection to the 21Aprime state far exceeds that to the underlying(nπ)σlowast state(s)mdashconsistent with the relative weakness of thelong wavelength tail in the UV absorption spectrum of 2-bromothiophenemdashand (ii) CndashS bond extension constitutes themajor decay pathway from the 21Aprime state and the subsequentnuclear motion samples the predicted region of CI with theS0 PES at extended CndashS bond lengths Radiationless transferat this CI yields highly vibrationally excited S0 moleculesThese have more than enough internal energy to decay notonly by RndashBr bond fission (yielding thienyl cofragments)but also to ring-open isomerise and decay to yield slowBr fragments in tandem with a range of C4H3S radicalproducts Figure 10 suggests similar energetics for CndashBr bondfission leading to ring closed (R) and some of the morestable ring opened radical products (eg H and I) Entropicconsiderations might favour the latter products but any seriousdiscussion of the likely product yields would require betterknowledge of the topology of the CI via which population isfunnelled to the biradical intermediate on the S0 PES and ofthe rate with which the latter can overcome the barrier to thestable ring-opened products featured on the right hand side ofFig 10

The data for 2-iodothiophene are less clear cut but againthe relative yield of slow I atoms increases upon tuning toshorter wavelengthsmdashconsistent with initial excitation to thecorresponding 21Aprime state and radiationless transfer to (andunimolecular decay from) highly vibrationally excited S0molecules formed by coupling through the predicted CI atextended RCndashS The ldquofastrdquo IIlowast feature also persists at shorterwavelengths however This could be explained by one orother or both of two mechanisms The first recognises the(relatively) greater partial absorption cross sections of theσCndashI

lowast larr nπ excitations in 2-iodothiophene (Fig 3) while thesecond assumes that following excitation to the 21Aprime state thethreshold energies for predissociation enabled by CndashI and CndashSbond extension are more similar than in 2-bromothiopheneThe evident reduction in the TKER and recoil anisotropyof the ldquofastrdquo I signal at shorter wavelengths hints that CndashXbond fission is a relatively more important decay channelfrom the 21Aprime state of 2-iodothiophene than in the case of2-bromothiophene

Finally we contrast the fast anisotropic Br and Brlowast

product velocity distributions formed in the 267 nm photolysis

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1372224331 On Tue 09 Jun 2015 131203

224303-10 Marchetti et al J Chem Phys 142 224303 (2015)

of 2-bromothiophene with the slow isotropic distributionsobserved when exciting 3-bromothiophene at this samewavelength9 The latter are reminiscent of that found whenexciting 2-bromothiophene at much shorter wavelengths(below sim245 nm) As noted previously all of our attemptsto characterise the analogue of biradical B on the S0 PES of3-bromothiophene minimised to a lower energy cyclopropeneadduct (an enantiomer of G) which will have ample internalenergy to decay further (eg to H + X products) Thepublished photolysis data for 3-bromothiophene may thusbe explicable if the corresponding σCndashS

lowast larr (nπ) PESs aresomewhat red-shifted cf 2-bromothiophene and provide anefficient route for funnelling photoexcited molecules to regionsof CI with the S0 PES and thence to CndashBr bond fission onthis latter PES Photolysis studies of 3-halothiophenes at arange of (longer) UV excitation wavelengths could be usefulin confirming (or refuting) this proposal

IV CONCLUSIONS

The near UV photofragmentation dynamics of jet-cooledgas phase 2-bromo- and 2-iodothiophene molecules havebeen investigated at many different excitation wavelengthsby measuring the velocity distributions of the XXlowast productsusing a VMI spectrometer equipped with a new purposedesigned ion optics assembly The new ion optics design isnot much more complicated than the traditional flat annularelectrode stack and offers a number of potentially significantadvantages stable velocity mapping from a larger interactionvolume protection from stray fields improved limiting veloc-ity resolution and natural compatibility with slice imagingThe present study addressing the photodissociation of a 9-atomic system does not provide an appropriate vehicle forillustrating the predicted velocity resolution As discussedin the supplementary material3 the ultimate test of theresolving power of the new ion optics assembly should involvephotodissociation studies of fully quantum state selectedparent molecules such as those reported by the Janssen groupfor methyl iodide46

The present imaging study reveals a marked change inphotofragmentation behaviour upon tuning to shorter excita-tion wavelengths particularly in the case of 2-bromothiopheneLong wavelength excitation results in prompt CndashX bondfission the resulting XXlowast products are formed withkinetic energies close to the maximum allowed by energyconservation and with near limiting parallel recoil anisotropyCompanion ab initio calculations show that these productsarise following excitation to one or more (nπ)σlowast states andserve to highlight the photophysical similarities with otheraryl iodides (and bromides) when excited at longer UVwavelengths4ndash8

At shorter wavelengths the anisotropic fast ring asso-ciated with these products fades in relative intensity and isprogressively replaced by a smaller isotropic feature thatsignifies the formation of much slower XXlowast atom productsThe appearance of this new channel roughly coincides withthe onset of more intense parent absorption to the 21Aprime excitedstate Ab initio theory suggests that this ππlowast excited statecan predissociate by interaction with neighbouring (nπ)σlowast

states the potentials for which decline in energy along boththe RCndashX and RCndashS stretch coordinates The 2-bromothiophenedata can be understood if the latter offers the lower energydecay route from the optically bright 21Aprime state to a CIwith the ground state PES at extended CndashS bond lengthsThe present DFTB3LYP6-311G(dp) calculations suggestthat the resulting biradicals have more than sufficient energyto sample the configuration space associated with severaldifferent isomeric forms of the starting molecule and to formboth ring closed and ring opened C4H3S radical productsfollowing subsequent CndashBr bond fission on the S0 PES

The data for 2-iodothiophene are less clear cut butagain the relative yield of slow I atoms increases markedlyupon tuning to shorter wavelengthsmdashconsistent with initialexcitation to the corresponding 21Aprime state and radiationlesstransfer to (and unimolecular decay from) highly vibrationallyexcited S0 molecules formed by coupling through the predictedCI at extended RCndashS The ldquofastrdquo IIlowast feature also persists atshorter wavelengths however This could be explained byone or other or both of two mechanisms The first recognisesthe (relatively) greater partial absorption cross sections ofthe σCndashI

lowast larr nπ excitations in 2-iodothiophene while thesecond assumes that following excitation to the 21Aprime state thethresholds for predissociation enabled by CndashI and CndashS bondextension are closer in energy than in 2-bromothiophene

It is interesting to compare the present findings withthose from recent condensed phase studies of thiophenonein acetonitrile solution wherein the photoexcited parentmolecules were similarly deduced to convert to highlyvibrationally excited levels of the ground (S0) state bycoupling via a CI at extended CndashS bond lengths19 Ringopening was confirmed in that case by the observation ofketene products but the time evolution of the parent bleachsignals indicated that the majority (sim60) of these highlyvibrationally excited S0 molecules relax and re-thermalise asthe ring closed thiophenone Such vibrational relaxation is notavailable in the present isolated molecule gas phase studies soeventual dissociation is assured on energetic grounds but therelative probabilities for forming ring opened or ring closedfragmentation products remains an open question

ACKNOWLEDGMENTS

The authors are grateful to EPSRC (Programme GrantNos EPG00224X and EPL005913) to the TSB (KnowledgeTransfer Partnership KTP008481 that stimulated the Bristol-Photek collaboration) to the Marie Curie Initial TrainingNetwork ICONIC (Contract Agreement No 238671) andto Rebecca Ingle Keith Rosser Dr Stuart Greaves DrJames Smith Dr Andreas Wenge Dr Eckart Wrede and DrDimitrios Zaouris for their many and varied contributions tothis work Data accessibility The underlying data for thispaper has been placed in the University of Bristolrsquos researchdata repository and can be accessed using the following DOI105523brisqwlb0o0e9tbd1bqojd8gxrnjj

1A L Sobolewski W Domcke C Dedonder-Lardeux and C Jouvet PhysChem Chem Phys 4 1093 (2002)

2M N R Ashfold G A King D Murdock M G D Nix T A A Oliverand A G Sage Phys Chem Chem Phys 12 1218 (2010)

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224303-4 Marchetti et al J Chem Phys 142 224303 (2015)

improvements in field definition in the later accelerationregions and provide a valuable counter against perturbingfields from the earthed support rods and any other externalinfluences

Further comparative simulations and sample Br andor Iatom images from the near UV photodissociation of jet-cooledIBr molecules recorded with the new ion optics assembly areincluded in the supplementary material3

B Ab initio calculations

Using the Molpro computational package38 the groundstate minimum energy geometries of 2-iodothiophene and 2-bromothiophene were optimised using Moslashller-Plesset secondorder perturbation theory (MP2) and Dunningrsquos contractedcc-pVDZ basis set (for the H C S and Br atoms)39 Forthe I atom a pseudopotential was added to the double-ζ(cc-pVDZ-pp) basis function along with a 46 electronrelativistic effective core potential40 Rigid body potentialenergy curves (PECs) were then calculated as a functionof the CndashX bond length RCndashX with the remainder of thenuclear framework fixed at the MP2 equilibrium geometry Cssymmetry was maintained at each point along this scan Spinorbit free (SOF) PECs were calculated using a state-averaged(SA) complete active space self-consistent field (CASSCF)reference wavefunction involving five 1Aprime four 1Aprimeprime five3Aprime and four 3Aprimeprime states The corresponding state energieswere calculated using the complete active space with secondorder perturbation theory (CASPT2) based on the above SA-CASSCF reference wavefunction with an imaginary levelshift of 05 Eh to aid convergence and circumvent the effectsof intruder states The active space for these calculationscomprised 12 electrons in the following nine orbitals threeoccupied π orbitals two unoccupied πlowast orbitals the px and pylone pairs centred on the X atom and the σ and σlowast orbitalslocalised around the CndashX bond Spin-orbit coupled states werethen calculated by evaluating the spin-orbit Hamiltonian (HSO)in a basis of ψelec these states are henceforth labelled using theextended irreducible representation including both spin-orbitfree and spin parts In order to account for dynamic correlationof electrons the obtained SOF CASPT2 energies were thenused in the diagonalisation of the spin-orbit coupling matrix

Gaussian 0941 was used to calculate ldquorelaxedrdquo PECsalong the C(2)ndashS ring opening coordinate (RCndashS) Thesecalculations started from the MP2 optimised ground stategeometry (above) RCndashS was then extended in 01 Aring incrementsto RCndashS = 37 Aring and at each value the remainder of thenuclear framework was optimised at the MP2LANL2DZ level(with Cs symmetry constraints) The excited states were thencalculated at these various relaxed ground state geometriesalong RCndashS using SOF CASPT2 These CASPT2 calculationsutilised the same state averaging active space basis set andimaginary level shift as the previously described calculationsalong RCndashX

Following C(2)ndashS bond extension various ground stateminima and transition state (TS) geometries linking potentialisomers were calculated using Density Functional The-ory (DFT) along with the Becke 3-parameter Lee-Yang-Parr (B3LYP) functional The default 6-311G(dp) basis

set embedded within Gaussian 09 was used for the 2-bromothiophene calculations whereas for 2-iodothiophenethe functions of the equivalent 6-311G(dp) basis set weregenerated artificially (and are listed in the supplementarymaterial3) The biradicals formed upon ring-opening in 2-bromo- and 2-iodothiophene were identified as local minimaand could thus be optimised in the normal way usingDFTB3LYP6-311G(dp) and a singlet spin multiplicityGiven the evident differences in the Br product recoilvelocity distributions from 267 nm photolysis of 2- and3-bromothiophene9 we also attempted to characterise theequivalent ring-opened biradical on the ground state PES of3-bromothiophene These calculations were unsuccessfulmdashminimising instead to a lower energy cyclopropene adductThus the lowest root of triplet spin multiplicity was used tooptimise this critical point in the case of 3-bromothiopheneand a linear interpolation in internal coordinate (LIIC) wasthen scanned (using the above level of DFT) between thering opened biradical and the cyclopropene adduct in order toestablish the energy profile along this coordinate

III RESULTS AND DISCUSSION

Near UV absorption spectra of the two title halothio-phenes are shown in Figure 3 along with an illustrativemolecular structure Both spectra show an intense band centredatsim230-240 nm and a tail stretching to longer wavelengths thatis much more evident in the case of 2-iodothiophene The 2-iodothiophene absorption profile agrees well with that reportedpreviously (in cyclohexane solution)10 but additionally showsa sequence of diffuse resonances separated by sim630 cmminus1

on the long wavelength side of the intense feature Analogywith bare thiophene (and the present ab initio calculationsvide infra) suggests that this feature is associated with oneor more πlowast larr π excitations and that these resonances arelikely attributable to symmetric (or antisymmetric) CndashSndashCstretch motions The long wavelength tail can be viewed asthe analogue of the σlowast larr nπ ldquoA-bandrdquo absorptions of otheralkyl (and aryl) halides The greater prominence and extent ofthis tail in the spectrum of 2-iodothiophene accords with thefindings of our recent studies of 4-bromo- and 4-iodophenol

FIG 3 Room temperature near-UV absorption spectra of 2-bromo- and2-iodothiophene recorded using the vapour above a small sample of the neatliquid in an atmosphere of air

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1372224331 On Tue 09 Jun 2015 131203

224303-5 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 4 Images of Br(2P32) products resulting from photolysis of jet-cooled2-bromothiophene at λ= (a) 2666 nm (b) 2606 nm (c) 2542 nm (d)2498 nm and (e) 2449 nm with the ε vector of the photolysis laser radiationaligned vertically in the plane of the images (as shown in panel (a)) TheTKER spectrum derived from analysis of each data set is shown to the rightof each image with the vertical arrow showing the TKERmax value assumingD0(RminusBr)= 29 000 cmminus1

nominally spin-forbidden singlet-triplet transitions appearwith much greater transition strength in the latter8

A Ion imaging data

The left hand panels in Figs 4ndash7 show raw images ofrespectively the Br and Brlowast fragments resulting from photol-ysis of 2-bromothiophene and the I and Ilowast fragments fromphotolysis of 2-iodothiophene recorded at several differentnear UV wavelengths For consistency only pancaked imagesare reported in the main paper All images of a given specieswere recorded using the same extraction voltages (ie therespective radii are reliable indicators of the relative velocity)and in all cases the electric (ε) vector of the photolysis laserradiation was vertical (ie parallel to the front face of thedetector) as shown on the first image in each figure Thecorresponding total kinetic energy release (TKER) spectra(shown to the right of each image) were derived using theappropriate Jacobian together with the assumption that theunobserved fragment partnering the BrI atom has chemicalformula C4SH3 and mass m = 8313 u

These data show a number of trends which we review byfirst focussing on 2-bromothiophene The Br image recordedat longest photolysis wavelengthmdasha one colour experimentat 2666 nmmdashshows a fast anisotropic recoil velocitydistribution peaking at TKER sim6000 cmminus1 with a full width

FIG 5 Images of Br(2P12) products resulting from photolysis of jet-cooled2-bromothiophene at λ= (a) 2667 nm (b) 2625 nm (c) 2542 nm and (d)2448 nm with the ε vector of the photolysis laser radiation aligned verticallyin the plane of the images (as shown in panel (a)) The TKER spectrumderived from analysis of each image is shown on the right with the verticalarrow indicating the TKERmax value assuming D0(RminusBr)= 29 000 cmminus1

half maximum (FWHM) of sim3000 cmminus1 and an associatedanisotropy parameter β sim 2 ie the limiting value for promptaxial recoil following excitation via a transition dipole moment(TDM) for which is aligned parallel to the breaking bondmdashasreported (following photolysis at 267 nm) by Zhang et al9

Moving to shorter wavelengths the anisotropic fast ringfades in relative intensity and is progressively replaced bya smaller isotropic featuremdashexemplified by the data recordedat 2449 nm (Fig 4(e)) The TKER spectrum derived from thisimage peaks close to zero

The Brlowast fragment images from photolysis of 2-bro-mothiophene (and the TKER spectra derived from them) reveala similar trend The images recorded at longer wavelengthsie at 2667 and 2625 nm (Figs 5(a) and 5(b)) are dominatedby an anisotropic feature peaking at TKER sim3500 cmminus1with a FWHM of sim3000 cmminus1 The β parameter is velocitydependent sim15 around the peak of the distribution anddeclining at lower TKER The weak Brlowast image taken atλ = 2448 nm in contrast is essentially isotropic and peaks atlow TKER

I and Ilowast images from 2-iodothiophene were recordedover a greater range of photolysis wavelengths and againshow similar trends A weak one-colour I atom signal wasdiscernible following excitation at 30368 nm As Fig 6(a)shows these I atoms display a narrow (sim2000 cmminus1 (FWHM))anisotropic (β sim 17) translational energy distribution peakingat TKER sim7000 cmminus1 which initially expands upon scanningto shorter wavelengths but then shrinks (in radius) becomesmore isotropic fades (in relative intensity) and is supple-mented by a slower isotropic component as the photolysiswavelength is reduced Nonetheless as Fig 6(f) shows a

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1372224331 On Tue 09 Jun 2015 131203

224303-6 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 6 Images of I(2P32) products resulting from photolysis of jet-cooled 2-iodothiophene at λ= (a) 3037 nm (b) 2779 nm (c) 2668 nm (d) 2547 nm(e) 2437 nm and (f) 2202 nm with the ε vector of the photolysis laserradiation aligned vertically in the plane of the images (as shown in panel(a)) The TKER spectrum derived from analysis of each data set is shownon the right with the vertical arrow showing the TKERmax value assumingD0(Rminus I)= 24 500 cmminus1

faster (albeit more anisotropic) component peaking at TKERsim7000 cmminus1 is still discernible at λ = 2202 nm

As in the cases of iodobenzene6 various fluorinatediodobenzenes7 and 4-iodophenol8 the Ilowast images measuredfollowing excitation of 2-iodothiophene at long wavelengthshow hints of product vibrational structure as illustrated inFig 7(a) (λ = 2774 nm) Following past precedent we assignthe most obvious peak in the TKER spectrum taken at longestwavelength (indicated by the arrow in Fig 7(a)) to formationof Ilowast atoms together with radical fragments in their ground(ie zero-point v = 0) vibrational level Such an attributionallows estimation of the CndashI bond strength via the usual energyconservation relationship

D0(R minus I) = Ephot minus A(I) minus TKERmax

where R represents the (α-)thienyl (sometimes also referredto as the thiophenyl) radical Ephot is the photolysis photonenergy A(I) is the spin-orbit energy of the Ilowast atom (7603 cmminus1)TKERmax is the TKER associated with this fastest featureand we neglect the small amount of internal excitation in

FIG 7 Images of I(2P12) products resulting from photolysis of jet-cooled 2-iodothiophene at λ= (a) 2774 nm (b) 2670 nm (c) 2547 nm (d) 2437 nmand (e) 2202 nm with the ε vector of the photolysis laser radiation alignedvertically in the plane of the images (as shown in panel (a)) The TKERspectrum derived from analysis of each data set is shown on the rightwith the vertical arrow showing the TKERmax value assuming D0(Rminus I)= 24 500 cmminus1

the jet-cooled parent molecules this analysis yields D0(R minus I)= 24 500 plusmn 500 cmminus1mdasha value that is sensibly consistent witha typical CndashI bond strength in other aromatic systems6ndash8 Abinitio calculations for the ground state thienyl radical provideclues as to the assignment of the vibrational peak observedin Fig 7(a) A unique assignment is not possible giventhe limited energy resolution but the observed wavenumberseparation (sim1500 cmminus1 from the peak assigned to v = 0products) is sensibly consistent with one quantum of the in-plane symmetric (and antisymmetric) CndashC stretching modesmdashsee the supplementary material3 We note that the formeris prominent in the previously reported RR spectra of2-iodothiophene (albeit when exciting at slightly shorterwavelengths)10

The short vertical arrows at the high TKER end of thespectra shown in Figs 6 and 7 indicate the maximum TKERspredicted for the I (or Ilowast) + R products arising at eachphotolysis wavelength given D0(R minus I) = 24 500 cmminus1 Thesedata emphasise that the mean TKER ⟨TKER⟩ associatedwith each channel is rather insensitive to changes in Ephotimplying progressively greater partitioning of the availableenergy (ie of Ephot minus D0(R minus I)) into internal excitation ofthe thienyl radical productmdasha conclusion that again is ingood accord with those reached from the earlier resonanceRaman studies10

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224303-7 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 8 Diabatic SO resolved PECs along RCndashX for the ground and lower lying excited states of (a) 2-bromothiophene and (b) 2-iodothiophene

None of the TKER distributions derived from the Br or Brlowast

images from photolysis of 2-bromothiophene show any hintsof product vibrational structure (as was the case also in theimaging study of 4-bromophenol photolysis8) Nonethelessif we assume similar dissociation dynamics to that revealedby 2-iodothiophene and that the TKERmax value for productsformed from 2-bromothiophene photolysis at the longestUV wavelengths similarly lies on the fast edge of the highTKER feature (as shown by the short vertical arrow in bothFigs 4(a) and 5(a)) we arrive at a CndashBr bond strength D0(R minus Br) sim 29 000 cmminus1 (given A(Br) = 3685 cmminus1) that isagain sensibly consistent with values reported for bro-mobenzene (3428 kJ molminus1 28 650 cmminus1 Ref 42) andfor 4-bromophenol (28 700 cmminus1 Ref 8) but substantiallylarger than an earlier suggested CndashBr bond strength in 2-bromothiophene43 As the other panels in Figs 4 and 5 showtuning to shorter photolysis wavelengths again results in arelative increase in the internal energy partitioned into thethienyl partner

B Ab initio calculations of CndashBrCndashI bond fissionand CndashS ring opening pathways in 2-bromothiopheneand 2-iodothiophene

Figures 8(a) and 8(b) present SO resolved PECs alongRCndashX for the ground and lower energy excited states of respec-tively 2-bromothiophene and 2-iodothiophene In both casesthe molecule has been constrained to planar geometries andthe PECs have been approximately ldquodiabatisedrdquo in terms of(nπ)σlowast and ππlowast states by inspecting the energies symmetriesand wavefunction coefficients The adiabatic SO resolvedpotentials from which these are derived are shown in thesupplementary material3 As in the case of the iodobenzenes67

and halophenols8 the lowest repulsive potentials all arise fromσCndashX

lowast larr nπ excitations these potentials correlate with theground (X2Aprime) state thienyl + XXlowast dissociation limits Thespin-orbit split analogues involving the first (A2Aprimeprime) excitedstate of the thienyl radical lie higher in energy while the(diabatically) bound interloper PECs arise from excitationsto ring centred πlowast orbitals The calculated vertical excitationenergies suggest that many of these excited states could besampled within the wavelength ranges explored in the presentstudy but analogy with the previous VMI studies of the XXlowast

products from photolysis of iodobenzenes and 4-iodo- and4-bromophenol suggests that the σlowast larr nπ TDMs returnedby the present calculations are likely to be of limited valuein deciding which are the optically ldquobrightrdquo states (illustrativeTDMs are listed in the supplementary material3) Rather asin those cases6ndash8 the observation that the faster X and Xlowast

products formed at long excitation wavelengths show nearlimiting parallel recoil anisotropy suggests that the populatedstates are those that gain intensity by vibronic interactionwith the higher lying intense CndashX bond centred σlowast larr σtransition

The predicted energy separations between the variousasymptotic limits merit comment The present calculationsreproduce the BrBrlowast and IIlowast spin-orbit splittings well butpredict a larger separation between the A and X statesof the thienyl radical (sim125 eV) than that derived fromphotodetachment studies of the thiophenide anion (sim06 eVRef 44) This is not surprising given that the PECs are derivedvia rigid body scans and thus preclude any structural relaxationof the ring Nonetheless the present calculations confirm theearlier suggestion that (nπ)σlowast excited states (ie states thatare dissociative with respect to CndashX bond extension and whichcorrelate to X or Xlowast products) are dominant contributors tothe long wavelength tail of the respective absorption spectraand locate the minimum of the first (spin-allowed) πlowast larr πtransition at sim53 eV (sim59 eV) for X = I (Br) in reasonableaccord with the observed absorption maxima (Fig 3)

Cuts through the first few SOF potentials of 2-bro-mothiophene and 2-iodothiophene along the RCndashS stretch(ring-opening) coordinate are shown in Figs 9(a) and 9(c)respectively The corresponding adiabatic SOF PECs alongRCndashX are shown alongside in Figs 9(b) and 9(d) The S0 stateis stable with respect to ring opening in both halothiophenesand its PEC thus rises steadily with increasing RCndashS Incontrast the PECs for the 11Aprimeprime and 21Aprimeprime states (and fortheir triplet analogues all of which arise via σCndashS

lowast larr (nπ)excitations) show obvious instability with respect to extendingRCndashS Notwithstanding the limitations of these SOF PECs(all of which are calculated at the MP2 relaxed ground stategeometries) they clearly point to the availability of exoergicpathways from the bright 1ππlowast (21Aprime) state via one or more(nπ)σCndashS

lowast PECs to extended CndashS bond lengths and regionsof conical intersection with the S0 state potential

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224303-8 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 9 Approximately diabatised SO free PECs along the RCndashS (ring-opening) coordinate and RCndashX stretch for the first few singlet (solid lines) and triplet(dashed lines) states of 2-bromothiophene ((a) and (b) respectively) and 2-iodothiophene ((c) and (d) respectively)

Figure 10 summarises some of the various possible fatesof 2-halothiophene molecules following non-adiabatic transferto the S0 PES The primary ring-opened structure is a biradical(structure B) that can exist as essentially degenerate cis andtrans isomersmdashboth of which are unstable with respect tothe original ground state halothiophene B can also isomeriseto various ring-opened isomers (including structures D-G)via transition states three of which that involve migrating(at most) one atom from the biradical structure (TS1 TS2and TS3) have been optimised As Fig 10 also shows arange of Br + radical (H I J K as well as R) dissociationlimits are calculated to lie below the energy of a 250 nmphoton (illustrated by the horizontal red line) The variousstructures and product pairs along with their energies (incmminus1 defined relative to that of the ground state minimum)are detailed below the figure along with the correspondingquantities for 2-iodothiophene (in parentheses) The calculatedRndashBr and RndashI bond strengths are seen to match well withthe experimental values derived in Sec III A Readersshould also note that the dashed lines at the far right ofFig 10 are simply intended to correlate various isomers toparticular dissociation products we have not checked for thepresence (or otherwise) of any barrier in the various fragmen-tation paths

C A plausible explanation for the observedwavelength dependent fragmentation dynamics

The X and Xlowast images measured at the longer UVwavelengths are very reminiscent of those reported previouslyfollowing photolysis of for example iodobenzenes67 and 4-iodo- and 4-bromophenol8 The X and Xlowast products recoilwith close to the maximum speed permitted by energyconservation and with close to limiting parallel anisotropyThe velocities of the faster XXlowast products increase littleupon tuning to shorter excitation wavelengths implying apreferential partitioning of the additional photon energy intointernal (vibrational) excitation of the thienyl radical partnerAs in the previously studied aryl iodides and bromides suchbehaviour is fully consistent with population of one or more(nπ)σlowast excited states and prompt CndashX bond fission Theprogressive increase in product vibrational excitation can beunderstood on Franck-Condon grounds given the changes inequilibrium ring geometry upon σlowast larr (nπ) excitation andsubsequent evolution to the radical product As noted abovethe near limiting parallel recoil anisotropy of the X and Xlowast

products can be understood if (as has been shown in detail inthe case of methyl iodide45) the participating (nπ)σlowast state(s)gains transition probability by vibronic interaction with a

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224303-9 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 10 Diagram showing the calculated relative energies of 2-bromothiophene (A) the biradical intermediate B formed upon CndashS bondfission TSs leading to various ring-opened isomers (D-G) and selectedBr + radical (H I J K) product limits The dashed lines at the far right of thefigure simply correlate given structures to particular dissociation productswe have not checked for the presence (or otherwise) of any barrier in thevarious fragmentation paths The various structures along with their energies(in cmminus1 defined relative to the ground state minimum) are detailed below to-gether with the corresponding quantities for 2-iodothiophene (in parentheses)The horizontal dashed line indicates the total energy reached upon absorptionof a 250 nm photon

higher lying CndashX bond localised σσlowast state and the subsequentbond fission occurs on a time scale that is fast compared to themolecular rotational period

The primary novelty of the present study however is theclear switch in energy disposal upon reducing the excitationwavelength This switch is revealed by the appearance (andgrowing dominance) of a slow isotropic component withinthe X and Xlowast imagesmdashmore obviously in the case of2-bromothiophenemdashand roughly coincides with the start ofthe obvious increase in parent absorption that we associatewith πlowast larr π absorption (Fig 3) Careful inspection of Figs 6and 7 also shows that the ldquofastrdquo component in the I and Ilowast

images slows and becomes less anisotropic as the photonenergy is increased further

The PES for this ππlowast state (the 21Aprime state in Fig 9)shows CIs (at geometries not far from the vertical FranckCondon region) with other excited states the potential energiesof which decline with increases in RCndashX andor RCndashS Thusit is reasonable to propose that excitation at these shorterwavelengths populates the (diabatically bound) 21Aprime statewhich can then decay by coupling to states that are dissociativewith respect to CndashX andor CndashS bond extension Which ofthese decay processes dominates (ie the CndashX bond fissionor the channel initiated by CndashS bond extension) will besensitively dependent upon the respective minimum energypathways from the 21Aprime state and the details of the couplingmodes that facilitate the population transfer

The almost total absence of ldquofastrdquo BrBrlowast products in theimages recorded following photolysis of 2-bromothiopheneat λ sim 245 nm suggests that (i) the partial absorption crosssection to the 21Aprime state far exceeds that to the underlying(nπ)σlowast state(s)mdashconsistent with the relative weakness of thelong wavelength tail in the UV absorption spectrum of 2-bromothiophenemdashand (ii) CndashS bond extension constitutes themajor decay pathway from the 21Aprime state and the subsequentnuclear motion samples the predicted region of CI with theS0 PES at extended CndashS bond lengths Radiationless transferat this CI yields highly vibrationally excited S0 moleculesThese have more than enough internal energy to decay notonly by RndashBr bond fission (yielding thienyl cofragments)but also to ring-open isomerise and decay to yield slowBr fragments in tandem with a range of C4H3S radicalproducts Figure 10 suggests similar energetics for CndashBr bondfission leading to ring closed (R) and some of the morestable ring opened radical products (eg H and I) Entropicconsiderations might favour the latter products but any seriousdiscussion of the likely product yields would require betterknowledge of the topology of the CI via which population isfunnelled to the biradical intermediate on the S0 PES and ofthe rate with which the latter can overcome the barrier to thestable ring-opened products featured on the right hand side ofFig 10

The data for 2-iodothiophene are less clear cut but againthe relative yield of slow I atoms increases upon tuning toshorter wavelengthsmdashconsistent with initial excitation to thecorresponding 21Aprime state and radiationless transfer to (andunimolecular decay from) highly vibrationally excited S0molecules formed by coupling through the predicted CI atextended RCndashS The ldquofastrdquo IIlowast feature also persists at shorterwavelengths however This could be explained by one orother or both of two mechanisms The first recognises the(relatively) greater partial absorption cross sections of theσCndashI

lowast larr nπ excitations in 2-iodothiophene (Fig 3) while thesecond assumes that following excitation to the 21Aprime state thethreshold energies for predissociation enabled by CndashI and CndashSbond extension are more similar than in 2-bromothiopheneThe evident reduction in the TKER and recoil anisotropyof the ldquofastrdquo I signal at shorter wavelengths hints that CndashXbond fission is a relatively more important decay channelfrom the 21Aprime state of 2-iodothiophene than in the case of2-bromothiophene

Finally we contrast the fast anisotropic Br and Brlowast

product velocity distributions formed in the 267 nm photolysis

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1372224331 On Tue 09 Jun 2015 131203

224303-10 Marchetti et al J Chem Phys 142 224303 (2015)

of 2-bromothiophene with the slow isotropic distributionsobserved when exciting 3-bromothiophene at this samewavelength9 The latter are reminiscent of that found whenexciting 2-bromothiophene at much shorter wavelengths(below sim245 nm) As noted previously all of our attemptsto characterise the analogue of biradical B on the S0 PES of3-bromothiophene minimised to a lower energy cyclopropeneadduct (an enantiomer of G) which will have ample internalenergy to decay further (eg to H + X products) Thepublished photolysis data for 3-bromothiophene may thusbe explicable if the corresponding σCndashS

lowast larr (nπ) PESs aresomewhat red-shifted cf 2-bromothiophene and provide anefficient route for funnelling photoexcited molecules to regionsof CI with the S0 PES and thence to CndashBr bond fission onthis latter PES Photolysis studies of 3-halothiophenes at arange of (longer) UV excitation wavelengths could be usefulin confirming (or refuting) this proposal

IV CONCLUSIONS

The near UV photofragmentation dynamics of jet-cooledgas phase 2-bromo- and 2-iodothiophene molecules havebeen investigated at many different excitation wavelengthsby measuring the velocity distributions of the XXlowast productsusing a VMI spectrometer equipped with a new purposedesigned ion optics assembly The new ion optics design isnot much more complicated than the traditional flat annularelectrode stack and offers a number of potentially significantadvantages stable velocity mapping from a larger interactionvolume protection from stray fields improved limiting veloc-ity resolution and natural compatibility with slice imagingThe present study addressing the photodissociation of a 9-atomic system does not provide an appropriate vehicle forillustrating the predicted velocity resolution As discussedin the supplementary material3 the ultimate test of theresolving power of the new ion optics assembly should involvephotodissociation studies of fully quantum state selectedparent molecules such as those reported by the Janssen groupfor methyl iodide46

The present imaging study reveals a marked change inphotofragmentation behaviour upon tuning to shorter excita-tion wavelengths particularly in the case of 2-bromothiopheneLong wavelength excitation results in prompt CndashX bondfission the resulting XXlowast products are formed withkinetic energies close to the maximum allowed by energyconservation and with near limiting parallel recoil anisotropyCompanion ab initio calculations show that these productsarise following excitation to one or more (nπ)σlowast states andserve to highlight the photophysical similarities with otheraryl iodides (and bromides) when excited at longer UVwavelengths4ndash8

At shorter wavelengths the anisotropic fast ring asso-ciated with these products fades in relative intensity and isprogressively replaced by a smaller isotropic feature thatsignifies the formation of much slower XXlowast atom productsThe appearance of this new channel roughly coincides withthe onset of more intense parent absorption to the 21Aprime excitedstate Ab initio theory suggests that this ππlowast excited statecan predissociate by interaction with neighbouring (nπ)σlowast

states the potentials for which decline in energy along boththe RCndashX and RCndashS stretch coordinates The 2-bromothiophenedata can be understood if the latter offers the lower energydecay route from the optically bright 21Aprime state to a CIwith the ground state PES at extended CndashS bond lengthsThe present DFTB3LYP6-311G(dp) calculations suggestthat the resulting biradicals have more than sufficient energyto sample the configuration space associated with severaldifferent isomeric forms of the starting molecule and to formboth ring closed and ring opened C4H3S radical productsfollowing subsequent CndashBr bond fission on the S0 PES

The data for 2-iodothiophene are less clear cut butagain the relative yield of slow I atoms increases markedlyupon tuning to shorter wavelengthsmdashconsistent with initialexcitation to the corresponding 21Aprime state and radiationlesstransfer to (and unimolecular decay from) highly vibrationallyexcited S0 molecules formed by coupling through the predictedCI at extended RCndashS The ldquofastrdquo IIlowast feature also persists atshorter wavelengths however This could be explained byone or other or both of two mechanisms The first recognisesthe (relatively) greater partial absorption cross sections ofthe σCndashI

lowast larr nπ excitations in 2-iodothiophene while thesecond assumes that following excitation to the 21Aprime state thethresholds for predissociation enabled by CndashI and CndashS bondextension are closer in energy than in 2-bromothiophene

It is interesting to compare the present findings withthose from recent condensed phase studies of thiophenonein acetonitrile solution wherein the photoexcited parentmolecules were similarly deduced to convert to highlyvibrationally excited levels of the ground (S0) state bycoupling via a CI at extended CndashS bond lengths19 Ringopening was confirmed in that case by the observation ofketene products but the time evolution of the parent bleachsignals indicated that the majority (sim60) of these highlyvibrationally excited S0 molecules relax and re-thermalise asthe ring closed thiophenone Such vibrational relaxation is notavailable in the present isolated molecule gas phase studies soeventual dissociation is assured on energetic grounds but therelative probabilities for forming ring opened or ring closedfragmentation products remains an open question

ACKNOWLEDGMENTS

The authors are grateful to EPSRC (Programme GrantNos EPG00224X and EPL005913) to the TSB (KnowledgeTransfer Partnership KTP008481 that stimulated the Bristol-Photek collaboration) to the Marie Curie Initial TrainingNetwork ICONIC (Contract Agreement No 238671) andto Rebecca Ingle Keith Rosser Dr Stuart Greaves DrJames Smith Dr Andreas Wenge Dr Eckart Wrede and DrDimitrios Zaouris for their many and varied contributions tothis work Data accessibility The underlying data for thispaper has been placed in the University of Bristolrsquos researchdata repository and can be accessed using the following DOI105523brisqwlb0o0e9tbd1bqojd8gxrnjj

1A L Sobolewski W Domcke C Dedonder-Lardeux and C Jouvet PhysChem Chem Phys 4 1093 (2002)

2M N R Ashfold G A King D Murdock M G D Nix T A A Oliverand A G Sage Phys Chem Chem Phys 12 1218 (2010)

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224303-5 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 4 Images of Br(2P32) products resulting from photolysis of jet-cooled2-bromothiophene at λ= (a) 2666 nm (b) 2606 nm (c) 2542 nm (d)2498 nm and (e) 2449 nm with the ε vector of the photolysis laser radiationaligned vertically in the plane of the images (as shown in panel (a)) TheTKER spectrum derived from analysis of each data set is shown to the rightof each image with the vertical arrow showing the TKERmax value assumingD0(RminusBr)= 29 000 cmminus1

nominally spin-forbidden singlet-triplet transitions appearwith much greater transition strength in the latter8

A Ion imaging data

The left hand panels in Figs 4ndash7 show raw images ofrespectively the Br and Brlowast fragments resulting from photol-ysis of 2-bromothiophene and the I and Ilowast fragments fromphotolysis of 2-iodothiophene recorded at several differentnear UV wavelengths For consistency only pancaked imagesare reported in the main paper All images of a given specieswere recorded using the same extraction voltages (ie therespective radii are reliable indicators of the relative velocity)and in all cases the electric (ε) vector of the photolysis laserradiation was vertical (ie parallel to the front face of thedetector) as shown on the first image in each figure Thecorresponding total kinetic energy release (TKER) spectra(shown to the right of each image) were derived using theappropriate Jacobian together with the assumption that theunobserved fragment partnering the BrI atom has chemicalformula C4SH3 and mass m = 8313 u

These data show a number of trends which we review byfirst focussing on 2-bromothiophene The Br image recordedat longest photolysis wavelengthmdasha one colour experimentat 2666 nmmdashshows a fast anisotropic recoil velocitydistribution peaking at TKER sim6000 cmminus1 with a full width

FIG 5 Images of Br(2P12) products resulting from photolysis of jet-cooled2-bromothiophene at λ= (a) 2667 nm (b) 2625 nm (c) 2542 nm and (d)2448 nm with the ε vector of the photolysis laser radiation aligned verticallyin the plane of the images (as shown in panel (a)) The TKER spectrumderived from analysis of each image is shown on the right with the verticalarrow indicating the TKERmax value assuming D0(RminusBr)= 29 000 cmminus1

half maximum (FWHM) of sim3000 cmminus1 and an associatedanisotropy parameter β sim 2 ie the limiting value for promptaxial recoil following excitation via a transition dipole moment(TDM) for which is aligned parallel to the breaking bondmdashasreported (following photolysis at 267 nm) by Zhang et al9

Moving to shorter wavelengths the anisotropic fast ringfades in relative intensity and is progressively replaced bya smaller isotropic featuremdashexemplified by the data recordedat 2449 nm (Fig 4(e)) The TKER spectrum derived from thisimage peaks close to zero

The Brlowast fragment images from photolysis of 2-bro-mothiophene (and the TKER spectra derived from them) reveala similar trend The images recorded at longer wavelengthsie at 2667 and 2625 nm (Figs 5(a) and 5(b)) are dominatedby an anisotropic feature peaking at TKER sim3500 cmminus1with a FWHM of sim3000 cmminus1 The β parameter is velocitydependent sim15 around the peak of the distribution anddeclining at lower TKER The weak Brlowast image taken atλ = 2448 nm in contrast is essentially isotropic and peaks atlow TKER

I and Ilowast images from 2-iodothiophene were recordedover a greater range of photolysis wavelengths and againshow similar trends A weak one-colour I atom signal wasdiscernible following excitation at 30368 nm As Fig 6(a)shows these I atoms display a narrow (sim2000 cmminus1 (FWHM))anisotropic (β sim 17) translational energy distribution peakingat TKER sim7000 cmminus1 which initially expands upon scanningto shorter wavelengths but then shrinks (in radius) becomesmore isotropic fades (in relative intensity) and is supple-mented by a slower isotropic component as the photolysiswavelength is reduced Nonetheless as Fig 6(f) shows a

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1372224331 On Tue 09 Jun 2015 131203

224303-6 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 6 Images of I(2P32) products resulting from photolysis of jet-cooled 2-iodothiophene at λ= (a) 3037 nm (b) 2779 nm (c) 2668 nm (d) 2547 nm(e) 2437 nm and (f) 2202 nm with the ε vector of the photolysis laserradiation aligned vertically in the plane of the images (as shown in panel(a)) The TKER spectrum derived from analysis of each data set is shownon the right with the vertical arrow showing the TKERmax value assumingD0(Rminus I)= 24 500 cmminus1

faster (albeit more anisotropic) component peaking at TKERsim7000 cmminus1 is still discernible at λ = 2202 nm

As in the cases of iodobenzene6 various fluorinatediodobenzenes7 and 4-iodophenol8 the Ilowast images measuredfollowing excitation of 2-iodothiophene at long wavelengthshow hints of product vibrational structure as illustrated inFig 7(a) (λ = 2774 nm) Following past precedent we assignthe most obvious peak in the TKER spectrum taken at longestwavelength (indicated by the arrow in Fig 7(a)) to formationof Ilowast atoms together with radical fragments in their ground(ie zero-point v = 0) vibrational level Such an attributionallows estimation of the CndashI bond strength via the usual energyconservation relationship

D0(R minus I) = Ephot minus A(I) minus TKERmax

where R represents the (α-)thienyl (sometimes also referredto as the thiophenyl) radical Ephot is the photolysis photonenergy A(I) is the spin-orbit energy of the Ilowast atom (7603 cmminus1)TKERmax is the TKER associated with this fastest featureand we neglect the small amount of internal excitation in

FIG 7 Images of I(2P12) products resulting from photolysis of jet-cooled 2-iodothiophene at λ= (a) 2774 nm (b) 2670 nm (c) 2547 nm (d) 2437 nmand (e) 2202 nm with the ε vector of the photolysis laser radiation alignedvertically in the plane of the images (as shown in panel (a)) The TKERspectrum derived from analysis of each data set is shown on the rightwith the vertical arrow showing the TKERmax value assuming D0(Rminus I)= 24 500 cmminus1

the jet-cooled parent molecules this analysis yields D0(R minus I)= 24 500 plusmn 500 cmminus1mdasha value that is sensibly consistent witha typical CndashI bond strength in other aromatic systems6ndash8 Abinitio calculations for the ground state thienyl radical provideclues as to the assignment of the vibrational peak observedin Fig 7(a) A unique assignment is not possible giventhe limited energy resolution but the observed wavenumberseparation (sim1500 cmminus1 from the peak assigned to v = 0products) is sensibly consistent with one quantum of the in-plane symmetric (and antisymmetric) CndashC stretching modesmdashsee the supplementary material3 We note that the formeris prominent in the previously reported RR spectra of2-iodothiophene (albeit when exciting at slightly shorterwavelengths)10

The short vertical arrows at the high TKER end of thespectra shown in Figs 6 and 7 indicate the maximum TKERspredicted for the I (or Ilowast) + R products arising at eachphotolysis wavelength given D0(R minus I) = 24 500 cmminus1 Thesedata emphasise that the mean TKER ⟨TKER⟩ associatedwith each channel is rather insensitive to changes in Ephotimplying progressively greater partitioning of the availableenergy (ie of Ephot minus D0(R minus I)) into internal excitation ofthe thienyl radical productmdasha conclusion that again is ingood accord with those reached from the earlier resonanceRaman studies10

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1372224331 On Tue 09 Jun 2015 131203

224303-7 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 8 Diabatic SO resolved PECs along RCndashX for the ground and lower lying excited states of (a) 2-bromothiophene and (b) 2-iodothiophene

None of the TKER distributions derived from the Br or Brlowast

images from photolysis of 2-bromothiophene show any hintsof product vibrational structure (as was the case also in theimaging study of 4-bromophenol photolysis8) Nonethelessif we assume similar dissociation dynamics to that revealedby 2-iodothiophene and that the TKERmax value for productsformed from 2-bromothiophene photolysis at the longestUV wavelengths similarly lies on the fast edge of the highTKER feature (as shown by the short vertical arrow in bothFigs 4(a) and 5(a)) we arrive at a CndashBr bond strength D0(R minus Br) sim 29 000 cmminus1 (given A(Br) = 3685 cmminus1) that isagain sensibly consistent with values reported for bro-mobenzene (3428 kJ molminus1 28 650 cmminus1 Ref 42) andfor 4-bromophenol (28 700 cmminus1 Ref 8) but substantiallylarger than an earlier suggested CndashBr bond strength in 2-bromothiophene43 As the other panels in Figs 4 and 5 showtuning to shorter photolysis wavelengths again results in arelative increase in the internal energy partitioned into thethienyl partner

B Ab initio calculations of CndashBrCndashI bond fissionand CndashS ring opening pathways in 2-bromothiopheneand 2-iodothiophene

Figures 8(a) and 8(b) present SO resolved PECs alongRCndashX for the ground and lower energy excited states of respec-tively 2-bromothiophene and 2-iodothiophene In both casesthe molecule has been constrained to planar geometries andthe PECs have been approximately ldquodiabatisedrdquo in terms of(nπ)σlowast and ππlowast states by inspecting the energies symmetriesand wavefunction coefficients The adiabatic SO resolvedpotentials from which these are derived are shown in thesupplementary material3 As in the case of the iodobenzenes67

and halophenols8 the lowest repulsive potentials all arise fromσCndashX

lowast larr nπ excitations these potentials correlate with theground (X2Aprime) state thienyl + XXlowast dissociation limits Thespin-orbit split analogues involving the first (A2Aprimeprime) excitedstate of the thienyl radical lie higher in energy while the(diabatically) bound interloper PECs arise from excitationsto ring centred πlowast orbitals The calculated vertical excitationenergies suggest that many of these excited states could besampled within the wavelength ranges explored in the presentstudy but analogy with the previous VMI studies of the XXlowast

products from photolysis of iodobenzenes and 4-iodo- and4-bromophenol suggests that the σlowast larr nπ TDMs returnedby the present calculations are likely to be of limited valuein deciding which are the optically ldquobrightrdquo states (illustrativeTDMs are listed in the supplementary material3) Rather asin those cases6ndash8 the observation that the faster X and Xlowast

products formed at long excitation wavelengths show nearlimiting parallel recoil anisotropy suggests that the populatedstates are those that gain intensity by vibronic interactionwith the higher lying intense CndashX bond centred σlowast larr σtransition

The predicted energy separations between the variousasymptotic limits merit comment The present calculationsreproduce the BrBrlowast and IIlowast spin-orbit splittings well butpredict a larger separation between the A and X statesof the thienyl radical (sim125 eV) than that derived fromphotodetachment studies of the thiophenide anion (sim06 eVRef 44) This is not surprising given that the PECs are derivedvia rigid body scans and thus preclude any structural relaxationof the ring Nonetheless the present calculations confirm theearlier suggestion that (nπ)σlowast excited states (ie states thatare dissociative with respect to CndashX bond extension and whichcorrelate to X or Xlowast products) are dominant contributors tothe long wavelength tail of the respective absorption spectraand locate the minimum of the first (spin-allowed) πlowast larr πtransition at sim53 eV (sim59 eV) for X = I (Br) in reasonableaccord with the observed absorption maxima (Fig 3)

Cuts through the first few SOF potentials of 2-bro-mothiophene and 2-iodothiophene along the RCndashS stretch(ring-opening) coordinate are shown in Figs 9(a) and 9(c)respectively The corresponding adiabatic SOF PECs alongRCndashX are shown alongside in Figs 9(b) and 9(d) The S0 stateis stable with respect to ring opening in both halothiophenesand its PEC thus rises steadily with increasing RCndashS Incontrast the PECs for the 11Aprimeprime and 21Aprimeprime states (and fortheir triplet analogues all of which arise via σCndashS

lowast larr (nπ)excitations) show obvious instability with respect to extendingRCndashS Notwithstanding the limitations of these SOF PECs(all of which are calculated at the MP2 relaxed ground stategeometries) they clearly point to the availability of exoergicpathways from the bright 1ππlowast (21Aprime) state via one or more(nπ)σCndashS

lowast PECs to extended CndashS bond lengths and regionsof conical intersection with the S0 state potential

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224303-8 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 9 Approximately diabatised SO free PECs along the RCndashS (ring-opening) coordinate and RCndashX stretch for the first few singlet (solid lines) and triplet(dashed lines) states of 2-bromothiophene ((a) and (b) respectively) and 2-iodothiophene ((c) and (d) respectively)

Figure 10 summarises some of the various possible fatesof 2-halothiophene molecules following non-adiabatic transferto the S0 PES The primary ring-opened structure is a biradical(structure B) that can exist as essentially degenerate cis andtrans isomersmdashboth of which are unstable with respect tothe original ground state halothiophene B can also isomeriseto various ring-opened isomers (including structures D-G)via transition states three of which that involve migrating(at most) one atom from the biradical structure (TS1 TS2and TS3) have been optimised As Fig 10 also shows arange of Br + radical (H I J K as well as R) dissociationlimits are calculated to lie below the energy of a 250 nmphoton (illustrated by the horizontal red line) The variousstructures and product pairs along with their energies (incmminus1 defined relative to that of the ground state minimum)are detailed below the figure along with the correspondingquantities for 2-iodothiophene (in parentheses) The calculatedRndashBr and RndashI bond strengths are seen to match well withthe experimental values derived in Sec III A Readersshould also note that the dashed lines at the far right ofFig 10 are simply intended to correlate various isomers toparticular dissociation products we have not checked for thepresence (or otherwise) of any barrier in the various fragmen-tation paths

C A plausible explanation for the observedwavelength dependent fragmentation dynamics

The X and Xlowast images measured at the longer UVwavelengths are very reminiscent of those reported previouslyfollowing photolysis of for example iodobenzenes67 and 4-iodo- and 4-bromophenol8 The X and Xlowast products recoilwith close to the maximum speed permitted by energyconservation and with close to limiting parallel anisotropyThe velocities of the faster XXlowast products increase littleupon tuning to shorter excitation wavelengths implying apreferential partitioning of the additional photon energy intointernal (vibrational) excitation of the thienyl radical partnerAs in the previously studied aryl iodides and bromides suchbehaviour is fully consistent with population of one or more(nπ)σlowast excited states and prompt CndashX bond fission Theprogressive increase in product vibrational excitation can beunderstood on Franck-Condon grounds given the changes inequilibrium ring geometry upon σlowast larr (nπ) excitation andsubsequent evolution to the radical product As noted abovethe near limiting parallel recoil anisotropy of the X and Xlowast

products can be understood if (as has been shown in detail inthe case of methyl iodide45) the participating (nπ)σlowast state(s)gains transition probability by vibronic interaction with a

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224303-9 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 10 Diagram showing the calculated relative energies of 2-bromothiophene (A) the biradical intermediate B formed upon CndashS bondfission TSs leading to various ring-opened isomers (D-G) and selectedBr + radical (H I J K) product limits The dashed lines at the far right of thefigure simply correlate given structures to particular dissociation productswe have not checked for the presence (or otherwise) of any barrier in thevarious fragmentation paths The various structures along with their energies(in cmminus1 defined relative to the ground state minimum) are detailed below to-gether with the corresponding quantities for 2-iodothiophene (in parentheses)The horizontal dashed line indicates the total energy reached upon absorptionof a 250 nm photon

higher lying CndashX bond localised σσlowast state and the subsequentbond fission occurs on a time scale that is fast compared to themolecular rotational period

The primary novelty of the present study however is theclear switch in energy disposal upon reducing the excitationwavelength This switch is revealed by the appearance (andgrowing dominance) of a slow isotropic component withinthe X and Xlowast imagesmdashmore obviously in the case of2-bromothiophenemdashand roughly coincides with the start ofthe obvious increase in parent absorption that we associatewith πlowast larr π absorption (Fig 3) Careful inspection of Figs 6and 7 also shows that the ldquofastrdquo component in the I and Ilowast

images slows and becomes less anisotropic as the photonenergy is increased further

The PES for this ππlowast state (the 21Aprime state in Fig 9)shows CIs (at geometries not far from the vertical FranckCondon region) with other excited states the potential energiesof which decline with increases in RCndashX andor RCndashS Thusit is reasonable to propose that excitation at these shorterwavelengths populates the (diabatically bound) 21Aprime statewhich can then decay by coupling to states that are dissociativewith respect to CndashX andor CndashS bond extension Which ofthese decay processes dominates (ie the CndashX bond fissionor the channel initiated by CndashS bond extension) will besensitively dependent upon the respective minimum energypathways from the 21Aprime state and the details of the couplingmodes that facilitate the population transfer

The almost total absence of ldquofastrdquo BrBrlowast products in theimages recorded following photolysis of 2-bromothiopheneat λ sim 245 nm suggests that (i) the partial absorption crosssection to the 21Aprime state far exceeds that to the underlying(nπ)σlowast state(s)mdashconsistent with the relative weakness of thelong wavelength tail in the UV absorption spectrum of 2-bromothiophenemdashand (ii) CndashS bond extension constitutes themajor decay pathway from the 21Aprime state and the subsequentnuclear motion samples the predicted region of CI with theS0 PES at extended CndashS bond lengths Radiationless transferat this CI yields highly vibrationally excited S0 moleculesThese have more than enough internal energy to decay notonly by RndashBr bond fission (yielding thienyl cofragments)but also to ring-open isomerise and decay to yield slowBr fragments in tandem with a range of C4H3S radicalproducts Figure 10 suggests similar energetics for CndashBr bondfission leading to ring closed (R) and some of the morestable ring opened radical products (eg H and I) Entropicconsiderations might favour the latter products but any seriousdiscussion of the likely product yields would require betterknowledge of the topology of the CI via which population isfunnelled to the biradical intermediate on the S0 PES and ofthe rate with which the latter can overcome the barrier to thestable ring-opened products featured on the right hand side ofFig 10

The data for 2-iodothiophene are less clear cut but againthe relative yield of slow I atoms increases upon tuning toshorter wavelengthsmdashconsistent with initial excitation to thecorresponding 21Aprime state and radiationless transfer to (andunimolecular decay from) highly vibrationally excited S0molecules formed by coupling through the predicted CI atextended RCndashS The ldquofastrdquo IIlowast feature also persists at shorterwavelengths however This could be explained by one orother or both of two mechanisms The first recognises the(relatively) greater partial absorption cross sections of theσCndashI

lowast larr nπ excitations in 2-iodothiophene (Fig 3) while thesecond assumes that following excitation to the 21Aprime state thethreshold energies for predissociation enabled by CndashI and CndashSbond extension are more similar than in 2-bromothiopheneThe evident reduction in the TKER and recoil anisotropyof the ldquofastrdquo I signal at shorter wavelengths hints that CndashXbond fission is a relatively more important decay channelfrom the 21Aprime state of 2-iodothiophene than in the case of2-bromothiophene

Finally we contrast the fast anisotropic Br and Brlowast

product velocity distributions formed in the 267 nm photolysis

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224303-10 Marchetti et al J Chem Phys 142 224303 (2015)

of 2-bromothiophene with the slow isotropic distributionsobserved when exciting 3-bromothiophene at this samewavelength9 The latter are reminiscent of that found whenexciting 2-bromothiophene at much shorter wavelengths(below sim245 nm) As noted previously all of our attemptsto characterise the analogue of biradical B on the S0 PES of3-bromothiophene minimised to a lower energy cyclopropeneadduct (an enantiomer of G) which will have ample internalenergy to decay further (eg to H + X products) Thepublished photolysis data for 3-bromothiophene may thusbe explicable if the corresponding σCndashS

lowast larr (nπ) PESs aresomewhat red-shifted cf 2-bromothiophene and provide anefficient route for funnelling photoexcited molecules to regionsof CI with the S0 PES and thence to CndashBr bond fission onthis latter PES Photolysis studies of 3-halothiophenes at arange of (longer) UV excitation wavelengths could be usefulin confirming (or refuting) this proposal

IV CONCLUSIONS

The near UV photofragmentation dynamics of jet-cooledgas phase 2-bromo- and 2-iodothiophene molecules havebeen investigated at many different excitation wavelengthsby measuring the velocity distributions of the XXlowast productsusing a VMI spectrometer equipped with a new purposedesigned ion optics assembly The new ion optics design isnot much more complicated than the traditional flat annularelectrode stack and offers a number of potentially significantadvantages stable velocity mapping from a larger interactionvolume protection from stray fields improved limiting veloc-ity resolution and natural compatibility with slice imagingThe present study addressing the photodissociation of a 9-atomic system does not provide an appropriate vehicle forillustrating the predicted velocity resolution As discussedin the supplementary material3 the ultimate test of theresolving power of the new ion optics assembly should involvephotodissociation studies of fully quantum state selectedparent molecules such as those reported by the Janssen groupfor methyl iodide46

The present imaging study reveals a marked change inphotofragmentation behaviour upon tuning to shorter excita-tion wavelengths particularly in the case of 2-bromothiopheneLong wavelength excitation results in prompt CndashX bondfission the resulting XXlowast products are formed withkinetic energies close to the maximum allowed by energyconservation and with near limiting parallel recoil anisotropyCompanion ab initio calculations show that these productsarise following excitation to one or more (nπ)σlowast states andserve to highlight the photophysical similarities with otheraryl iodides (and bromides) when excited at longer UVwavelengths4ndash8

At shorter wavelengths the anisotropic fast ring asso-ciated with these products fades in relative intensity and isprogressively replaced by a smaller isotropic feature thatsignifies the formation of much slower XXlowast atom productsThe appearance of this new channel roughly coincides withthe onset of more intense parent absorption to the 21Aprime excitedstate Ab initio theory suggests that this ππlowast excited statecan predissociate by interaction with neighbouring (nπ)σlowast

states the potentials for which decline in energy along boththe RCndashX and RCndashS stretch coordinates The 2-bromothiophenedata can be understood if the latter offers the lower energydecay route from the optically bright 21Aprime state to a CIwith the ground state PES at extended CndashS bond lengthsThe present DFTB3LYP6-311G(dp) calculations suggestthat the resulting biradicals have more than sufficient energyto sample the configuration space associated with severaldifferent isomeric forms of the starting molecule and to formboth ring closed and ring opened C4H3S radical productsfollowing subsequent CndashBr bond fission on the S0 PES

The data for 2-iodothiophene are less clear cut butagain the relative yield of slow I atoms increases markedlyupon tuning to shorter wavelengthsmdashconsistent with initialexcitation to the corresponding 21Aprime state and radiationlesstransfer to (and unimolecular decay from) highly vibrationallyexcited S0 molecules formed by coupling through the predictedCI at extended RCndashS The ldquofastrdquo IIlowast feature also persists atshorter wavelengths however This could be explained byone or other or both of two mechanisms The first recognisesthe (relatively) greater partial absorption cross sections ofthe σCndashI

lowast larr nπ excitations in 2-iodothiophene while thesecond assumes that following excitation to the 21Aprime state thethresholds for predissociation enabled by CndashI and CndashS bondextension are closer in energy than in 2-bromothiophene

It is interesting to compare the present findings withthose from recent condensed phase studies of thiophenonein acetonitrile solution wherein the photoexcited parentmolecules were similarly deduced to convert to highlyvibrationally excited levels of the ground (S0) state bycoupling via a CI at extended CndashS bond lengths19 Ringopening was confirmed in that case by the observation ofketene products but the time evolution of the parent bleachsignals indicated that the majority (sim60) of these highlyvibrationally excited S0 molecules relax and re-thermalise asthe ring closed thiophenone Such vibrational relaxation is notavailable in the present isolated molecule gas phase studies soeventual dissociation is assured on energetic grounds but therelative probabilities for forming ring opened or ring closedfragmentation products remains an open question

ACKNOWLEDGMENTS

The authors are grateful to EPSRC (Programme GrantNos EPG00224X and EPL005913) to the TSB (KnowledgeTransfer Partnership KTP008481 that stimulated the Bristol-Photek collaboration) to the Marie Curie Initial TrainingNetwork ICONIC (Contract Agreement No 238671) andto Rebecca Ingle Keith Rosser Dr Stuart Greaves DrJames Smith Dr Andreas Wenge Dr Eckart Wrede and DrDimitrios Zaouris for their many and varied contributions tothis work Data accessibility The underlying data for thispaper has been placed in the University of Bristolrsquos researchdata repository and can be accessed using the following DOI105523brisqwlb0o0e9tbd1bqojd8gxrnjj

1A L Sobolewski W Domcke C Dedonder-Lardeux and C Jouvet PhysChem Chem Phys 4 1093 (2002)

2M N R Ashfold G A King D Murdock M G D Nix T A A Oliverand A G Sage Phys Chem Chem Phys 12 1218 (2010)

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224303-6 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 6 Images of I(2P32) products resulting from photolysis of jet-cooled 2-iodothiophene at λ= (a) 3037 nm (b) 2779 nm (c) 2668 nm (d) 2547 nm(e) 2437 nm and (f) 2202 nm with the ε vector of the photolysis laserradiation aligned vertically in the plane of the images (as shown in panel(a)) The TKER spectrum derived from analysis of each data set is shownon the right with the vertical arrow showing the TKERmax value assumingD0(Rminus I)= 24 500 cmminus1

faster (albeit more anisotropic) component peaking at TKERsim7000 cmminus1 is still discernible at λ = 2202 nm

As in the cases of iodobenzene6 various fluorinatediodobenzenes7 and 4-iodophenol8 the Ilowast images measuredfollowing excitation of 2-iodothiophene at long wavelengthshow hints of product vibrational structure as illustrated inFig 7(a) (λ = 2774 nm) Following past precedent we assignthe most obvious peak in the TKER spectrum taken at longestwavelength (indicated by the arrow in Fig 7(a)) to formationof Ilowast atoms together with radical fragments in their ground(ie zero-point v = 0) vibrational level Such an attributionallows estimation of the CndashI bond strength via the usual energyconservation relationship

D0(R minus I) = Ephot minus A(I) minus TKERmax

where R represents the (α-)thienyl (sometimes also referredto as the thiophenyl) radical Ephot is the photolysis photonenergy A(I) is the spin-orbit energy of the Ilowast atom (7603 cmminus1)TKERmax is the TKER associated with this fastest featureand we neglect the small amount of internal excitation in

FIG 7 Images of I(2P12) products resulting from photolysis of jet-cooled 2-iodothiophene at λ= (a) 2774 nm (b) 2670 nm (c) 2547 nm (d) 2437 nmand (e) 2202 nm with the ε vector of the photolysis laser radiation alignedvertically in the plane of the images (as shown in panel (a)) The TKERspectrum derived from analysis of each data set is shown on the rightwith the vertical arrow showing the TKERmax value assuming D0(Rminus I)= 24 500 cmminus1

the jet-cooled parent molecules this analysis yields D0(R minus I)= 24 500 plusmn 500 cmminus1mdasha value that is sensibly consistent witha typical CndashI bond strength in other aromatic systems6ndash8 Abinitio calculations for the ground state thienyl radical provideclues as to the assignment of the vibrational peak observedin Fig 7(a) A unique assignment is not possible giventhe limited energy resolution but the observed wavenumberseparation (sim1500 cmminus1 from the peak assigned to v = 0products) is sensibly consistent with one quantum of the in-plane symmetric (and antisymmetric) CndashC stretching modesmdashsee the supplementary material3 We note that the formeris prominent in the previously reported RR spectra of2-iodothiophene (albeit when exciting at slightly shorterwavelengths)10

The short vertical arrows at the high TKER end of thespectra shown in Figs 6 and 7 indicate the maximum TKERspredicted for the I (or Ilowast) + R products arising at eachphotolysis wavelength given D0(R minus I) = 24 500 cmminus1 Thesedata emphasise that the mean TKER ⟨TKER⟩ associatedwith each channel is rather insensitive to changes in Ephotimplying progressively greater partitioning of the availableenergy (ie of Ephot minus D0(R minus I)) into internal excitation ofthe thienyl radical productmdasha conclusion that again is ingood accord with those reached from the earlier resonanceRaman studies10

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1372224331 On Tue 09 Jun 2015 131203

224303-7 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 8 Diabatic SO resolved PECs along RCndashX for the ground and lower lying excited states of (a) 2-bromothiophene and (b) 2-iodothiophene

None of the TKER distributions derived from the Br or Brlowast

images from photolysis of 2-bromothiophene show any hintsof product vibrational structure (as was the case also in theimaging study of 4-bromophenol photolysis8) Nonethelessif we assume similar dissociation dynamics to that revealedby 2-iodothiophene and that the TKERmax value for productsformed from 2-bromothiophene photolysis at the longestUV wavelengths similarly lies on the fast edge of the highTKER feature (as shown by the short vertical arrow in bothFigs 4(a) and 5(a)) we arrive at a CndashBr bond strength D0(R minus Br) sim 29 000 cmminus1 (given A(Br) = 3685 cmminus1) that isagain sensibly consistent with values reported for bro-mobenzene (3428 kJ molminus1 28 650 cmminus1 Ref 42) andfor 4-bromophenol (28 700 cmminus1 Ref 8) but substantiallylarger than an earlier suggested CndashBr bond strength in 2-bromothiophene43 As the other panels in Figs 4 and 5 showtuning to shorter photolysis wavelengths again results in arelative increase in the internal energy partitioned into thethienyl partner

B Ab initio calculations of CndashBrCndashI bond fissionand CndashS ring opening pathways in 2-bromothiopheneand 2-iodothiophene

Figures 8(a) and 8(b) present SO resolved PECs alongRCndashX for the ground and lower energy excited states of respec-tively 2-bromothiophene and 2-iodothiophene In both casesthe molecule has been constrained to planar geometries andthe PECs have been approximately ldquodiabatisedrdquo in terms of(nπ)σlowast and ππlowast states by inspecting the energies symmetriesand wavefunction coefficients The adiabatic SO resolvedpotentials from which these are derived are shown in thesupplementary material3 As in the case of the iodobenzenes67

and halophenols8 the lowest repulsive potentials all arise fromσCndashX

lowast larr nπ excitations these potentials correlate with theground (X2Aprime) state thienyl + XXlowast dissociation limits Thespin-orbit split analogues involving the first (A2Aprimeprime) excitedstate of the thienyl radical lie higher in energy while the(diabatically) bound interloper PECs arise from excitationsto ring centred πlowast orbitals The calculated vertical excitationenergies suggest that many of these excited states could besampled within the wavelength ranges explored in the presentstudy but analogy with the previous VMI studies of the XXlowast

products from photolysis of iodobenzenes and 4-iodo- and4-bromophenol suggests that the σlowast larr nπ TDMs returnedby the present calculations are likely to be of limited valuein deciding which are the optically ldquobrightrdquo states (illustrativeTDMs are listed in the supplementary material3) Rather asin those cases6ndash8 the observation that the faster X and Xlowast

products formed at long excitation wavelengths show nearlimiting parallel recoil anisotropy suggests that the populatedstates are those that gain intensity by vibronic interactionwith the higher lying intense CndashX bond centred σlowast larr σtransition

The predicted energy separations between the variousasymptotic limits merit comment The present calculationsreproduce the BrBrlowast and IIlowast spin-orbit splittings well butpredict a larger separation between the A and X statesof the thienyl radical (sim125 eV) than that derived fromphotodetachment studies of the thiophenide anion (sim06 eVRef 44) This is not surprising given that the PECs are derivedvia rigid body scans and thus preclude any structural relaxationof the ring Nonetheless the present calculations confirm theearlier suggestion that (nπ)σlowast excited states (ie states thatare dissociative with respect to CndashX bond extension and whichcorrelate to X or Xlowast products) are dominant contributors tothe long wavelength tail of the respective absorption spectraand locate the minimum of the first (spin-allowed) πlowast larr πtransition at sim53 eV (sim59 eV) for X = I (Br) in reasonableaccord with the observed absorption maxima (Fig 3)

Cuts through the first few SOF potentials of 2-bro-mothiophene and 2-iodothiophene along the RCndashS stretch(ring-opening) coordinate are shown in Figs 9(a) and 9(c)respectively The corresponding adiabatic SOF PECs alongRCndashX are shown alongside in Figs 9(b) and 9(d) The S0 stateis stable with respect to ring opening in both halothiophenesand its PEC thus rises steadily with increasing RCndashS Incontrast the PECs for the 11Aprimeprime and 21Aprimeprime states (and fortheir triplet analogues all of which arise via σCndashS

lowast larr (nπ)excitations) show obvious instability with respect to extendingRCndashS Notwithstanding the limitations of these SOF PECs(all of which are calculated at the MP2 relaxed ground stategeometries) they clearly point to the availability of exoergicpathways from the bright 1ππlowast (21Aprime) state via one or more(nπ)σCndashS

lowast PECs to extended CndashS bond lengths and regionsof conical intersection with the S0 state potential

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1372224331 On Tue 09 Jun 2015 131203

224303-8 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 9 Approximately diabatised SO free PECs along the RCndashS (ring-opening) coordinate and RCndashX stretch for the first few singlet (solid lines) and triplet(dashed lines) states of 2-bromothiophene ((a) and (b) respectively) and 2-iodothiophene ((c) and (d) respectively)

Figure 10 summarises some of the various possible fatesof 2-halothiophene molecules following non-adiabatic transferto the S0 PES The primary ring-opened structure is a biradical(structure B) that can exist as essentially degenerate cis andtrans isomersmdashboth of which are unstable with respect tothe original ground state halothiophene B can also isomeriseto various ring-opened isomers (including structures D-G)via transition states three of which that involve migrating(at most) one atom from the biradical structure (TS1 TS2and TS3) have been optimised As Fig 10 also shows arange of Br + radical (H I J K as well as R) dissociationlimits are calculated to lie below the energy of a 250 nmphoton (illustrated by the horizontal red line) The variousstructures and product pairs along with their energies (incmminus1 defined relative to that of the ground state minimum)are detailed below the figure along with the correspondingquantities for 2-iodothiophene (in parentheses) The calculatedRndashBr and RndashI bond strengths are seen to match well withthe experimental values derived in Sec III A Readersshould also note that the dashed lines at the far right ofFig 10 are simply intended to correlate various isomers toparticular dissociation products we have not checked for thepresence (or otherwise) of any barrier in the various fragmen-tation paths

C A plausible explanation for the observedwavelength dependent fragmentation dynamics

The X and Xlowast images measured at the longer UVwavelengths are very reminiscent of those reported previouslyfollowing photolysis of for example iodobenzenes67 and 4-iodo- and 4-bromophenol8 The X and Xlowast products recoilwith close to the maximum speed permitted by energyconservation and with close to limiting parallel anisotropyThe velocities of the faster XXlowast products increase littleupon tuning to shorter excitation wavelengths implying apreferential partitioning of the additional photon energy intointernal (vibrational) excitation of the thienyl radical partnerAs in the previously studied aryl iodides and bromides suchbehaviour is fully consistent with population of one or more(nπ)σlowast excited states and prompt CndashX bond fission Theprogressive increase in product vibrational excitation can beunderstood on Franck-Condon grounds given the changes inequilibrium ring geometry upon σlowast larr (nπ) excitation andsubsequent evolution to the radical product As noted abovethe near limiting parallel recoil anisotropy of the X and Xlowast

products can be understood if (as has been shown in detail inthe case of methyl iodide45) the participating (nπ)σlowast state(s)gains transition probability by vibronic interaction with a

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203

224303-9 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 10 Diagram showing the calculated relative energies of 2-bromothiophene (A) the biradical intermediate B formed upon CndashS bondfission TSs leading to various ring-opened isomers (D-G) and selectedBr + radical (H I J K) product limits The dashed lines at the far right of thefigure simply correlate given structures to particular dissociation productswe have not checked for the presence (or otherwise) of any barrier in thevarious fragmentation paths The various structures along with their energies(in cmminus1 defined relative to the ground state minimum) are detailed below to-gether with the corresponding quantities for 2-iodothiophene (in parentheses)The horizontal dashed line indicates the total energy reached upon absorptionof a 250 nm photon

higher lying CndashX bond localised σσlowast state and the subsequentbond fission occurs on a time scale that is fast compared to themolecular rotational period

The primary novelty of the present study however is theclear switch in energy disposal upon reducing the excitationwavelength This switch is revealed by the appearance (andgrowing dominance) of a slow isotropic component withinthe X and Xlowast imagesmdashmore obviously in the case of2-bromothiophenemdashand roughly coincides with the start ofthe obvious increase in parent absorption that we associatewith πlowast larr π absorption (Fig 3) Careful inspection of Figs 6and 7 also shows that the ldquofastrdquo component in the I and Ilowast

images slows and becomes less anisotropic as the photonenergy is increased further

The PES for this ππlowast state (the 21Aprime state in Fig 9)shows CIs (at geometries not far from the vertical FranckCondon region) with other excited states the potential energiesof which decline with increases in RCndashX andor RCndashS Thusit is reasonable to propose that excitation at these shorterwavelengths populates the (diabatically bound) 21Aprime statewhich can then decay by coupling to states that are dissociativewith respect to CndashX andor CndashS bond extension Which ofthese decay processes dominates (ie the CndashX bond fissionor the channel initiated by CndashS bond extension) will besensitively dependent upon the respective minimum energypathways from the 21Aprime state and the details of the couplingmodes that facilitate the population transfer

The almost total absence of ldquofastrdquo BrBrlowast products in theimages recorded following photolysis of 2-bromothiopheneat λ sim 245 nm suggests that (i) the partial absorption crosssection to the 21Aprime state far exceeds that to the underlying(nπ)σlowast state(s)mdashconsistent with the relative weakness of thelong wavelength tail in the UV absorption spectrum of 2-bromothiophenemdashand (ii) CndashS bond extension constitutes themajor decay pathway from the 21Aprime state and the subsequentnuclear motion samples the predicted region of CI with theS0 PES at extended CndashS bond lengths Radiationless transferat this CI yields highly vibrationally excited S0 moleculesThese have more than enough internal energy to decay notonly by RndashBr bond fission (yielding thienyl cofragments)but also to ring-open isomerise and decay to yield slowBr fragments in tandem with a range of C4H3S radicalproducts Figure 10 suggests similar energetics for CndashBr bondfission leading to ring closed (R) and some of the morestable ring opened radical products (eg H and I) Entropicconsiderations might favour the latter products but any seriousdiscussion of the likely product yields would require betterknowledge of the topology of the CI via which population isfunnelled to the biradical intermediate on the S0 PES and ofthe rate with which the latter can overcome the barrier to thestable ring-opened products featured on the right hand side ofFig 10

The data for 2-iodothiophene are less clear cut but againthe relative yield of slow I atoms increases upon tuning toshorter wavelengthsmdashconsistent with initial excitation to thecorresponding 21Aprime state and radiationless transfer to (andunimolecular decay from) highly vibrationally excited S0molecules formed by coupling through the predicted CI atextended RCndashS The ldquofastrdquo IIlowast feature also persists at shorterwavelengths however This could be explained by one orother or both of two mechanisms The first recognises the(relatively) greater partial absorption cross sections of theσCndashI

lowast larr nπ excitations in 2-iodothiophene (Fig 3) while thesecond assumes that following excitation to the 21Aprime state thethreshold energies for predissociation enabled by CndashI and CndashSbond extension are more similar than in 2-bromothiopheneThe evident reduction in the TKER and recoil anisotropyof the ldquofastrdquo I signal at shorter wavelengths hints that CndashXbond fission is a relatively more important decay channelfrom the 21Aprime state of 2-iodothiophene than in the case of2-bromothiophene

Finally we contrast the fast anisotropic Br and Brlowast

product velocity distributions formed in the 267 nm photolysis

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1372224331 On Tue 09 Jun 2015 131203

224303-10 Marchetti et al J Chem Phys 142 224303 (2015)

of 2-bromothiophene with the slow isotropic distributionsobserved when exciting 3-bromothiophene at this samewavelength9 The latter are reminiscent of that found whenexciting 2-bromothiophene at much shorter wavelengths(below sim245 nm) As noted previously all of our attemptsto characterise the analogue of biradical B on the S0 PES of3-bromothiophene minimised to a lower energy cyclopropeneadduct (an enantiomer of G) which will have ample internalenergy to decay further (eg to H + X products) Thepublished photolysis data for 3-bromothiophene may thusbe explicable if the corresponding σCndashS

lowast larr (nπ) PESs aresomewhat red-shifted cf 2-bromothiophene and provide anefficient route for funnelling photoexcited molecules to regionsof CI with the S0 PES and thence to CndashBr bond fission onthis latter PES Photolysis studies of 3-halothiophenes at arange of (longer) UV excitation wavelengths could be usefulin confirming (or refuting) this proposal

IV CONCLUSIONS

The near UV photofragmentation dynamics of jet-cooledgas phase 2-bromo- and 2-iodothiophene molecules havebeen investigated at many different excitation wavelengthsby measuring the velocity distributions of the XXlowast productsusing a VMI spectrometer equipped with a new purposedesigned ion optics assembly The new ion optics design isnot much more complicated than the traditional flat annularelectrode stack and offers a number of potentially significantadvantages stable velocity mapping from a larger interactionvolume protection from stray fields improved limiting veloc-ity resolution and natural compatibility with slice imagingThe present study addressing the photodissociation of a 9-atomic system does not provide an appropriate vehicle forillustrating the predicted velocity resolution As discussedin the supplementary material3 the ultimate test of theresolving power of the new ion optics assembly should involvephotodissociation studies of fully quantum state selectedparent molecules such as those reported by the Janssen groupfor methyl iodide46

The present imaging study reveals a marked change inphotofragmentation behaviour upon tuning to shorter excita-tion wavelengths particularly in the case of 2-bromothiopheneLong wavelength excitation results in prompt CndashX bondfission the resulting XXlowast products are formed withkinetic energies close to the maximum allowed by energyconservation and with near limiting parallel recoil anisotropyCompanion ab initio calculations show that these productsarise following excitation to one or more (nπ)σlowast states andserve to highlight the photophysical similarities with otheraryl iodides (and bromides) when excited at longer UVwavelengths4ndash8

At shorter wavelengths the anisotropic fast ring asso-ciated with these products fades in relative intensity and isprogressively replaced by a smaller isotropic feature thatsignifies the formation of much slower XXlowast atom productsThe appearance of this new channel roughly coincides withthe onset of more intense parent absorption to the 21Aprime excitedstate Ab initio theory suggests that this ππlowast excited statecan predissociate by interaction with neighbouring (nπ)σlowast

states the potentials for which decline in energy along boththe RCndashX and RCndashS stretch coordinates The 2-bromothiophenedata can be understood if the latter offers the lower energydecay route from the optically bright 21Aprime state to a CIwith the ground state PES at extended CndashS bond lengthsThe present DFTB3LYP6-311G(dp) calculations suggestthat the resulting biradicals have more than sufficient energyto sample the configuration space associated with severaldifferent isomeric forms of the starting molecule and to formboth ring closed and ring opened C4H3S radical productsfollowing subsequent CndashBr bond fission on the S0 PES

The data for 2-iodothiophene are less clear cut butagain the relative yield of slow I atoms increases markedlyupon tuning to shorter wavelengthsmdashconsistent with initialexcitation to the corresponding 21Aprime state and radiationlesstransfer to (and unimolecular decay from) highly vibrationallyexcited S0 molecules formed by coupling through the predictedCI at extended RCndashS The ldquofastrdquo IIlowast feature also persists atshorter wavelengths however This could be explained byone or other or both of two mechanisms The first recognisesthe (relatively) greater partial absorption cross sections ofthe σCndashI

lowast larr nπ excitations in 2-iodothiophene while thesecond assumes that following excitation to the 21Aprime state thethresholds for predissociation enabled by CndashI and CndashS bondextension are closer in energy than in 2-bromothiophene

It is interesting to compare the present findings withthose from recent condensed phase studies of thiophenonein acetonitrile solution wherein the photoexcited parentmolecules were similarly deduced to convert to highlyvibrationally excited levels of the ground (S0) state bycoupling via a CI at extended CndashS bond lengths19 Ringopening was confirmed in that case by the observation ofketene products but the time evolution of the parent bleachsignals indicated that the majority (sim60) of these highlyvibrationally excited S0 molecules relax and re-thermalise asthe ring closed thiophenone Such vibrational relaxation is notavailable in the present isolated molecule gas phase studies soeventual dissociation is assured on energetic grounds but therelative probabilities for forming ring opened or ring closedfragmentation products remains an open question

ACKNOWLEDGMENTS

The authors are grateful to EPSRC (Programme GrantNos EPG00224X and EPL005913) to the TSB (KnowledgeTransfer Partnership KTP008481 that stimulated the Bristol-Photek collaboration) to the Marie Curie Initial TrainingNetwork ICONIC (Contract Agreement No 238671) andto Rebecca Ingle Keith Rosser Dr Stuart Greaves DrJames Smith Dr Andreas Wenge Dr Eckart Wrede and DrDimitrios Zaouris for their many and varied contributions tothis work Data accessibility The underlying data for thispaper has been placed in the University of Bristolrsquos researchdata repository and can be accessed using the following DOI105523brisqwlb0o0e9tbd1bqojd8gxrnjj

1A L Sobolewski W Domcke C Dedonder-Lardeux and C Jouvet PhysChem Chem Phys 4 1093 (2002)

2M N R Ashfold G A King D Murdock M G D Nix T A A Oliverand A G Sage Phys Chem Chem Phys 12 1218 (2010)

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1372224331 On Tue 09 Jun 2015 131203

224303-7 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 8 Diabatic SO resolved PECs along RCndashX for the ground and lower lying excited states of (a) 2-bromothiophene and (b) 2-iodothiophene

None of the TKER distributions derived from the Br or Brlowast

images from photolysis of 2-bromothiophene show any hintsof product vibrational structure (as was the case also in theimaging study of 4-bromophenol photolysis8) Nonethelessif we assume similar dissociation dynamics to that revealedby 2-iodothiophene and that the TKERmax value for productsformed from 2-bromothiophene photolysis at the longestUV wavelengths similarly lies on the fast edge of the highTKER feature (as shown by the short vertical arrow in bothFigs 4(a) and 5(a)) we arrive at a CndashBr bond strength D0(R minus Br) sim 29 000 cmminus1 (given A(Br) = 3685 cmminus1) that isagain sensibly consistent with values reported for bro-mobenzene (3428 kJ molminus1 28 650 cmminus1 Ref 42) andfor 4-bromophenol (28 700 cmminus1 Ref 8) but substantiallylarger than an earlier suggested CndashBr bond strength in 2-bromothiophene43 As the other panels in Figs 4 and 5 showtuning to shorter photolysis wavelengths again results in arelative increase in the internal energy partitioned into thethienyl partner

B Ab initio calculations of CndashBrCndashI bond fissionand CndashS ring opening pathways in 2-bromothiopheneand 2-iodothiophene

Figures 8(a) and 8(b) present SO resolved PECs alongRCndashX for the ground and lower energy excited states of respec-tively 2-bromothiophene and 2-iodothiophene In both casesthe molecule has been constrained to planar geometries andthe PECs have been approximately ldquodiabatisedrdquo in terms of(nπ)σlowast and ππlowast states by inspecting the energies symmetriesand wavefunction coefficients The adiabatic SO resolvedpotentials from which these are derived are shown in thesupplementary material3 As in the case of the iodobenzenes67

and halophenols8 the lowest repulsive potentials all arise fromσCndashX

lowast larr nπ excitations these potentials correlate with theground (X2Aprime) state thienyl + XXlowast dissociation limits Thespin-orbit split analogues involving the first (A2Aprimeprime) excitedstate of the thienyl radical lie higher in energy while the(diabatically) bound interloper PECs arise from excitationsto ring centred πlowast orbitals The calculated vertical excitationenergies suggest that many of these excited states could besampled within the wavelength ranges explored in the presentstudy but analogy with the previous VMI studies of the XXlowast

products from photolysis of iodobenzenes and 4-iodo- and4-bromophenol suggests that the σlowast larr nπ TDMs returnedby the present calculations are likely to be of limited valuein deciding which are the optically ldquobrightrdquo states (illustrativeTDMs are listed in the supplementary material3) Rather asin those cases6ndash8 the observation that the faster X and Xlowast

products formed at long excitation wavelengths show nearlimiting parallel recoil anisotropy suggests that the populatedstates are those that gain intensity by vibronic interactionwith the higher lying intense CndashX bond centred σlowast larr σtransition

The predicted energy separations between the variousasymptotic limits merit comment The present calculationsreproduce the BrBrlowast and IIlowast spin-orbit splittings well butpredict a larger separation between the A and X statesof the thienyl radical (sim125 eV) than that derived fromphotodetachment studies of the thiophenide anion (sim06 eVRef 44) This is not surprising given that the PECs are derivedvia rigid body scans and thus preclude any structural relaxationof the ring Nonetheless the present calculations confirm theearlier suggestion that (nπ)σlowast excited states (ie states thatare dissociative with respect to CndashX bond extension and whichcorrelate to X or Xlowast products) are dominant contributors tothe long wavelength tail of the respective absorption spectraand locate the minimum of the first (spin-allowed) πlowast larr πtransition at sim53 eV (sim59 eV) for X = I (Br) in reasonableaccord with the observed absorption maxima (Fig 3)

Cuts through the first few SOF potentials of 2-bro-mothiophene and 2-iodothiophene along the RCndashS stretch(ring-opening) coordinate are shown in Figs 9(a) and 9(c)respectively The corresponding adiabatic SOF PECs alongRCndashX are shown alongside in Figs 9(b) and 9(d) The S0 stateis stable with respect to ring opening in both halothiophenesand its PEC thus rises steadily with increasing RCndashS Incontrast the PECs for the 11Aprimeprime and 21Aprimeprime states (and fortheir triplet analogues all of which arise via σCndashS

lowast larr (nπ)excitations) show obvious instability with respect to extendingRCndashS Notwithstanding the limitations of these SOF PECs(all of which are calculated at the MP2 relaxed ground stategeometries) they clearly point to the availability of exoergicpathways from the bright 1ππlowast (21Aprime) state via one or more(nπ)σCndashS

lowast PECs to extended CndashS bond lengths and regionsof conical intersection with the S0 state potential

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1372224331 On Tue 09 Jun 2015 131203

224303-8 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 9 Approximately diabatised SO free PECs along the RCndashS (ring-opening) coordinate and RCndashX stretch for the first few singlet (solid lines) and triplet(dashed lines) states of 2-bromothiophene ((a) and (b) respectively) and 2-iodothiophene ((c) and (d) respectively)

Figure 10 summarises some of the various possible fatesof 2-halothiophene molecules following non-adiabatic transferto the S0 PES The primary ring-opened structure is a biradical(structure B) that can exist as essentially degenerate cis andtrans isomersmdashboth of which are unstable with respect tothe original ground state halothiophene B can also isomeriseto various ring-opened isomers (including structures D-G)via transition states three of which that involve migrating(at most) one atom from the biradical structure (TS1 TS2and TS3) have been optimised As Fig 10 also shows arange of Br + radical (H I J K as well as R) dissociationlimits are calculated to lie below the energy of a 250 nmphoton (illustrated by the horizontal red line) The variousstructures and product pairs along with their energies (incmminus1 defined relative to that of the ground state minimum)are detailed below the figure along with the correspondingquantities for 2-iodothiophene (in parentheses) The calculatedRndashBr and RndashI bond strengths are seen to match well withthe experimental values derived in Sec III A Readersshould also note that the dashed lines at the far right ofFig 10 are simply intended to correlate various isomers toparticular dissociation products we have not checked for thepresence (or otherwise) of any barrier in the various fragmen-tation paths

C A plausible explanation for the observedwavelength dependent fragmentation dynamics

The X and Xlowast images measured at the longer UVwavelengths are very reminiscent of those reported previouslyfollowing photolysis of for example iodobenzenes67 and 4-iodo- and 4-bromophenol8 The X and Xlowast products recoilwith close to the maximum speed permitted by energyconservation and with close to limiting parallel anisotropyThe velocities of the faster XXlowast products increase littleupon tuning to shorter excitation wavelengths implying apreferential partitioning of the additional photon energy intointernal (vibrational) excitation of the thienyl radical partnerAs in the previously studied aryl iodides and bromides suchbehaviour is fully consistent with population of one or more(nπ)σlowast excited states and prompt CndashX bond fission Theprogressive increase in product vibrational excitation can beunderstood on Franck-Condon grounds given the changes inequilibrium ring geometry upon σlowast larr (nπ) excitation andsubsequent evolution to the radical product As noted abovethe near limiting parallel recoil anisotropy of the X and Xlowast

products can be understood if (as has been shown in detail inthe case of methyl iodide45) the participating (nπ)σlowast state(s)gains transition probability by vibronic interaction with a

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203

224303-9 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 10 Diagram showing the calculated relative energies of 2-bromothiophene (A) the biradical intermediate B formed upon CndashS bondfission TSs leading to various ring-opened isomers (D-G) and selectedBr + radical (H I J K) product limits The dashed lines at the far right of thefigure simply correlate given structures to particular dissociation productswe have not checked for the presence (or otherwise) of any barrier in thevarious fragmentation paths The various structures along with their energies(in cmminus1 defined relative to the ground state minimum) are detailed below to-gether with the corresponding quantities for 2-iodothiophene (in parentheses)The horizontal dashed line indicates the total energy reached upon absorptionof a 250 nm photon

higher lying CndashX bond localised σσlowast state and the subsequentbond fission occurs on a time scale that is fast compared to themolecular rotational period

The primary novelty of the present study however is theclear switch in energy disposal upon reducing the excitationwavelength This switch is revealed by the appearance (andgrowing dominance) of a slow isotropic component withinthe X and Xlowast imagesmdashmore obviously in the case of2-bromothiophenemdashand roughly coincides with the start ofthe obvious increase in parent absorption that we associatewith πlowast larr π absorption (Fig 3) Careful inspection of Figs 6and 7 also shows that the ldquofastrdquo component in the I and Ilowast

images slows and becomes less anisotropic as the photonenergy is increased further

The PES for this ππlowast state (the 21Aprime state in Fig 9)shows CIs (at geometries not far from the vertical FranckCondon region) with other excited states the potential energiesof which decline with increases in RCndashX andor RCndashS Thusit is reasonable to propose that excitation at these shorterwavelengths populates the (diabatically bound) 21Aprime statewhich can then decay by coupling to states that are dissociativewith respect to CndashX andor CndashS bond extension Which ofthese decay processes dominates (ie the CndashX bond fissionor the channel initiated by CndashS bond extension) will besensitively dependent upon the respective minimum energypathways from the 21Aprime state and the details of the couplingmodes that facilitate the population transfer

The almost total absence of ldquofastrdquo BrBrlowast products in theimages recorded following photolysis of 2-bromothiopheneat λ sim 245 nm suggests that (i) the partial absorption crosssection to the 21Aprime state far exceeds that to the underlying(nπ)σlowast state(s)mdashconsistent with the relative weakness of thelong wavelength tail in the UV absorption spectrum of 2-bromothiophenemdashand (ii) CndashS bond extension constitutes themajor decay pathway from the 21Aprime state and the subsequentnuclear motion samples the predicted region of CI with theS0 PES at extended CndashS bond lengths Radiationless transferat this CI yields highly vibrationally excited S0 moleculesThese have more than enough internal energy to decay notonly by RndashBr bond fission (yielding thienyl cofragments)but also to ring-open isomerise and decay to yield slowBr fragments in tandem with a range of C4H3S radicalproducts Figure 10 suggests similar energetics for CndashBr bondfission leading to ring closed (R) and some of the morestable ring opened radical products (eg H and I) Entropicconsiderations might favour the latter products but any seriousdiscussion of the likely product yields would require betterknowledge of the topology of the CI via which population isfunnelled to the biradical intermediate on the S0 PES and ofthe rate with which the latter can overcome the barrier to thestable ring-opened products featured on the right hand side ofFig 10

The data for 2-iodothiophene are less clear cut but againthe relative yield of slow I atoms increases upon tuning toshorter wavelengthsmdashconsistent with initial excitation to thecorresponding 21Aprime state and radiationless transfer to (andunimolecular decay from) highly vibrationally excited S0molecules formed by coupling through the predicted CI atextended RCndashS The ldquofastrdquo IIlowast feature also persists at shorterwavelengths however This could be explained by one orother or both of two mechanisms The first recognises the(relatively) greater partial absorption cross sections of theσCndashI

lowast larr nπ excitations in 2-iodothiophene (Fig 3) while thesecond assumes that following excitation to the 21Aprime state thethreshold energies for predissociation enabled by CndashI and CndashSbond extension are more similar than in 2-bromothiopheneThe evident reduction in the TKER and recoil anisotropyof the ldquofastrdquo I signal at shorter wavelengths hints that CndashXbond fission is a relatively more important decay channelfrom the 21Aprime state of 2-iodothiophene than in the case of2-bromothiophene

Finally we contrast the fast anisotropic Br and Brlowast

product velocity distributions formed in the 267 nm photolysis

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203

224303-10 Marchetti et al J Chem Phys 142 224303 (2015)

of 2-bromothiophene with the slow isotropic distributionsobserved when exciting 3-bromothiophene at this samewavelength9 The latter are reminiscent of that found whenexciting 2-bromothiophene at much shorter wavelengths(below sim245 nm) As noted previously all of our attemptsto characterise the analogue of biradical B on the S0 PES of3-bromothiophene minimised to a lower energy cyclopropeneadduct (an enantiomer of G) which will have ample internalenergy to decay further (eg to H + X products) Thepublished photolysis data for 3-bromothiophene may thusbe explicable if the corresponding σCndashS

lowast larr (nπ) PESs aresomewhat red-shifted cf 2-bromothiophene and provide anefficient route for funnelling photoexcited molecules to regionsof CI with the S0 PES and thence to CndashBr bond fission onthis latter PES Photolysis studies of 3-halothiophenes at arange of (longer) UV excitation wavelengths could be usefulin confirming (or refuting) this proposal

IV CONCLUSIONS

The near UV photofragmentation dynamics of jet-cooledgas phase 2-bromo- and 2-iodothiophene molecules havebeen investigated at many different excitation wavelengthsby measuring the velocity distributions of the XXlowast productsusing a VMI spectrometer equipped with a new purposedesigned ion optics assembly The new ion optics design isnot much more complicated than the traditional flat annularelectrode stack and offers a number of potentially significantadvantages stable velocity mapping from a larger interactionvolume protection from stray fields improved limiting veloc-ity resolution and natural compatibility with slice imagingThe present study addressing the photodissociation of a 9-atomic system does not provide an appropriate vehicle forillustrating the predicted velocity resolution As discussedin the supplementary material3 the ultimate test of theresolving power of the new ion optics assembly should involvephotodissociation studies of fully quantum state selectedparent molecules such as those reported by the Janssen groupfor methyl iodide46

The present imaging study reveals a marked change inphotofragmentation behaviour upon tuning to shorter excita-tion wavelengths particularly in the case of 2-bromothiopheneLong wavelength excitation results in prompt CndashX bondfission the resulting XXlowast products are formed withkinetic energies close to the maximum allowed by energyconservation and with near limiting parallel recoil anisotropyCompanion ab initio calculations show that these productsarise following excitation to one or more (nπ)σlowast states andserve to highlight the photophysical similarities with otheraryl iodides (and bromides) when excited at longer UVwavelengths4ndash8

At shorter wavelengths the anisotropic fast ring asso-ciated with these products fades in relative intensity and isprogressively replaced by a smaller isotropic feature thatsignifies the formation of much slower XXlowast atom productsThe appearance of this new channel roughly coincides withthe onset of more intense parent absorption to the 21Aprime excitedstate Ab initio theory suggests that this ππlowast excited statecan predissociate by interaction with neighbouring (nπ)σlowast

states the potentials for which decline in energy along boththe RCndashX and RCndashS stretch coordinates The 2-bromothiophenedata can be understood if the latter offers the lower energydecay route from the optically bright 21Aprime state to a CIwith the ground state PES at extended CndashS bond lengthsThe present DFTB3LYP6-311G(dp) calculations suggestthat the resulting biradicals have more than sufficient energyto sample the configuration space associated with severaldifferent isomeric forms of the starting molecule and to formboth ring closed and ring opened C4H3S radical productsfollowing subsequent CndashBr bond fission on the S0 PES

The data for 2-iodothiophene are less clear cut butagain the relative yield of slow I atoms increases markedlyupon tuning to shorter wavelengthsmdashconsistent with initialexcitation to the corresponding 21Aprime state and radiationlesstransfer to (and unimolecular decay from) highly vibrationallyexcited S0 molecules formed by coupling through the predictedCI at extended RCndashS The ldquofastrdquo IIlowast feature also persists atshorter wavelengths however This could be explained byone or other or both of two mechanisms The first recognisesthe (relatively) greater partial absorption cross sections ofthe σCndashI

lowast larr nπ excitations in 2-iodothiophene while thesecond assumes that following excitation to the 21Aprime state thethresholds for predissociation enabled by CndashI and CndashS bondextension are closer in energy than in 2-bromothiophene

It is interesting to compare the present findings withthose from recent condensed phase studies of thiophenonein acetonitrile solution wherein the photoexcited parentmolecules were similarly deduced to convert to highlyvibrationally excited levels of the ground (S0) state bycoupling via a CI at extended CndashS bond lengths19 Ringopening was confirmed in that case by the observation ofketene products but the time evolution of the parent bleachsignals indicated that the majority (sim60) of these highlyvibrationally excited S0 molecules relax and re-thermalise asthe ring closed thiophenone Such vibrational relaxation is notavailable in the present isolated molecule gas phase studies soeventual dissociation is assured on energetic grounds but therelative probabilities for forming ring opened or ring closedfragmentation products remains an open question

ACKNOWLEDGMENTS

The authors are grateful to EPSRC (Programme GrantNos EPG00224X and EPL005913) to the TSB (KnowledgeTransfer Partnership KTP008481 that stimulated the Bristol-Photek collaboration) to the Marie Curie Initial TrainingNetwork ICONIC (Contract Agreement No 238671) andto Rebecca Ingle Keith Rosser Dr Stuart Greaves DrJames Smith Dr Andreas Wenge Dr Eckart Wrede and DrDimitrios Zaouris for their many and varied contributions tothis work Data accessibility The underlying data for thispaper has been placed in the University of Bristolrsquos researchdata repository and can be accessed using the following DOI105523brisqwlb0o0e9tbd1bqojd8gxrnjj

1A L Sobolewski W Domcke C Dedonder-Lardeux and C Jouvet PhysChem Chem Phys 4 1093 (2002)

2M N R Ashfold G A King D Murdock M G D Nix T A A Oliverand A G Sage Phys Chem Chem Phys 12 1218 (2010)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203

224303-8 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 9 Approximately diabatised SO free PECs along the RCndashS (ring-opening) coordinate and RCndashX stretch for the first few singlet (solid lines) and triplet(dashed lines) states of 2-bromothiophene ((a) and (b) respectively) and 2-iodothiophene ((c) and (d) respectively)

Figure 10 summarises some of the various possible fatesof 2-halothiophene molecules following non-adiabatic transferto the S0 PES The primary ring-opened structure is a biradical(structure B) that can exist as essentially degenerate cis andtrans isomersmdashboth of which are unstable with respect tothe original ground state halothiophene B can also isomeriseto various ring-opened isomers (including structures D-G)via transition states three of which that involve migrating(at most) one atom from the biradical structure (TS1 TS2and TS3) have been optimised As Fig 10 also shows arange of Br + radical (H I J K as well as R) dissociationlimits are calculated to lie below the energy of a 250 nmphoton (illustrated by the horizontal red line) The variousstructures and product pairs along with their energies (incmminus1 defined relative to that of the ground state minimum)are detailed below the figure along with the correspondingquantities for 2-iodothiophene (in parentheses) The calculatedRndashBr and RndashI bond strengths are seen to match well withthe experimental values derived in Sec III A Readersshould also note that the dashed lines at the far right ofFig 10 are simply intended to correlate various isomers toparticular dissociation products we have not checked for thepresence (or otherwise) of any barrier in the various fragmen-tation paths

C A plausible explanation for the observedwavelength dependent fragmentation dynamics

The X and Xlowast images measured at the longer UVwavelengths are very reminiscent of those reported previouslyfollowing photolysis of for example iodobenzenes67 and 4-iodo- and 4-bromophenol8 The X and Xlowast products recoilwith close to the maximum speed permitted by energyconservation and with close to limiting parallel anisotropyThe velocities of the faster XXlowast products increase littleupon tuning to shorter excitation wavelengths implying apreferential partitioning of the additional photon energy intointernal (vibrational) excitation of the thienyl radical partnerAs in the previously studied aryl iodides and bromides suchbehaviour is fully consistent with population of one or more(nπ)σlowast excited states and prompt CndashX bond fission Theprogressive increase in product vibrational excitation can beunderstood on Franck-Condon grounds given the changes inequilibrium ring geometry upon σlowast larr (nπ) excitation andsubsequent evolution to the radical product As noted abovethe near limiting parallel recoil anisotropy of the X and Xlowast

products can be understood if (as has been shown in detail inthe case of methyl iodide45) the participating (nπ)σlowast state(s)gains transition probability by vibronic interaction with a

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203

224303-9 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 10 Diagram showing the calculated relative energies of 2-bromothiophene (A) the biradical intermediate B formed upon CndashS bondfission TSs leading to various ring-opened isomers (D-G) and selectedBr + radical (H I J K) product limits The dashed lines at the far right of thefigure simply correlate given structures to particular dissociation productswe have not checked for the presence (or otherwise) of any barrier in thevarious fragmentation paths The various structures along with their energies(in cmminus1 defined relative to the ground state minimum) are detailed below to-gether with the corresponding quantities for 2-iodothiophene (in parentheses)The horizontal dashed line indicates the total energy reached upon absorptionof a 250 nm photon

higher lying CndashX bond localised σσlowast state and the subsequentbond fission occurs on a time scale that is fast compared to themolecular rotational period

The primary novelty of the present study however is theclear switch in energy disposal upon reducing the excitationwavelength This switch is revealed by the appearance (andgrowing dominance) of a slow isotropic component withinthe X and Xlowast imagesmdashmore obviously in the case of2-bromothiophenemdashand roughly coincides with the start ofthe obvious increase in parent absorption that we associatewith πlowast larr π absorption (Fig 3) Careful inspection of Figs 6and 7 also shows that the ldquofastrdquo component in the I and Ilowast

images slows and becomes less anisotropic as the photonenergy is increased further

The PES for this ππlowast state (the 21Aprime state in Fig 9)shows CIs (at geometries not far from the vertical FranckCondon region) with other excited states the potential energiesof which decline with increases in RCndashX andor RCndashS Thusit is reasonable to propose that excitation at these shorterwavelengths populates the (diabatically bound) 21Aprime statewhich can then decay by coupling to states that are dissociativewith respect to CndashX andor CndashS bond extension Which ofthese decay processes dominates (ie the CndashX bond fissionor the channel initiated by CndashS bond extension) will besensitively dependent upon the respective minimum energypathways from the 21Aprime state and the details of the couplingmodes that facilitate the population transfer

The almost total absence of ldquofastrdquo BrBrlowast products in theimages recorded following photolysis of 2-bromothiopheneat λ sim 245 nm suggests that (i) the partial absorption crosssection to the 21Aprime state far exceeds that to the underlying(nπ)σlowast state(s)mdashconsistent with the relative weakness of thelong wavelength tail in the UV absorption spectrum of 2-bromothiophenemdashand (ii) CndashS bond extension constitutes themajor decay pathway from the 21Aprime state and the subsequentnuclear motion samples the predicted region of CI with theS0 PES at extended CndashS bond lengths Radiationless transferat this CI yields highly vibrationally excited S0 moleculesThese have more than enough internal energy to decay notonly by RndashBr bond fission (yielding thienyl cofragments)but also to ring-open isomerise and decay to yield slowBr fragments in tandem with a range of C4H3S radicalproducts Figure 10 suggests similar energetics for CndashBr bondfission leading to ring closed (R) and some of the morestable ring opened radical products (eg H and I) Entropicconsiderations might favour the latter products but any seriousdiscussion of the likely product yields would require betterknowledge of the topology of the CI via which population isfunnelled to the biradical intermediate on the S0 PES and ofthe rate with which the latter can overcome the barrier to thestable ring-opened products featured on the right hand side ofFig 10

The data for 2-iodothiophene are less clear cut but againthe relative yield of slow I atoms increases upon tuning toshorter wavelengthsmdashconsistent with initial excitation to thecorresponding 21Aprime state and radiationless transfer to (andunimolecular decay from) highly vibrationally excited S0molecules formed by coupling through the predicted CI atextended RCndashS The ldquofastrdquo IIlowast feature also persists at shorterwavelengths however This could be explained by one orother or both of two mechanisms The first recognises the(relatively) greater partial absorption cross sections of theσCndashI

lowast larr nπ excitations in 2-iodothiophene (Fig 3) while thesecond assumes that following excitation to the 21Aprime state thethreshold energies for predissociation enabled by CndashI and CndashSbond extension are more similar than in 2-bromothiopheneThe evident reduction in the TKER and recoil anisotropyof the ldquofastrdquo I signal at shorter wavelengths hints that CndashXbond fission is a relatively more important decay channelfrom the 21Aprime state of 2-iodothiophene than in the case of2-bromothiophene

Finally we contrast the fast anisotropic Br and Brlowast

product velocity distributions formed in the 267 nm photolysis

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203

224303-10 Marchetti et al J Chem Phys 142 224303 (2015)

of 2-bromothiophene with the slow isotropic distributionsobserved when exciting 3-bromothiophene at this samewavelength9 The latter are reminiscent of that found whenexciting 2-bromothiophene at much shorter wavelengths(below sim245 nm) As noted previously all of our attemptsto characterise the analogue of biradical B on the S0 PES of3-bromothiophene minimised to a lower energy cyclopropeneadduct (an enantiomer of G) which will have ample internalenergy to decay further (eg to H + X products) Thepublished photolysis data for 3-bromothiophene may thusbe explicable if the corresponding σCndashS

lowast larr (nπ) PESs aresomewhat red-shifted cf 2-bromothiophene and provide anefficient route for funnelling photoexcited molecules to regionsof CI with the S0 PES and thence to CndashBr bond fission onthis latter PES Photolysis studies of 3-halothiophenes at arange of (longer) UV excitation wavelengths could be usefulin confirming (or refuting) this proposal

IV CONCLUSIONS

The near UV photofragmentation dynamics of jet-cooledgas phase 2-bromo- and 2-iodothiophene molecules havebeen investigated at many different excitation wavelengthsby measuring the velocity distributions of the XXlowast productsusing a VMI spectrometer equipped with a new purposedesigned ion optics assembly The new ion optics design isnot much more complicated than the traditional flat annularelectrode stack and offers a number of potentially significantadvantages stable velocity mapping from a larger interactionvolume protection from stray fields improved limiting veloc-ity resolution and natural compatibility with slice imagingThe present study addressing the photodissociation of a 9-atomic system does not provide an appropriate vehicle forillustrating the predicted velocity resolution As discussedin the supplementary material3 the ultimate test of theresolving power of the new ion optics assembly should involvephotodissociation studies of fully quantum state selectedparent molecules such as those reported by the Janssen groupfor methyl iodide46

The present imaging study reveals a marked change inphotofragmentation behaviour upon tuning to shorter excita-tion wavelengths particularly in the case of 2-bromothiopheneLong wavelength excitation results in prompt CndashX bondfission the resulting XXlowast products are formed withkinetic energies close to the maximum allowed by energyconservation and with near limiting parallel recoil anisotropyCompanion ab initio calculations show that these productsarise following excitation to one or more (nπ)σlowast states andserve to highlight the photophysical similarities with otheraryl iodides (and bromides) when excited at longer UVwavelengths4ndash8

At shorter wavelengths the anisotropic fast ring asso-ciated with these products fades in relative intensity and isprogressively replaced by a smaller isotropic feature thatsignifies the formation of much slower XXlowast atom productsThe appearance of this new channel roughly coincides withthe onset of more intense parent absorption to the 21Aprime excitedstate Ab initio theory suggests that this ππlowast excited statecan predissociate by interaction with neighbouring (nπ)σlowast

states the potentials for which decline in energy along boththe RCndashX and RCndashS stretch coordinates The 2-bromothiophenedata can be understood if the latter offers the lower energydecay route from the optically bright 21Aprime state to a CIwith the ground state PES at extended CndashS bond lengthsThe present DFTB3LYP6-311G(dp) calculations suggestthat the resulting biradicals have more than sufficient energyto sample the configuration space associated with severaldifferent isomeric forms of the starting molecule and to formboth ring closed and ring opened C4H3S radical productsfollowing subsequent CndashBr bond fission on the S0 PES

The data for 2-iodothiophene are less clear cut butagain the relative yield of slow I atoms increases markedlyupon tuning to shorter wavelengthsmdashconsistent with initialexcitation to the corresponding 21Aprime state and radiationlesstransfer to (and unimolecular decay from) highly vibrationallyexcited S0 molecules formed by coupling through the predictedCI at extended RCndashS The ldquofastrdquo IIlowast feature also persists atshorter wavelengths however This could be explained byone or other or both of two mechanisms The first recognisesthe (relatively) greater partial absorption cross sections ofthe σCndashI

lowast larr nπ excitations in 2-iodothiophene while thesecond assumes that following excitation to the 21Aprime state thethresholds for predissociation enabled by CndashI and CndashS bondextension are closer in energy than in 2-bromothiophene

It is interesting to compare the present findings withthose from recent condensed phase studies of thiophenonein acetonitrile solution wherein the photoexcited parentmolecules were similarly deduced to convert to highlyvibrationally excited levels of the ground (S0) state bycoupling via a CI at extended CndashS bond lengths19 Ringopening was confirmed in that case by the observation ofketene products but the time evolution of the parent bleachsignals indicated that the majority (sim60) of these highlyvibrationally excited S0 molecules relax and re-thermalise asthe ring closed thiophenone Such vibrational relaxation is notavailable in the present isolated molecule gas phase studies soeventual dissociation is assured on energetic grounds but therelative probabilities for forming ring opened or ring closedfragmentation products remains an open question

ACKNOWLEDGMENTS

The authors are grateful to EPSRC (Programme GrantNos EPG00224X and EPL005913) to the TSB (KnowledgeTransfer Partnership KTP008481 that stimulated the Bristol-Photek collaboration) to the Marie Curie Initial TrainingNetwork ICONIC (Contract Agreement No 238671) andto Rebecca Ingle Keith Rosser Dr Stuart Greaves DrJames Smith Dr Andreas Wenge Dr Eckart Wrede and DrDimitrios Zaouris for their many and varied contributions tothis work Data accessibility The underlying data for thispaper has been placed in the University of Bristolrsquos researchdata repository and can be accessed using the following DOI105523brisqwlb0o0e9tbd1bqojd8gxrnjj

1A L Sobolewski W Domcke C Dedonder-Lardeux and C Jouvet PhysChem Chem Phys 4 1093 (2002)

2M N R Ashfold G A King D Murdock M G D Nix T A A Oliverand A G Sage Phys Chem Chem Phys 12 1218 (2010)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203

224303-9 Marchetti et al J Chem Phys 142 224303 (2015)

FIG 10 Diagram showing the calculated relative energies of 2-bromothiophene (A) the biradical intermediate B formed upon CndashS bondfission TSs leading to various ring-opened isomers (D-G) and selectedBr + radical (H I J K) product limits The dashed lines at the far right of thefigure simply correlate given structures to particular dissociation productswe have not checked for the presence (or otherwise) of any barrier in thevarious fragmentation paths The various structures along with their energies(in cmminus1 defined relative to the ground state minimum) are detailed below to-gether with the corresponding quantities for 2-iodothiophene (in parentheses)The horizontal dashed line indicates the total energy reached upon absorptionof a 250 nm photon

higher lying CndashX bond localised σσlowast state and the subsequentbond fission occurs on a time scale that is fast compared to themolecular rotational period

The primary novelty of the present study however is theclear switch in energy disposal upon reducing the excitationwavelength This switch is revealed by the appearance (andgrowing dominance) of a slow isotropic component withinthe X and Xlowast imagesmdashmore obviously in the case of2-bromothiophenemdashand roughly coincides with the start ofthe obvious increase in parent absorption that we associatewith πlowast larr π absorption (Fig 3) Careful inspection of Figs 6and 7 also shows that the ldquofastrdquo component in the I and Ilowast

images slows and becomes less anisotropic as the photonenergy is increased further

The PES for this ππlowast state (the 21Aprime state in Fig 9)shows CIs (at geometries not far from the vertical FranckCondon region) with other excited states the potential energiesof which decline with increases in RCndashX andor RCndashS Thusit is reasonable to propose that excitation at these shorterwavelengths populates the (diabatically bound) 21Aprime statewhich can then decay by coupling to states that are dissociativewith respect to CndashX andor CndashS bond extension Which ofthese decay processes dominates (ie the CndashX bond fissionor the channel initiated by CndashS bond extension) will besensitively dependent upon the respective minimum energypathways from the 21Aprime state and the details of the couplingmodes that facilitate the population transfer

The almost total absence of ldquofastrdquo BrBrlowast products in theimages recorded following photolysis of 2-bromothiopheneat λ sim 245 nm suggests that (i) the partial absorption crosssection to the 21Aprime state far exceeds that to the underlying(nπ)σlowast state(s)mdashconsistent with the relative weakness of thelong wavelength tail in the UV absorption spectrum of 2-bromothiophenemdashand (ii) CndashS bond extension constitutes themajor decay pathway from the 21Aprime state and the subsequentnuclear motion samples the predicted region of CI with theS0 PES at extended CndashS bond lengths Radiationless transferat this CI yields highly vibrationally excited S0 moleculesThese have more than enough internal energy to decay notonly by RndashBr bond fission (yielding thienyl cofragments)but also to ring-open isomerise and decay to yield slowBr fragments in tandem with a range of C4H3S radicalproducts Figure 10 suggests similar energetics for CndashBr bondfission leading to ring closed (R) and some of the morestable ring opened radical products (eg H and I) Entropicconsiderations might favour the latter products but any seriousdiscussion of the likely product yields would require betterknowledge of the topology of the CI via which population isfunnelled to the biradical intermediate on the S0 PES and ofthe rate with which the latter can overcome the barrier to thestable ring-opened products featured on the right hand side ofFig 10

The data for 2-iodothiophene are less clear cut but againthe relative yield of slow I atoms increases upon tuning toshorter wavelengthsmdashconsistent with initial excitation to thecorresponding 21Aprime state and radiationless transfer to (andunimolecular decay from) highly vibrationally excited S0molecules formed by coupling through the predicted CI atextended RCndashS The ldquofastrdquo IIlowast feature also persists at shorterwavelengths however This could be explained by one orother or both of two mechanisms The first recognises the(relatively) greater partial absorption cross sections of theσCndashI

lowast larr nπ excitations in 2-iodothiophene (Fig 3) while thesecond assumes that following excitation to the 21Aprime state thethreshold energies for predissociation enabled by CndashI and CndashSbond extension are more similar than in 2-bromothiopheneThe evident reduction in the TKER and recoil anisotropyof the ldquofastrdquo I signal at shorter wavelengths hints that CndashXbond fission is a relatively more important decay channelfrom the 21Aprime state of 2-iodothiophene than in the case of2-bromothiophene

Finally we contrast the fast anisotropic Br and Brlowast

product velocity distributions formed in the 267 nm photolysis

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203

224303-10 Marchetti et al J Chem Phys 142 224303 (2015)

of 2-bromothiophene with the slow isotropic distributionsobserved when exciting 3-bromothiophene at this samewavelength9 The latter are reminiscent of that found whenexciting 2-bromothiophene at much shorter wavelengths(below sim245 nm) As noted previously all of our attemptsto characterise the analogue of biradical B on the S0 PES of3-bromothiophene minimised to a lower energy cyclopropeneadduct (an enantiomer of G) which will have ample internalenergy to decay further (eg to H + X products) Thepublished photolysis data for 3-bromothiophene may thusbe explicable if the corresponding σCndashS

lowast larr (nπ) PESs aresomewhat red-shifted cf 2-bromothiophene and provide anefficient route for funnelling photoexcited molecules to regionsof CI with the S0 PES and thence to CndashBr bond fission onthis latter PES Photolysis studies of 3-halothiophenes at arange of (longer) UV excitation wavelengths could be usefulin confirming (or refuting) this proposal

IV CONCLUSIONS

The near UV photofragmentation dynamics of jet-cooledgas phase 2-bromo- and 2-iodothiophene molecules havebeen investigated at many different excitation wavelengthsby measuring the velocity distributions of the XXlowast productsusing a VMI spectrometer equipped with a new purposedesigned ion optics assembly The new ion optics design isnot much more complicated than the traditional flat annularelectrode stack and offers a number of potentially significantadvantages stable velocity mapping from a larger interactionvolume protection from stray fields improved limiting veloc-ity resolution and natural compatibility with slice imagingThe present study addressing the photodissociation of a 9-atomic system does not provide an appropriate vehicle forillustrating the predicted velocity resolution As discussedin the supplementary material3 the ultimate test of theresolving power of the new ion optics assembly should involvephotodissociation studies of fully quantum state selectedparent molecules such as those reported by the Janssen groupfor methyl iodide46

The present imaging study reveals a marked change inphotofragmentation behaviour upon tuning to shorter excita-tion wavelengths particularly in the case of 2-bromothiopheneLong wavelength excitation results in prompt CndashX bondfission the resulting XXlowast products are formed withkinetic energies close to the maximum allowed by energyconservation and with near limiting parallel recoil anisotropyCompanion ab initio calculations show that these productsarise following excitation to one or more (nπ)σlowast states andserve to highlight the photophysical similarities with otheraryl iodides (and bromides) when excited at longer UVwavelengths4ndash8

At shorter wavelengths the anisotropic fast ring asso-ciated with these products fades in relative intensity and isprogressively replaced by a smaller isotropic feature thatsignifies the formation of much slower XXlowast atom productsThe appearance of this new channel roughly coincides withthe onset of more intense parent absorption to the 21Aprime excitedstate Ab initio theory suggests that this ππlowast excited statecan predissociate by interaction with neighbouring (nπ)σlowast

states the potentials for which decline in energy along boththe RCndashX and RCndashS stretch coordinates The 2-bromothiophenedata can be understood if the latter offers the lower energydecay route from the optically bright 21Aprime state to a CIwith the ground state PES at extended CndashS bond lengthsThe present DFTB3LYP6-311G(dp) calculations suggestthat the resulting biradicals have more than sufficient energyto sample the configuration space associated with severaldifferent isomeric forms of the starting molecule and to formboth ring closed and ring opened C4H3S radical productsfollowing subsequent CndashBr bond fission on the S0 PES

The data for 2-iodothiophene are less clear cut butagain the relative yield of slow I atoms increases markedlyupon tuning to shorter wavelengthsmdashconsistent with initialexcitation to the corresponding 21Aprime state and radiationlesstransfer to (and unimolecular decay from) highly vibrationallyexcited S0 molecules formed by coupling through the predictedCI at extended RCndashS The ldquofastrdquo IIlowast feature also persists atshorter wavelengths however This could be explained byone or other or both of two mechanisms The first recognisesthe (relatively) greater partial absorption cross sections ofthe σCndashI

lowast larr nπ excitations in 2-iodothiophene while thesecond assumes that following excitation to the 21Aprime state thethresholds for predissociation enabled by CndashI and CndashS bondextension are closer in energy than in 2-bromothiophene

It is interesting to compare the present findings withthose from recent condensed phase studies of thiophenonein acetonitrile solution wherein the photoexcited parentmolecules were similarly deduced to convert to highlyvibrationally excited levels of the ground (S0) state bycoupling via a CI at extended CndashS bond lengths19 Ringopening was confirmed in that case by the observation ofketene products but the time evolution of the parent bleachsignals indicated that the majority (sim60) of these highlyvibrationally excited S0 molecules relax and re-thermalise asthe ring closed thiophenone Such vibrational relaxation is notavailable in the present isolated molecule gas phase studies soeventual dissociation is assured on energetic grounds but therelative probabilities for forming ring opened or ring closedfragmentation products remains an open question

ACKNOWLEDGMENTS

The authors are grateful to EPSRC (Programme GrantNos EPG00224X and EPL005913) to the TSB (KnowledgeTransfer Partnership KTP008481 that stimulated the Bristol-Photek collaboration) to the Marie Curie Initial TrainingNetwork ICONIC (Contract Agreement No 238671) andto Rebecca Ingle Keith Rosser Dr Stuart Greaves DrJames Smith Dr Andreas Wenge Dr Eckart Wrede and DrDimitrios Zaouris for their many and varied contributions tothis work Data accessibility The underlying data for thispaper has been placed in the University of Bristolrsquos researchdata repository and can be accessed using the following DOI105523brisqwlb0o0e9tbd1bqojd8gxrnjj

1A L Sobolewski W Domcke C Dedonder-Lardeux and C Jouvet PhysChem Chem Phys 4 1093 (2002)

2M N R Ashfold G A King D Murdock M G D Nix T A A Oliverand A G Sage Phys Chem Chem Phys 12 1218 (2010)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203

224303-10 Marchetti et al J Chem Phys 142 224303 (2015)

of 2-bromothiophene with the slow isotropic distributionsobserved when exciting 3-bromothiophene at this samewavelength9 The latter are reminiscent of that found whenexciting 2-bromothiophene at much shorter wavelengths(below sim245 nm) As noted previously all of our attemptsto characterise the analogue of biradical B on the S0 PES of3-bromothiophene minimised to a lower energy cyclopropeneadduct (an enantiomer of G) which will have ample internalenergy to decay further (eg to H + X products) Thepublished photolysis data for 3-bromothiophene may thusbe explicable if the corresponding σCndashS

lowast larr (nπ) PESs aresomewhat red-shifted cf 2-bromothiophene and provide anefficient route for funnelling photoexcited molecules to regionsof CI with the S0 PES and thence to CndashBr bond fission onthis latter PES Photolysis studies of 3-halothiophenes at arange of (longer) UV excitation wavelengths could be usefulin confirming (or refuting) this proposal

IV CONCLUSIONS

The near UV photofragmentation dynamics of jet-cooledgas phase 2-bromo- and 2-iodothiophene molecules havebeen investigated at many different excitation wavelengthsby measuring the velocity distributions of the XXlowast productsusing a VMI spectrometer equipped with a new purposedesigned ion optics assembly The new ion optics design isnot much more complicated than the traditional flat annularelectrode stack and offers a number of potentially significantadvantages stable velocity mapping from a larger interactionvolume protection from stray fields improved limiting veloc-ity resolution and natural compatibility with slice imagingThe present study addressing the photodissociation of a 9-atomic system does not provide an appropriate vehicle forillustrating the predicted velocity resolution As discussedin the supplementary material3 the ultimate test of theresolving power of the new ion optics assembly should involvephotodissociation studies of fully quantum state selectedparent molecules such as those reported by the Janssen groupfor methyl iodide46

The present imaging study reveals a marked change inphotofragmentation behaviour upon tuning to shorter excita-tion wavelengths particularly in the case of 2-bromothiopheneLong wavelength excitation results in prompt CndashX bondfission the resulting XXlowast products are formed withkinetic energies close to the maximum allowed by energyconservation and with near limiting parallel recoil anisotropyCompanion ab initio calculations show that these productsarise following excitation to one or more (nπ)σlowast states andserve to highlight the photophysical similarities with otheraryl iodides (and bromides) when excited at longer UVwavelengths4ndash8

At shorter wavelengths the anisotropic fast ring asso-ciated with these products fades in relative intensity and isprogressively replaced by a smaller isotropic feature thatsignifies the formation of much slower XXlowast atom productsThe appearance of this new channel roughly coincides withthe onset of more intense parent absorption to the 21Aprime excitedstate Ab initio theory suggests that this ππlowast excited statecan predissociate by interaction with neighbouring (nπ)σlowast

states the potentials for which decline in energy along boththe RCndashX and RCndashS stretch coordinates The 2-bromothiophenedata can be understood if the latter offers the lower energydecay route from the optically bright 21Aprime state to a CIwith the ground state PES at extended CndashS bond lengthsThe present DFTB3LYP6-311G(dp) calculations suggestthat the resulting biradicals have more than sufficient energyto sample the configuration space associated with severaldifferent isomeric forms of the starting molecule and to formboth ring closed and ring opened C4H3S radical productsfollowing subsequent CndashBr bond fission on the S0 PES

The data for 2-iodothiophene are less clear cut butagain the relative yield of slow I atoms increases markedlyupon tuning to shorter wavelengthsmdashconsistent with initialexcitation to the corresponding 21Aprime state and radiationlesstransfer to (and unimolecular decay from) highly vibrationallyexcited S0 molecules formed by coupling through the predictedCI at extended RCndashS The ldquofastrdquo IIlowast feature also persists atshorter wavelengths however This could be explained byone or other or both of two mechanisms The first recognisesthe (relatively) greater partial absorption cross sections ofthe σCndashI

lowast larr nπ excitations in 2-iodothiophene while thesecond assumes that following excitation to the 21Aprime state thethresholds for predissociation enabled by CndashI and CndashS bondextension are closer in energy than in 2-bromothiophene

It is interesting to compare the present findings withthose from recent condensed phase studies of thiophenonein acetonitrile solution wherein the photoexcited parentmolecules were similarly deduced to convert to highlyvibrationally excited levels of the ground (S0) state bycoupling via a CI at extended CndashS bond lengths19 Ringopening was confirmed in that case by the observation ofketene products but the time evolution of the parent bleachsignals indicated that the majority (sim60) of these highlyvibrationally excited S0 molecules relax and re-thermalise asthe ring closed thiophenone Such vibrational relaxation is notavailable in the present isolated molecule gas phase studies soeventual dissociation is assured on energetic grounds but therelative probabilities for forming ring opened or ring closedfragmentation products remains an open question

ACKNOWLEDGMENTS

The authors are grateful to EPSRC (Programme GrantNos EPG00224X and EPL005913) to the TSB (KnowledgeTransfer Partnership KTP008481 that stimulated the Bristol-Photek collaboration) to the Marie Curie Initial TrainingNetwork ICONIC (Contract Agreement No 238671) andto Rebecca Ingle Keith Rosser Dr Stuart Greaves DrJames Smith Dr Andreas Wenge Dr Eckart Wrede and DrDimitrios Zaouris for their many and varied contributions tothis work Data accessibility The underlying data for thispaper has been placed in the University of Bristolrsquos researchdata repository and can be accessed using the following DOI105523brisqwlb0o0e9tbd1bqojd8gxrnjj

1A L Sobolewski W Domcke C Dedonder-Lardeux and C Jouvet PhysChem Chem Phys 4 1093 (2002)

2M N R Ashfold G A King D Murdock M G D Nix T A A Oliverand A G Sage Phys Chem Chem Phys 12 1218 (2010)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203

224303-11 Marchetti et al J Chem Phys 142 224303 (2015)

3See supplementary material at httpdxdoiorg10106314921315 foradditional details regarding the design and implementation of the new ionoptics assembly the 6ndash311G(dp) basis set used for the 2-iodothiophenecalculations adiabatic SO resolved potentials along RCndashX for 2-bromo-and 2-iodothiophene and calculated TDMs and vibrational wavenumbers forthe ground states of 2-bromo- and 2-iodothiophene and the thienyl radical

4K L Han and G Z He J Photochem Photobiol C 8 55 (2007)5X-P Zhang Z-R Wei Y Tang T-J Chao B Zhang and K-C LinChemPhysChem 9 1130 (2008)

6A G Sage T A A Oliver D Murdock M D Crow G A D Ritchie J NHarvey and M N R Ashfold Phys Chem Chem Phys 13 8075 (2011)

7D Murdock M B Crow G A D Ritchie and M N R Ashfold J ChemPhys 136 124313 (2012)

8A G Sage T A A Oliver G A King D Murdock J N Harvey and MN R Ashfold J Chem Phys 138 164318 (2013)

9F Zhang Z Cao X Qin Y Liu Y Wang and B Zhang Acta Phys-ChimSin 24 1335 (2008)

10H-L Zhu J Liu X M Zheng and D L Phillips J Chem Phys 125054510 (2006)

11X-F Wu X M Zheng H-G Wang Y Y Zhao X G Guan D L PhillipsX Chen and W H Fang J Chem Phys 133 134507 (2010)

12H A Wiebe and J Heicklen Can J Chem 47 2965 (1969)13C-W Hsu C-L Liao Z-X Ma and C Y Ng J Phys Chem 99 1760

(1995)14F Qi O Sorkhabi A H Rizvi and A G Suits J Phys Chem A 103 8351

(1999)15S Salzmann M Kleinschmidt J Tatchen R Weinkauf and C M Marian

Phys Chem Chem Phys 10 380 (2008)16N Gavrilov S Salzmann and C M Marian Chem Phys 349 269 (2008)17G Cui and W Fang J Phys Chem A 115 11544 (2011)18M Stenrup Chem Phys 397 18 (2012)19D Murdock S J Harris J Luke M P Grubb A J Orr-Ewing and M N

R Ashfold Phys Chem Chem Phys 16 21271 (2014)20M Stenrup and Aring Larson Chem Phys 379 6 (2011)21E V Gromov C Leacuteveque F Gatti I Burghardt and H Koumlppel J Chem

Phys 135 164305 (2011)22C M Krauter J Moumlhring T Buckup M Pernpointner and M Motzkus

Phys Chem Chem Phys 15 17846 (2013)23S Prager I Burghardt and A Dreuw J Phys Chem A 118 1339 (2014)

and references therein24D Tuna A L Sobolewski and W Domcke Phys Chem Chem Phys 16

38 (2014)

25E Wrede S Laubach S Schulenburg A Brown E R Wouters A J Orr-Ewing and M N R Ashfold J Chem Phys 114 2629 (2001)

26S Arepalli N Presser D Robie and R J Gordon Chem Phys Lett 11764 (1985)

27A Gedanken M B Robin and Y Yafet J Chem Phys 76 4798 (1982)28A M Wenge T N V Karsili J R Diaz M I Cotterell B Marchetti R N

Dixon and M N R Ashfold ldquoTuning photochemistry Substituent effectson πσ state mediated bond fission in thioanisolesrdquo Phys Chem ChemPhys (to be published)

29See httpsimioncom for calculating electric fields and the trajectories ofcharged particles in those fields

30T J Obernhuber U Kensy and B Dick Phys Chem Chem Phys 5 2799(2003)

31V Dribinski A B Potter I Federov and H Reisler Chem Phys Lett 385233 (2004)

32T Nishide and T Suzuki J Phys Chem A 108 7863 (2004)33J G Izquierdo G A Amaral F Ausfelder F J Aoiz and L Banares

ChemPhysChem 7 1682 (2006)34I Wilkinson and B J Whitaker J Chem Phys 129 154312 (2008)35W S Hopkins S M Hamilton P D McNaughten and S R Mackenzie

Chem Phys Lett 483 10 (2009)36R A Livingstone J O F Thompson M Illina R J Donaldson B J

Sussman M J Paterson and D Townsend J Chem Phys 137 184304(2012)

37A M Wenge A Schmaunz U Kensy and B Dick Phys Chem ChemPhys 14 7076 (2012)

38H-J Werner P J Knowles G Knizia F R Manby M Schuumltz et alversion 20101 a package of ab initio programs University of CardiffCardiff UK 2010 see httpwwwmolpronet

39T H Dunning Jr J Chem Phys 90 1007 (1989)40K A Peterson B C Shepler D Figgen and H Stoll J Phys Chem A 110

13877 (2006)41M J Frisch G W Trucks H B Schlegel et al 09 Revision D01

Gaussian Inc Wallingford CT 201342H Zhang R-S Zhang G-J Wang K-L Han G-Z He and N-Q Lou J

Chem Phys 110 2922 (1999)43C Paacuterkaacutenyi Pure Appl Chem 55 331 (1983)44L M Culberson and A Sanov J Chem Phys 134 204306 (2011)45A B Alekseyev H-P Liebermann and R J Buenker J Chem Phys 126

234102 (2007)46L Lipciuc J B Buijs and M H M Janssen Phys Chem Chem Phys 8

219 (2006)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to IP

1372224331 On Tue 09 Jun 2015 131203