Jet Spectroscopy and Molecular Dynamics

Edîted by
University of Reading
First edition 1995
© 1995 Springer Science+Business Media New York Originally published by B1ackie Academic & Professional in 1995 Softcover reprint of the hardcover Ist edition 1995
ISBN 978-94-010-4573-5 ISBN 978-94-011-1314-4 (eBook) DOI 10.1007/978-94-011-1314-4
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Preface
The conditions which obtain in a supersonic jet have been referred to as those of a fourth state of matter. This may be something of an exaggeration but it does go some way towards conveying the reason for the excitement generated among those working in many branches of spectroscopy and dynamics who saw it as a means of obtaining information which could previously only have been dreamt of.
In an effusive atomic or molecular beam, the species concerned could be investigated under conditions which removed pressure broadening and much of Doppler broadening from the resulting spectra. The effusive beam was in many ways the precursor of the supersonic jet but suffered by comparison in that particles in the beam have a Maxwellian velocity distribution which is the same as that of those in the reservoir of gas forming the beam. Such an effusive beam can be produced by pumping atoms or molecules through a narrow (c. 20 11m) slit or pinhole with a pressure of only a few torr on the high pressure side of the aperture.
In the early 1950s it was found that, if the gas being pumped through the small aperture is atomic, typically helium or argon, and the pressure is greatly increased to a few atmospheres, the many collisions occurring in, and just downstream of, the pinhole or slit result in an extremely low translational temperature of the gas, of the order of I K, and a so-called supersonic jet results. If particularly uniform temperature and velocity are required, the outer regions of the conical jet may be removed with a skimmer to form a supersonic beam.
When molecules are injected into the carrier gas, they attain a very similar translational temperature. However, because rotational energy levels are more widely spaced than translational levels, the rotational temperature is somewhat higher, typically of the order of 10K. This can be reduced further by increasing the pressure of the carrier gas. Vibrational energy levels are still more widely spaced and typical vibrational temperatures are of the order of 100 K but may vary among the vibrational modes of a polyatomic molecule.
This 'fourth state of matter' consists, therefore, of molecules which are generally extremely cold, colder than could previously have been contemplated, and which have different translational, rotational and vibrational temperatures.
In studies of molecular spectroscopy and dynamics these conditions have resulted in several major advantages. One of these is that very weakly bound species, such as van der Waals and hydrogen-bonded complexes and clusters, are held together at the typically low vibrational temperatures which obtain.
VI PREFACE
This allows the investigation of their spectroscopy and dynamics to a level of precision which was never previously approached. Another advantage is that spectra of very much larger molecules can be rotationally resolved. At room temperature the spectra of large molecules tend to be overcrowded, even to the extent of creating a pseudocontinuum, due to very closely-spaced rotational energy levels and an abundance of low-lying vibrational levels all of which are appreciably populated. In a skimmed supersonic beam, for example, individual rotational transitions in the electronic spectrum of a molecule as large as naphthalene or carbazole can be observed. This allows a detailed investigation of the molecular structure, from a rotational analysis, and of the vibrational and rotational dependence of the dynamics of far larger molecules than was previously possible.
Lasers, which were developed from the 1960s onwards, have proved to be an extremely important tool in investigations of the spectroscopy and dynamics of molecules in supersonic jets or beams. Of particular importance are the dye lasers, for the visible and ultraviolet regions, and the diode lasers, for the near infrared. For the study of microwave spectra of jet-cooled molecules, Fourier transform techniques have proved essential.
In the late 1970s the pulsed supersonic jet or beam was developed whereas earlier ones were continuous. Originally the pulsed jet was used in conjunction with a pulsed laser to conserve material and to give greater cooling of the molecules. The increased cooling was possible because of the less stringent pumping requirements allowing higher pressures to be used before the pinhole or slit. However, it was soon realised that the shortness of the laser pulses, firstly a few nanoseconds in length and then picoseconds and femtoseconds, allowed studies of the molecular dynamics to be made on extremely short timescales and at vibrational or even rotational resolution.
The contributors to this volume are all international authorities on their subjects and we are extremely grateful to them for devoting a considerable amount of time in employing their expertise to make it a success. The spectroscopy of molecules, free radicals and clusters in supersonic jets and beams is covered from the microwave region, through the near infrared to the visible and ultraviolet regions. Aspects of molecular dynamics include rotational coherence phenomena, intramolecular vibrational relaxation, relaxation processes in van der Waals clusters, internal relaxation dynamics and the effects of optically dark states. The study of spectroscopy and dynamics of molecules in supersonic jets continues to develop rapidly and we hope that the present volume serves to give a general picture of the present state ofthe art and to convey much of the excitement which has been generated.
J.M.H. D.P.
Chemistry Department, Texas A&M University, College Station, Texas 77843-3255, USA
Laboratorium fur Physikalische Chemie, Eidgenossische Technische Hochschule, CH-8092 Zurich, Switzerland
Laboratorium fur Physikalische Chemie, Eidgenossische Technische Hochschule, CH-8092 Zurich, Switzerland
R.P.A. Bettens Laboratorium fur Physikalische Chemie, Eidgenossische Technische Hochschule, CH-8092 Zurich, Switzerland
J.W. Bevan
T. Biirgi
P.M. Felker
J.M. Hollas
s. Leutwyler
W.L. Meerts
T.A. Miller
".J. Neusser
D. Phillips
Chemistry Department, Texas A&M University, College Station, Texas 77843-3255, USA
Institut fur Anorganische, Analytische und Physikalische Chemie, Freiestra13e 3, CH-3000 Bern 9, Switzerland
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024-1569, USA
Chemistry Department, University of Reading, Whiteknights, Reading, RG6 2AD, UK
Institut fUr Anorganische, Analytische und Physikalische Chemie, Freiestra13e 3, CH-3000 Bern 9, Switzerland
Department of Molecular and Laser Physics, University of Nijmegen, Toemooiveld, 6525 ED Nijmegen, The Netherlands
Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA
Institut fur Physikalische und Theoretische Chemie, Technische UniversiUit Miinchen, LichtenbergstraBe 4,85748 Garching, Germany
Chemistry Department, Imperial College, University of London, London SW7 2AY, UK
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
VIII CONTRIBUTORS
L.H. Spangler Department of Chemistry, Montana State University, Bozeman, Montana 59717, USA
R. Sussmann Institut fUr Physikalische und Theoretische Chemie, Technische Universitat Miinchen, Lichtenbergstraf3e 4,85748 Garching, Germany
A.G. Taylor Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, UK
M.R. Topp Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA
T.G. Wright Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA
Xue Qing Tan Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA
A.H. Zewail Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA
Contents
1 Rotational spectroscopy of weakly bound complexes F.L. BETTENS, R.P.A. BETTENS and A. BAUDER
I
1.1 Introduction I 1.2 Experimental techniques 2
1.2.1 Molecular beam electric resonance 2 1.2.2 Pulsed nozzle Fourier transform microwave spectroscopy 2 1.2.3 Electric-resonance optothermal spectroscopy 5
1.3 Van der Waals complexes 5 1.3.1 Aromatic molecule...(rare gas)•• n = I. 2, complexes 5 1.3.2 Force field and derived properties of aromatic molecule.. ·rare gas
complexes 9 1.3.3 Aromatic molecule diatomic molecule complexes II 1.3.4 Aromatic molecule triatomic molecule complexes 13 1.3.5 Larger complexes containing at least one aromatic molecule 15
1.4 Hydrogen bonded complexes 16 1.4.1 Complexes involving water 17 1.4.2 Complexes not involving water 19
1.5 Conclusion and outlook 22 References 24
2 Infrared spectroscopy in supersonic free jets and molecular beams 29 J. ARNO and lW. BEVAN
2.1 Introduction 29 2.2 Supersonic free jets and molecular beams 30
2.2.1 Structure and properties of continuous supersonic free jets 31 2.2.2 Cluster formation 33 2.2.3 Pulsed nozzle supersonic jets 35 2.2.4 Slit supersonic jets 36 2.2.5 Supersonic molecular beams 37
2.3 Instrumentation and techniques for infrared spectroscopy in supersonic jets and molecular beams 37 2.3.1 Fourier transform spectroscopy 38 2.3.2 Laser-based spectroscopy 42
2.4 Applications of FTIR supersonic jet spectroscopy 47 2.5 Applications of infrared laser spectrometers in supersonic jets and molecular
beams 52 2.5.1 Fixed frequency lasers 52 2.5.2 Laser sideband spectrometers 52 2.5.3 Semiconductor diode lasers 56 2.5.4 Tunable lasers based on non-linear mixing techniques 60 2.5.5 F center lasers 63 2.5.6 Other laser spectrometers 65
Acknowledgements 66 ~~= ~
x CONTENTS
3 Electronic spectroscopy of free radicals in supersonic jets 74 XUE QING TAN, T.G. WRIGHT and T.A. MILLER
3.1 Introduction 3.2 Experimental approaches
3.2.1 Apparatus overview 3.2.2 Radical production methods 3.2.3 LIF experiments 3.2.4 REMPI and ZEKE experiments
3.3 Radicals studied 3.3.1 LIF of di- and triatomic radicals 3.3.2 REMPI of di- and triatomic radicals 3.3.3 Small hydrocarbon radicals 3.3.4 Alkoxy radicals and their derivatives 3.3.5 Aromatic radicals 3.3.6 Organometallic radicals
3.4 Conclusion Acknowledgement References
74 75 75 77 81 81 82 82 89 94 96
100 107 112 112 113
4 Structure of weakly bound complexes from electronic spectra 118 H.J. NEUSSER and R. SUSSMANN
4.1 Introduction 4.2 Experimental
4.2.1 General remarks 4.2.2 Mass-selective detection 4.2.3 Experimental set-up
4.3 Spectroscopy of dimers 4.3.1 Benzene-noble gas dimers 4.3.2 Dimers of fluorene and noble gas atoms
4.4 Spectroscopy and structure of trimers 4.4.1 Benzene-noble gas trimers 4.4.2 Carbazole-noble gas trimers
4.5 Benzene-molecule dimers 4.6 Concluding remarks Acknowledgements References
118 120 120 121 122 124 124 134 137 137 139 145 148 148 149
5 Jet spectra of aromatic molecules in hydrogen bonded microsolvant clusters 151 A.G. TAYLOR, T. BORGI and S. LEUTWYLER
5.1 Introduction 151 5.2 Aromatic molecule/H 20 complexes 152
5.2.1 Hydroxyaromatics 152 5.2.2 N-aromatic molecules 159 5.2.3 Benzene and toluene 161 5.2.4 Cyanobenzenes 161 5.2.5 Tautomerising molecules 163
5.3 Aromatic molecule/NH 3 complexes 164 5.3.1 Hydroxyaromatics 164 5.3.2 N-aromatic molecules 167 5.3.3 Aromatic molecules/NH 3 168 5.3.4 Molecules which undergo tautomerism 169
5.4 Comparison of experimental data and results of ab initio calculations 169
CONTENTS XI
5.4.1 Hydrogen bond energies, geometric parameters and atom charges 170 5.4.2 Vibrational frequencies 176
References 179
181
6.1 Introduction 181 6.2 Alignment recurrences: the free rotational dynamics of dipole-excited species 182
6.2.1 Definitions and nomenclature 182 6.2.2 The effect of resonant, short-pulse excitation 185 6.2.3 The orientational probability density 188 6.2.4 Alignment recurrences 189
6.3 Rotational coherence phenomena: observable manifestations of free rotational dynamics 193 6.3.1 Probing of transient alignment 193 6.3.2 Rotational coherence effects in symmetric tops 198 6.3.3 Asymmetric tops 204
6.4 Rotational coherence spectroscopy 210 6.5 Results from experiment 213 6.6 Summary and conclusion 217 Acknowledgements 219 References 219
7 Ultrafast dynamics of IVR in molecules and reactions P.M. FELKER and A.H. ZEWAIL
7.1 Introduction 7.2 Theoretical description of vibrational coherence and IVR
7.2.1 Two-level IVR 7.2.2 IVR between N levels 7.2.3 Types and regions of IVR
7.3 Applications to molecular systems: non-reactive 7.3.1 Anthracene 7.3.2 9-d t -Anthracene and dID-anthracene 7.3.3 trans-Stilbene 7.3.4 Alkylanilines: 'ring and tail' systems 7.3.5 p-Difluorobenzene 7.3.6 Techniques and other molecules
7.4 Effects of rotations on IVR: mismatches of rotational constants 7.5 IVR in reactions
7.5.1 Vibrational predissociation in I2-X complexes 7.5.2 t-Stilbene van der Waals complexes 7.5.3 Hydrogen-bonded systems 7.5.4 Electron transfer reactions 7.5.5 IVR in consecutive reactions 7.5.6 Ground-state reactions 7.5.7 Isomerization reactions
7.6 Rotational coherence dynamics and IVR 7.6.1 Discussion of the phenomenon 7.6.2 Time-resolved fluorescence 7.6.3 Pump-probe fluorescence gain (PPFG) 7.6.4 Pump-probe fluorescence depletion (PPFD) 7.6.5 Pump-probe ionization gain (pPIG) 7.6.6 Saturation effects 7.6.7 Rotational coherence in reactions
Acknowledgements References
222
222 224 225 228 230 232 232 242 244 248 255 255 256 261 264 267 270 272 276 278 278 279 279 281 287 294 296 301 302 306 306
XII CONTENTS
8 Fast relaxation processes in jet-cooled van der Waals clusters involving large aromatic molecules 309 M.R. TOPP
8.1 Introduction 309 8.2 Experimental procedures 310
8.2.1 Fluorescence excitation and dispersed emission spectroscopy 310 8.2.2 Picosecond time-resolved fluorescence spectroscopy 311 8.2.3 Hole-burning spectroscopy 313 8.2.4 Hot-band spectroscopy 314
8.3 Excited-state dynamics of jet-cooled aromatic molecules 320 8.3.1 Perylene 321 8.3.2 2,5-Diphenylfuran and 2,5-diphenyloxazole (PPO) 324 8.3.3 9,9'-Bifluorenyl 327
8.4 The effect of cluster formation on fluorescence lifetimes 330 8.4.1 Perylene complexes 330 8.4.2 Hydrogen-bonded interactions 332 8.4.3 Xanthione 334
8.5 Vibrational relaxation 337 8.5.1 Vibronic excitation of perylene aggregates 337 8.5.2 Predissociation of argon and methane complexes of perylene 344 8.5.3 Flexible molecules 351
8.6 Vibrationally-induced conformational relaxation: perylene complexes with alklyl halides 355 8.6.1 Time-resolved fluorescence 355 8.6.2 Rotational coherence spectroscopy 358
Acknowledgements 362 References 362
9 Internal rotation dynamics from electronic spectroscopy in supersonic jets and beams L.H. SPANGLER and D.W. PRATT
9.1 Introduction 9.2 Terms in the Hamiltonian and their spectral consequences
9.2.1 Vibrationally resolved experiments 9.2.2 Symmetry considerations 9.2.3 Rotationally resolved experiments
9.3 Methyl rotor barriers: where do they come from? 9.3.1 Substituent effects 9.3.2 Effects of electronic excitation 9.3.3 Long-range interactions
9.4 Summary Acknowledgements References
10.1 Introduction 10.2 Experiment
10.2.1 High-resolution cw laser set-up 10.2.2 Fourier transform limited pulsed laser experiments
10.3 Pyrazine 10.3.1 Excitation spectra 10.3.2 Absorption spectra 10.3.3 Fourier transform limited pulsed laser spectra
366
366 367 368 374 378 387 390 393 394 395 396 397
399
CONTENTS
10.4 The phosphorescence spectrum of naphthalene 10.5 The singlet-triplet perturbation in the Al Au state of acetylene 10.6 Conclusion Acknowledgements References
Index
X111
434
1 Rotational spectroscopy of weakly bound complexes F.L. BETTENS, R.P.A. BETTENS and A. BAUDER
1.1 Introduction
The first indication of the formation of complexes between molecules came from investigations of the equation of state of real gases through the work of van der Waals. During collisions, molecules or rare gas atoms may form short-lived complexes which are likely to be dissociated by the next collision. The binding energy of such complexes between stable closed shell molecules or rare gas atoms is small, much smaller than that for a typical chemical bond. Weakly bound molecular complexes can now be investigated with different spectroscopic techniques. They are produced most efficiently in supersonic expansions where the strong adiabatic cooling favours their formation. The spectroscopic characterization is then performed in the collision-free region of the emerging jet or molecular beam.
The binding energies of molecular complexes cover a substantial range. At the low end there are the van der Waals complexes, whereas the hydrogen bonded complexes show higher values. The geometric structure of a complex is of primary importance besides the binding energy. Rotational spectroscopy in the microwave and radio frequency range proved to be an excellent method, not only for the determination of the structure, but also for the study of large amplitude intermolecular vibrations. Motions of the subunits of a complex between equivalent minima on the potential energy surface give rise to splittings of the rotational transitions. The binding in van der Waals complexes follows different rules from those established for chemical bonds. Although a number of characteristic binding patterns emerged from the studies of complexes, unexpected structures are still uncovered and new patterns are yet to be determined.
Probably the earliest measurement of the rotational spectrum ofa complex, the hydrogen bonded complex between two different carboxylic acids, was made in the static gas by Costain and Srivastava [1, 2]. Only a low resolution spectrum was observed which confirmed the qualitative structure of the com plex. This pioneering study was later extended to additional carboxylic acid complexes by Bellot and Wilson [3]. Strongly hydrogen bonded complexes involving HF and HCN were also characterized from their rotational spectra in the static gas [4-6]. The observation of the hydrogen fluoride dimer in a molecular beam, by Dyke et ai. [7], marked the beginning of the use of the molecular beam electric resonance method for the study of complexes.
2 JET SPECTROSCOPY AND MOLECULAR DYNAMICS
The earlier work in this field was covered in the reviews edited by Weber [8] and by Halberstadt and Janda [9]. Although most complexes studied so far were formed from two subunits only, rotational spectroscopy is not restricted to this class. More recently, a small number of studies were reported which dealt successfully with complexes of up to five subunits [10-21].
The experimental techniques which are available for the investigation of rotational spectra of complexes are explained in section 1.2. It is necessary to concentrate the following survey of complexes on those areas where important developments took place. We discuss, in section 1.3, complexes which involve an aromatic molecule as the primary subunit. Section 1.4 is devoted to recent results coming from the study of hydrogen bonded species.
1.2 Experimental techniques
1.2.1 Molecular beam electric resonance
Dyke et al. [7] made the first successful observation of completely resolved rotational transitions of a complex, the dimer of hydrogen fluoride; for these measurements the molecular beam electric resonance (MBER) method was employed. This method was originally developed to measure the spectra of atoms and molecules in the radio frequency and microwave range [22]. In the instrument, the molecular beam passes three field regions. The molecules, in appropriate states, are focused or defocused in quadrupolar or hexapolar fields in the first and third region. Radio frequency or microwave fields in the second region change the state of the focused molecules. A mass spectrometer acts as a selective particle detector for the molecules passing the three field regions and reaching the detector.
The use of molecular beams produced in a supersonic expansion [23 - 28] greatly improved the sensitivity for studies of complexes. The strong cooling in supersonic expansions of translational and rotational degrees of freedom favoured the detection oflow J transitions. At the same time, the concentration of weakly bound complexes was greatly enhanced. The initial examples of rotational spectra of complexes were all based on the MBER method in the pioneering investigations by Klemperer and his co-workers.
1.2.2 Pulsed nozzle Fourier transform microwave spectroscopy
Later, Flygare [29] adapted pulsed Fourier transform microwave (FTMW) spectroscopy to the study of molecular complexes in a pulsed supersonic expansion. Pulsed FTMW spectroscopy, developed by Ekkers and Flygare [30], offered the same improvement in sensitivity that revolutionized nuclear magnetic resonance spectroscopy [31]. Combined with a Fabry-Perot cavity, pulsed FTMW spectroscopy lent itself ideally to measurements of rotational
ROTATIONAL SPECTROSCOPY OF WEAKLY BOUND COMPLEXES 3
transitions in a pulsed beam, as demonstrated by Balle et al. [32]. A crucial advantage was the increased number of molecules, by up to three orders of magnitude, compared with a continuous beam for the same pumping speed. The molecules travelled essentially without collisions through the cavity. The operation of this new spectrometer was fully described by Campbell et al. [33] who discussed properties of the supersonic expansion in the cavity [34]. In a subsequent paper, technical details of the instrument were reported by Balle and Flygare [35]. Pulsed nozzle FTMW spectroscopy was very successful in observing molecular complexes. It had some advantages over the MBER method with respect to ease of operation.
Permanent electric dipole moments are important properties of complexes for investigating intermolecular interactions. Shortly after the first FTMW spectrometer was built, its design was modified and two plates parallel to both the beam and the cavity axis were inserted for the generation of a static Stark field [36]. This enabled dipole moments to be determined from the splittings of rotational transitions. A modified version for generating the Stark field was presented later as a cage-like structure with a number of parallel wires instead of the plates [37]. By feeding the wires with appropriately selected voltages, the field direction was able to be rotated with respect to the fixed polarization of the microwave radiation. A special cavity was also built which fitted in a wide bore superconducting magnet for the measurements of Zeeman splittings of complexes [38, 39]. Further extensions were made to the original design which were aimed at double resonance experiments. These experiments proved useful for confirming assignments in complicated microwave spectra. Asecond Fabry- Perot cavity was arranged perpendicularly to the main cavity for the application of the pump power [40]. Introducing the molecular beam through one of the mirrors of the Fabry-Perot cavity increased the resolution compared with the original perpendicular injection of the beam [41,42].
Heated nozzles were constructed for increasing the vapour pressure of a substance which is seeded directly into the rare gas in the nozzle [43, 44]. Methods developed earlier for the vaporization of solids for ultraviolet spectroscopy were adapted for FTMW spectroscopy. This enabled pure rotational spectra of refractory oxides to be observed by pulsed laser ablation [45-47]. Furthermore, special nozzles were built to support an electric discharge of the seeded rare gas [48-53]. Unstable species or molecules in excited vibrational states were formed in the discharge, including complexes between radicals and rare gas atoms. Molecular complexes as intermediates of highly reactive reagents were studied by mixing the two seeded reagents at the outlet of the pulsed nozzle [54, 55].
A brief description of a state-of-the-art FTMW spectrometer [56-59], represented by the block diagram in Figure 1.1, and its operation is given below. The output power of a microwave synthesizer is split into two parts. The frequency of the first part is shifted by 30 MHz in a single-sideband
Figure 1.1 Block diagram of a pulsed nozzle FTMW spectrometer. I, Microwave synthesizer; 2, power splitter; 3, single-sideband modulator; 4, coaxial isolator; 5, PIN diode switch; 6, medium power microwave amplifier; 7, adjustable attenuator; 8, coaxial directional coupler; 9, power monitor; 10, waveguide circulator; 11, Fabry-Perot resonator cavity; 12, cavity tuning monitor; 13, low-noise microwave amplifier; 14, microwave bandpass filter; 15, microwave mixer; 16, intermediate frequency amplifier; 17, radio frequency bandpass filter; 18, radio frequency mixer; 19, broadband amplifier; 20, lowpass filter; 21, 12-bit analog-to-digital converter; 22, personal computer; 23, stepping motor drive; 24, electromechanical valve; 25, valve driver; 26, programmable pulse generator; 27, radio frequency synthesizer; 28, frequency tripler; 29, 10 MHz
frequency standard.
modulator. A microwave pulse of IllS duration is formed from this signal with a pair of PIN diode switches. This signal is amplified to the necessary power level. The pulse is applied to the Fabry-Perot cavity via a circulator. Coupling is accomplished either through a circular iris from a waveguide connection with a tuner, for critical coupling, or through a coaxial antenna. The microwave pulse polarizes the molecules in the beam. When the stored microwave energy in the cavity is decayed sufficiently, after the end of the applied pulse, the radiation of the molecules is extracted from the cavity and directed via the circulator to the detection system. A third PIN switch, which is closed during the application of the microwave pulse, protects the sensitive detection system. The signals from the molecules are first amplified in a low-noise microwave amplifier. The amplifier signal is then mixed with the second part of the power from the synthesizer down to the frequency range of 27.5-32.5 MHz. After further amplification, the signals are mixed a second time with a frequency of 27.5 MHz which is coherently derived from the 30 MHz single-sideband modulation frequency. The signals in the 0-5 MHz range are digitized with a 12-bit analog-to-digital converter at a rate of 10 MHz for 128-4096 channels, depending on the desired resolution. The
digitized signals from a number of microwave pulses are added in the memory of a fast signal processor which is part of a personal computer. The power or amplitude spectrum in the frequency domain is calculated with the fast Fourier transform from the accumulated signals in the time domain.
The useful spectral range for a given cavity setting is only 0.3-1 MHz, depending on the cavity quality factor. For a broader frequency range, the cavity is tuned automatically by a stepping motor such that the spectra ranges are combined to form a spectrum covering several hundred MHz. Simultaneously during such sweeps, the exciting microwave frequency is always set to the centre of the cavity response.
The pulsed molecular beam is generated with the help ofan electromechanical valve. Pulse durations range between 0.2 and 2 ms. Stagnation pressures of 0.5-5 bar, in the gas reservoir before the valve, induce the supersonic expansion through a nozzle into the evacuated Fabry- Perot cavity. Nozzles are either thin circular openings, with a diameter ofabout 0.5 mm, or conically shaped ducts with an opening angle of about 20· [14]. The full expansion from the nozzle enters the cavity usually without a skimmer. Monomers for the formation of complexes are seeded to rare gases at ratios of around 1% or less. Rare gases such as argon, neon or helium produce the strongest cooling (usually < 10 K) of the translational and rotational degrees of freedom with argon being the most efficient. Gas pulses are repeated at a rate of 1-10 Hz depending on the pumping speed of the diffusion pump-mechanical pump combination. Up to 16 microwave pulses may be applied to a single gas pulse in order to improve the signal-to-noise ratio [60]; however, this occurs with some loss of resolution.
1.2.3 Electric-resonance optothermal spectroscopy
At millimeter-wave frequencies, the limited availability of components and their high costs make the original design of the pulsed nozzle FTMW spectrometer less attractive. Electric-resonance optothermal spectroscopy (EROS) was tested successfully as an alternative method for observing rotational spectra of complexes at higher frequencies [61]. The method was initially developed mainly for observing high resolution infrared spectra [62]. It depends on the detection of the additional energy transported by the molecules in a collimated beam irradiated by monochromatic radiation, e.g. from a laser. A low temperature bolometer serves as the sensitive detector.
1.3 Van der Waals complexes
1.3.1 Aromatic molecule···(rare gas)n' n = 1,2, complexes
Up to the present time, all the complexes between an aromatic molecule and one or two rare gas atoms that were studied in the microwave range have
been measured using a pulsed nozzle FTMW spectrometer. The aromatic monomers in these complexes were either five or six membered ring systems. The rare gas atoms were always found to attach above the plane of the aromatic molecule or on both sides of the plane in those cases where two rare gas atoms bind to the 1t system. No aromatic molecule···rare gas complex has been observed where a rare gas atom is bound in the ring plane. So far the microwave spectra of the hetero-aromatic molecule···(rare gas)n' n = 1,2, complexes have all been those of centrifugally distorted asymmetric tops. These spectra had no direct evidence of large amplitude motions, i.e. no unexplained splittings or perturbed energy levels. To date, the equilibrium structure of the hetero-aromatic molecules in these complexes have all possessed Czv symmetry; consequently the symmetry of the observed (rare gas)l complexes have been found, or assumed, to be Cs ' Thus, of the three possible structural parameters that would be needed to specify the position of the rare gas atom in these Cs complexes, only two are required. Another consequence of the Cs symmetry is that the observed spectra were found to consist of two dipolar types of transitions which were weak J1.a type and either strong J1.b or strong J1.c type transitions. The structures of the (rare gas)z complexes were very similar to the corresponding (rare gas)l complexes, but in the (rare gas)z cases the complexes possessed C zv symmetry and so have only one dipole transition type present in their spectra. These complexes also require only two structural parameters in order to define them. Planar aromatic monomers with at least a three-fold symmetry axis are special cases. These monomers do not possess a permanent dipole moment. The resulting centrifugally distorted symmetric top spectra of these complexes are entirely due to the induced dipole in the rare gas and monomers. Thus, in these complexes only a distance is required to specify the structure of the complex. Complexes of this (rare gas)z type [63], with a rare gas atom attached above and below the plane along the axis of symmetry, cannot be observed with rotational spectroscopy because such complexes possess no permanent dipole moment.
Before the structures determined for the above complexes are presented, two structural parameters must be defined. Rem is the distance between the centre-of-mass of the aromatic molecule and that of the rare gas atom. () is the angle between the normal to the plane of the aromatic molecule and Rem. () is defined to be positive if directed towards the hetero-atom. The sign of () can only be determined from the spectroscopic data if more than one isotopic species of the complex has been measured.
In 1982, the first aromatic molecule···rare gas complex to be measured in the microwave region was reported by Kukolich and Shea [64]. The complex was the parent species of furan ... Ar. In the following year this brief report was extended by Kukolich [65]. Apart from the parent species, Kukolich measured four transitions of the furan-2-d ..·Ar complex. Using the rotational constants of this complex, Kukolich was able to establish that ewas positive,
i.e. Ar was tilted toward the oxygen of furan. The results found by Kukolich were confirmed by Spycher et al. [66, 67] who measured the complexes furan-3-d···Ar, furan-3,4-d2 ···Ar and furan-d4 ···Ar. In their analysis of the structure, the latter authors employed a method developed by Klots et al. [68] which incorporates the large amplitude bending motions of the rare gas. Accurate dipole moment measurements were also carried out on furan and furan···Ar by Oh et al. [69].
Pyrrole···Ar [70], pyridine···Ar and pyridine···Kr [68] were the next aromatic molecule···rare gas complexes to be measured. As various isotopic species of these complexes were measured, including isotopes of Kr, the sign of () could be established. Accurate dipole moment measurements were also carried out in the pyrrole···Ar work. For a discussion of the differences in the dipole moments between the furan···Ar and pyrrole···Ar complexes and their monomers see the work ofOh et al. [69]. Pyridine-ds·..Ar was measured by Spycher et al. [67, 71] who again employed the method of Klots et al. [68] for their structural analysis. From this analysis the effect of the large amplitude bending motions of Ar was clearly seen.
Klots et al. [68] first considered the effects of the van der Waals bending motions upon the observed moments of inertia in the structural determination of pyridine ..·Ar, pyridine ..·Kr and furan .. ·Ar. In all the Cs aromatic molecule .. ·rare gas complexes (as well as the C2v (rare gash complexes) the van der Waals vibrational effects are clearly evident when the difference between the planar moment of inertia of the complex about the axis perpendicular to the mirror plane and this same axis in the aromatic monomer is taken (this axis is denoted here as the x-axis). The planar moment of inertia about axis aisdefinedas p. = t(-I. + I p + I y)where a, {3,Y are the principal inertial axes x, y, z or a cyclic permutation. The above difference, denoted here as !:lP~, would be zero if there were no van der Waals vibrational effects incorporated within the moments of inertia of the complex. This is because Px does not depend upon atoms which lie in the yz inertial plane, i.e. the mirror plane, and so the rare gas, which lies in this plane, should not contribute to Px' The value of !:lP~ was found to be non-zero, and ranged from - 0.5 to - 0.7 u A2 in the above complexes. Thus, while there was no direct evidence in the spectra of these complexes for the van der Waals vibrations being of large amplitude these 'modes' are large enough in amplitude to significantly affect the moments of inertia expected for infinitesimal amplitudes of vibration. The above authors found that the average angular displacement of Ar and Kr in pyridine along the x-axis was approximately Y.
In 1990, the pure rotational spectrum of benzene-h6 •..Ar and benzene d6 .. • Ar was measured by Brupbacher and Bauder [72]. The dipole moment was determined to be 0.12(4) D which, to first order, depends only on the polarizability of Ar and on the electric field associated with the benzene charge distribution. In 1992, the spectra of benzene'" Kr with various isotopes of Kr were measured by Klots et al. [73]. These workers also determined
the dipole moment of the benzene··· 84Kr complex, 0.136(2) D, and the 83Kr quadrupople coupling constants in the benzene'"83Kr complex. These results yielded the electric field gradient induced at the Kr. Very recently, the spectra of the benzene .. ·Ne [74, 21] and benzene ..·Xe [74] were measured which, apart from He, completed the list of rare gas atoms complexed with benzene. As expected, it was found that Ne was quite weakly bound to benzene giving a rather weak rotational spectrum, and so the complex with He would be expected to show a very weak spectrum indeed. This complex would possess a very small dipole moment and, due to the weak binding, very few complexes would be formed in the supersonic expansion.
Reports of the investigation on the spectra of fluorobenzene .. ·Ar [75] and 1,2-difluorobenzene..·Ar [76] appeared in 1992 and 1993, respectively, with accurate data not only for the rotational constants but also for the centrifugal distortion constants. While these complexes are not hetero-aromatic molecule ..· rare gas complexes, both substituted benzene monomers possessed C2v symmetry and the resulting complexes were of Cs symmetry. No isotopic species of these complexes were measured, and so the sign of the angle () was not determined experimentally.
In 1992 the rotational spectrum of furan'" Ar2 was reported by Spycher et ai. [20]. The rotational constants, from JJ.b type rotational transitions, were consistent with a C2v structure which was very similar to the structure of furan ..·Ar. The sign of the angle () was not determined from the data of a single isotopic species. Later, several isotopic species were measured [66, 67] and the sign and magnitude of () was confirmed to be very similar to furan", Ar. The second example of an aromatic molecule..·(rare gash complex, which was pyridine..·Ar2 , was studied recently with several isotopic species [67, 71]. The observed asymmetric top spectra were consistent with a C 2v structure with the Ar atoms again attached above and below the ring plane of the aromatic molecule. The structural parameters were found to be very similar to the pyridine..·Ar complex.
Finally, the centrifugally distorted symmetric top spectrum of 1,2,3 trioxane..·Ar was measured by Legon and Lister in 1993 [77]. Although the monomer is not aromatic it seems appropriate to include this complex here. 1,2,3-trioxane is a symmetric top; the resulting complex was also found to be a symmetric top. These authors developed a procedure for determining the van der Waals force field for similar symmetric top complexes from the measured quartic centrifugal distortion constants. The high symmetry ensures that only two independent force constants are required in the harmonic approximation, a stretching force constant and a force constant for the doubly degenerate bending vibration of Ar. Legon and Lister determined these force constants and calculated the fundamental frequencies and rms displacements of the two vibrations. Their analysis is accurate within the framework of small amplitude vibrations. The authors also applied their procedure to benzene-h6 • ..Ar and benzene-d6 .. • Ar.
Table 1.1 The'0 and pseudo-,e structures for rare gas complexes'
Complex Ro 80 Req b 8eq
b Ref.c
benzene.··Ne 346 0 0 [74] benzene...Ar 359 0 0 [72] benzene... 84Kr 368 0 0 [73] benzene... 129Xe 383 0 0 [74] furan···Ar 354 11.0 347 10.4 [65] pyridine...Ar 355 5.5 349 4.8 [68] pyrrole ...Ar 356 7.5 349 6.9 [70] fluorobenzene ...Ar 359 ±6.7 352 ±6.2 [75] I,2-difluorobenzene...Ar 359 ±8.9 353 ±8.0 [76] 1,2,3-trioxane...Ar 363 0 0 [77] 1,I-difluoroethylene... Ar 352 ±16.6 345 ±16.1 [171]
• Distances in pm. Angles in degrees. b See text. C References to experimental work, '0 structures of benzene.·· rare gas complexes calculated from equation (2) in [77]. Bo for benzene was taken as 5688.916 MHz [172,173]. For the remaining complexes Ro and 80 were determined from the experimental values of P, and Pz (cf. text) without any account being taken of the van der Waals vibrations. The sign of the angle 8 is ambiguous for those complexes where no isotopic species were measured.
The ro structures for the complexes discussed above can be found in Table 1.1. For the asymmetric top complexes, in order that a consistent set of results is presented which can be compared with the results of the symmetric top complexes, we have determined the parameters ourselves from the experimental results in the literature. The r0 values of Rem and (J were determined from the observed planar moments of inertia about the y and z axes (i.e. Py and Pz; Px contains no direct structural information).
1.3.2 Force field and derived properties of aromatic molecule···rare gas complexes
The quartic centrifugal distortion constants of complexes have proved to be a valuable source of information regarding the harmonic force field [78]. Legon and Lister [77] introduced a procedure for determining the force field of symmetric top monomer···rare gas complexes as discussed above. Bettens et al. [79] have developed a method for the determination of the four independent force constants in complexes with Cs symmetry, as well as (rare gas)2 complexes with C 2v symmetry and, using an assumed anharmonic potential, were able to estimate the equilibrium structure. Both of the above procedures are accurate within the small amplitude motion approximation. We have included in Table 1.1 the estimated re parameters from the work of Bettens et al. [79] for comparison with the ro structures. We have also applied this same analysis to fluorobenzene···Ar [75] and 1,2-difluoroben zene···Ar [76] and present the derived results here. From the force field the fundamental frequencies of the van der Waals modes and the rms displacements
of the rare gas atom in the complex are calculated. The binding energy, e, of the rare gas atom can also be crudely estimated from the stretching force constant and the Rem distance from the pseudodiatomic Lennard-Jones 6-12 potential relationship, kRR = 72e/R;m. Table 1.2 gives the force constants and binding energies for the aromatic molecule·;·rare gas complexes as well as the results of 1,2,3-trioxane···Ar and 1,1-difluoroethylene···Ar for comparison. The results for the symmetric top benzene··· 84Kr were derived here by following the procedure of Legon and Lister. The two fluorinated benzene··· Ar complexes were analysed by us using the procedure of Bettens et aI., and are presented here for the first time.
It can be noticed from this table that the out-of-plane bending force constant, kn , (X is defined in Table 1.2) determined from the inertial analysis of the fluorinated benzenes disagrees significantly with the value obtained from the centrifugal distortion analysis. This difference is difficult to reconcile considering the results of the remaining asymmetric top complexes which
Table 1.2 Force constants and binding energies for rare gas complexes'
Complex kRR k99 [k"y·d. [kxx]inerlia kR9 Noteb
benzene···Ne 0.87 0.133 0.133 0 73 c benzene...Ar 2.78 0.371 0.371 0 251 d benzene... 84Kr 3.49 0.435 0.435 0 330 e benzene... 129Xe 4.20 0.499 0.499 0 431 c furan ..·Ar 3.06 0.265 0.543 0.352 -0.106 258 f pyridine...Ar 3.05 0.389 0.382 0.306 -0.044 259 f pyrrole...Ar 3.27 0.365 0.504 0.322 -0.105 278 f fluorobenzene· ..Ar 2.90 0.336 0.566 0.086 +0.230; 252 g 1,2-difluorobenzene...Ar 3.00 0.321 0.409 0.182 +0.381; 261 h 1,2,3-trioxane...Ar 1.96 0.065 0.065 0 181 d 1,I-difluoroethylene...Ar 2.09 0.119 0.093 0.092 ±0.016; 174 f
'Force constants in N m -I. Force constants involving an angular internal displacement coordinate have been converted to N m -, using the ro (benzene ... rare gas complexes) or r, (remaining complexes) centre-of-mass distances given in Table 1.1. Binding energy, e, in cm- 1
The distances used in the calculation of the binding energy are the same as those used in the force constant conversion. The angle X is analogous to 0, but describes the out-of-mirror-plane displacement of the rare gas atom. X = 0' at equilibrium. kRR , k99 and [k,Jc.d. have been determined from a centrifugal distortion analysis. [k,,];n"t;' has been determined from an inertial analysis, cf. [79]. b References to the experimental work for these complexes can be found in Table 1.1. c From [74]. d From [77]. 'Determined here using the method of [77]. For the 8 0 used for benzene see note c of Table 1.1. (From [79]. g Determined here using the method of [79]. The rotational constants used for the monomer were taken from [174]. The quartic centrifugal distortion constants used for the monomer were taken from the constants of fluorobenzene-4-d [175]. h Determined here using the method of [79]. The rotational and quartic centrifugal distortion constants used for the monomer were taken from [176]. ;The sign of kR9 cannot be determined from one isotopic species; the ± is with respect to the angle 0 in Table 1.1.
showed much smaller deviations. The reason for this large discrepancy lies in the very large values of LlP~ observed for these complexes (- 1.321 u A2
and - 1.278 u A2 for f1uorobenzene'" Ar and 1,2-difluorobenzene'" Ar, respectively). However, the small amplitude motion model which used the force constants from the centrifugal distortion analysis accounts for 63% in f1uorobenzene···Ar and 60% in 1,2-difluorobenzene···Ar of the observed large values of LlP?
1.3.3 Aromatic molecule"'diatomic molecule complexes
About 10 years ago the rotational spectra of the first aromatic mol ecule···diatomic molecule complexes were reported. The diatomic monomers were the halide acids HCl and HF. The complexes reported were benzene", HCl [80, 81], benzene·.. HF [82] and furan···HCl [83, 84]. The microwave measurements of benzene··· HF were performed with an MBER spectrometer. A pulsed nozzle FTMW spectrometer was used to measure the spectra of the other two complexes. The spectra of both benzene", HX complexes were those of centrifugally distorted symmetric tops while the spectrum of the furan'" HCl complex was that of a centrifugally distorted asymmetric top. In the benzene··· HX complexes, the measured spectroscopic constants were explained by a structure where HX lay above the plane of benzene with evidence of the equilibrium position of H and X situated on the C6 symmetry axis of benzene. Measurements of isotopic species of these complexes showed that the H of HX was located between the X atom and the benzene ring. Furan··· HCl was found to be planar (inertial defect, Llo ~ 1.8 u A2
) with significant in-plane bending motion(s) (Llo positive). From the measurements of isotopic species of this complex, HCl was determined to lie along the a inertial axis of furan, bisecting the oxygen-carbon angle with the H of HC} being hydrogen bonded to the oxygen of furan. A summary of the determined structures, stretching force constants and estimated binding energies of these complexes as well as the remaining aromatic molecule···diatomic molecule complexes can be found in Table 1.3. The spectroscopic constants determined for the benzene···HCl complexes also indicated that the average HCl axis tilt was ~23° away from the benzene C6 axis as the HCl wagged above benzene in its zero point motion. The corresponding average angle determined for benzene··· DF was 22(8)". For furan'" H 35Cl the authors determined, from the quadrupole coupling constants, an average in-plane bending angle of 17.09(8)" and an out-of-plane bending angle of 15.47(11)". This bending refers to excursions of the H atom away from the a inertial axis of the complex. The above difference in the bending angles was consistent with the positive value of Llo.
The remaining two benzene ..·diatomic molecule complexes that have been reported were both measured with pulsed nozzle FTMW spectrometers. These complexes are benzene.. · 15N 2 [85], and very recently benzene .. ·CO
Table 1.3 The ro or r, structure, stretching force constant and binding energy of aromatic molecule-diatomic molecule complexes'
Complex Type Rem Ob pe kRR Note
benzene...HF above-plane 313 0.0 0.0 7.3 500 d benzene...H 35CI above-plane 359.38 0.0 0.0 8.0 720 e furan ... H 35Cl in-plane 436.4(7) 6.70(6) 883 f benzene.. · 15N2 above-plane 349.8 0.0 -90 2.65 227 g benzene...CO above-plane 344.8(3) 0.0 85.4(2) 2.55 211 h pyrrole...CO above-plane 349.9(1) -2.6(1) 68.3(9) 3.4 293
• Rem in pm. Angles in degrees. kRR in N m - 1, Ii in cm - 1.
b In the above-plane complexes, 0 is the tilt angle between the normal to the plane of the aromatic molecule and Rem' e In the above-plane complexes, p is the angle between the axis of the diatomic molecule and the normal to the plane of the aromatic molecule. It should be noted that in benzene ... 15N 2 and benzene...CO free or nearly free internal rotation was observed. p is measured to the Iigher atom in the diatomic molecule and is positive when directed towards the hetero-atom in the aromatic molecule. dExperimental work and kRR from [82]. ro Rem calculated here from R F and RH given in [82]. Ii
calculated here from kRR and Rem' • All data from [81], ro structure. f Experimental work from [84]. This complex has a planar C2v structure with HCl hydrogen bonded to the oxygen offuran. The r0 Rem distances given in [84] were averaged and one standard deviation was calculated. kRR and Ii from [84]. • All data except Ii from [85]. Ii calculated from kRR and Rem' ro structure. h Experimental work and r, structure from [86]. kRR calculated here treating CO as a point mass and applying the procedure of [77]. Ii calculated here from kRR and Rem. i All data from [87]. The structure is an ro structure and kRR and Ii were derived from this ro structure in a manner similar to that for the above complexes. These values are quoted here rather than the results of the full centrifugal distortion analysis given in [87] for comparative purposes. The structure presented here is the more likely Structure I given in [87].
and various isotopic species [86]. Again, centrifugally distorted symmetric top spectra were observed. It was interpreted from the spectroscopic evidence that the diatomic molecules in these complexes executed free or almost free internal rotation approximately parallel to the plane of benzene. For benzene .. · 15N 2' additional transitions, probably due to the first excited internal rotational state, were observed but were not analysed in detail [85]. For benzene···CO, no transitions other than those of the ground internal rotational state were observed.
Very recently, the spectra of pyrrole···CO [87] and pyridine"'CO [88] and various isotopic species ofeach were measured and assigned using pulsed nozzle FTMW spectroscopy. The centrifugally distorted asymmetric top spectra of both complexes showed no evidence of large amplitude motions. In pyrrole"'CO, the CO which was located above the pyrrole plane did not exhibit any internal rotation. However, a centrifugal distortion analysis revealed that the two torsional motions of CO above pyrrole were the lowest frequency modes. The CO was found to be significantly tilted towards the pyrrole ring with C closer than O. Unfortunately, the authors were not able
to determine unambiguously whether the carbon end of CO was pointed toward the nitrogen ofpyrrole or in the opposite direction. In the pyridine·..CO complex, CO was located relatively rigidly in the ring plane of pyridine. The inertial defect was found to be ~o ;:::; -0.76 u A2 indicating large out-of-plane motions which is in contrast to the large in-plane motions exhibited in furan··· HCI. CO was found not to lie along the a-axis of the pyridine monomer which was confirmed by the presence of a weak J1b spectrum.
The apparent structural trend for complexes of the aromatic mol ecule···diatomic molecule type is that, if a hydrogen is missing in the ring plane of the aromatic monomer (e.g. in furan and pyridine), the diatomic molecule prefers to bind in the plane of the monomer. It also appears that if the aromatic monomer possesses a permanent dipole moment then the diatomic molecule tends to be better localized about a specific position with respect to the aromatic monomer than the benzene···diatomic molecule complexes.
1.3.4 Aromatic molecule···triatomic molecule complexes
To date there have been six complexes of the aromatic molecule···triatomic molecule type measured in the microwave range. Except where otherwise stated, all of these complexes have been measured using pulsed nozzle FTMW spectrometers. Only two different triatomic monomers, S02 and H20, have been incorporated in these complexes. The measured complexes were benzene···S02[89,90], toluene···S02 [91], furan· ..S02[92], pyridine···S02 [89], benzene.·.H 20 [93, 94] and pyrrole···H20 [95]. Various isotopic species were also measured for all of these complexes. For benzene···S02, in addition to the ground state centrifugally distorted asymmetric top spectrum, transition due to a slightly hindered internal rotation of S02 (up to m = ± 5) above the benzene plane were assigned using the principal axis method. An internal rotational Hamiltonian with centrifugal distortion was used to analyse the assigned transitions. The barrier height to the six-fold internal rotation ofS02above benzene was determined to be V6 = 0.277(2) em -1. The Rem distance was determined to be 348.5(I)pm, and the average angle between the C2 axis of S02 and Rem was 44(6t with the sulphur atom closer to benzene than the oxygen atoms. Although the sign of the angle between the perpendicular to Rem and the plane of benzene could not be unambiguously determined, its magnitude was found to be 12(1r.
In the spectrum of toluene···S02 [91], all three dipole moment transition types were observed because S02 was found to be in a rather unsymmetrical position above the ring plane. The spectra due to two different 180 species were assigned along with other isotopic species, and a preferred structure was reported. The S02 monomer was found to lie above the plane of toluene at an Rem distance of 337.0(I)pm. The projection of the C2 axis of S02 on the aromatic plane made an angle of 47.0(1)" with the C3 axis of the methyl
group of toluene. The plane of SOz was tilted toward the toluene ring with the sulphur atom directed towards the ring plane. Transitions due to the m = 0 and ± I internal rotation states of the methyl group were assigned and analysed by applying a Hamiltonian similar to that used for benzene··· SOz' The V3 barrier to internal rotation was determined to be 83.236(2) cm - '. This represented a substantial increase in barrier height for the internal rotation of the methyl group in the toluene monomer where V6 = 4.88(3) cm -, [96]. It was also found that the m = 0 state c-type transitions for the parent species and toluene-CD 3···SOzexhibited small splittings (-100 kHz) which indicated a reorientation tunnelling motion of SOz with respect to the aromatic ring.
As in toluene···SOz, the spectra of the furan···SO z [92J complexes contained transitions due to the presence of all three dipole moment components. A preferred structure was reported which was similar to that of toluene···SOz. The Rem was determined to be 343(I)pm and the two Cz axes of the monomers were skewed by -65". Small splittings were also observed for the Jlc and some Jlb type transitions. These were attributed to a tunnelling motion between two equivalent forms of the complex.
In the spectrum of pyridine···SOz [89J there was no evidence of facile internal rotation. The complex was found to be a centrifugally distorted asymmetric top with the plane of SOz more nearly perpendicular to the ring plane than in benzene···SOz' The complex had Cs symmetry with the mirror plane passing through the Cz axes of both monomers. The plane of SOz and the ring plane of pyridine were found to be tilted at ±9(8t and 74(4t, respectively, to the perpendicular of Rem with the nitrogen end of pyridine closest to the sulphur of SOz. The Rem was found to be 401(3) pm.
As has been found in many complexes involving HzO, the spectra of the two aromatic molecule··· HzO complexes exhibited signs of significant internal motion. Frequency modulated and Stark modulated direct absorption spectroscopy of jet-cooled benzene···HzO was used in the observation of this complex for J ~ 4 [93]. The low J transitions between 0 and 4 were observed with a pulsed nozzle FTMW spectrometer [94]. Among the many transitions observed for benzene··· HzO, those due to the ground (m = 0) and first excited (m = ± I) internal rotational states were assigned in both works. These internal rotational states were correlated with the jKp.K
o = 000 and
jKp.K o
= 101 , 111' respectively, of the free HzO molecule. Transitions due to these two states were analysed with a simple model. The obtained spectroscopic constants showed that the hydrogens of HzO were hydrogen bonded to the n system. In benzene···HzO, with the centre-of-mass of HzO situated on the C6 axis of benzene, the Rem distance was found to be 334.7(S)pm [93J or 332.9 pm [94]. The latter workers also determined that the a-axis of the complex corresponded with, or very nearly with, the C6 axis of benzene. The average tilt angle of the Cz axis of HzO with respect to the C6 axis of benzene, as it internally rotated almost freely above the plane of the ring, was 20(ISt [93J or 37" (measured from the a-axis) [94].
All of the observed transitions of pyrrole--- H20 [95] were split into doublets by an internal motion. The authors presented evidence that these doublets were, in fact, tunnelling doublets and not due to the presence of a rotational spectrum associated with an excited vibrational state. Likely pathways for the internal motion were discussed. An in-plane structure, rather than an above-plane structure, best explained the spectroscopic constants of this complex. Unlike benzene---H 20, where the protons of H 20 were donated to the n system of benzene, pyrrole acted as a proton donor via a hydrogen bond to the oxygen of H 20.
1.3.5 Larger complexes containing at least one aromatic molecule
There remain three complexes not yet discussed which do not fit into the above classifications. These complexes all involve benzene as one of the aromatic monomers. The complexes studied were the benzene--- NH 3 complex [97]; the benzene dimer [98] and the Ne---benzene---H 20 complex [21]. The jet-cooled spectrum of the benzene--- NH 3 complex, and various isotopic species, were recorded using both a Stark modulated microwave absorption spectrometer and an FTMW spectrometer. The microwave spectrum was that ofa centrifugally distorted symmetric top. The results ofresonance-enhanced two-photon ionization spectroscopy, also reported in [97], showed two bands, one assigned as being due to the m = 0 interval rotational state of NH 3 and the other the m = 1. As in benzene---H20, the m = 0 and m = 1 states correlated with j(NH3 ) = 0 and 1 for free NH 3 , respectively. However, in the microwave spectrum, no transitions due to the m = 1state were observed. The derived structure was one where the hydrogens of NH 3 were weakly hydrogen bonded to the n system of benzene. The distance from the nitrogen atom to the benzene plane was found to be 359.0(5) pm. The vibrationally averaged tilt angle of the C3 axis of NH 3 with respect to the C6 axis of benzene was approximately 58°.
The spectrum ofthe parent species of the benzene dimer [98] was measured using a pulsed nozzle FTMW spectrometer. Each transition was observed to be a symmetrical quartet with a 3:1 ratio between the splitting frequencies. The authors suggested that these splittings were tunnelling splittings and were indicative of two similar pathways because of the small 3:1 ratio. Average line centres of these quartets were determined and fitted to a centrifugally distorted symmetric top Hamiltonian whereby a very large DJK = 869(5) kHz resulted. The structure was determined to be T-shaped with Rem = 496 pm.
The spectrum of the jet-cooled Ne---benzene---H 20 complex [21], and various isotopic species, were measured using an FTMW spectrometer. Transitions due to the m = 0 and m = 1 internal rotational states of H 20 were observed. The spectra, due to the m = 0 state, for each of the isotopic species were that of centrifugally distorted symmetric tops. The transitions arising from the m = 1 state were not analysed. Ne and H 20 were found to
be positioned on opposite sides of the ring. The binding of HzO and Ne to benzene appeared to be little affected by each other's presence. The Rem for the Ne···benzene subunit was determined to be 339.1 pm, and located on the C6 axis of benzene. Similarily HzO, situated on the other side of benzene, was at an Rem of 333.4 pm. The vibrationally averaged tilt angle of the Cz axis of HzO with respect to the C6 axis of benzene was determined to be 35°.
1.4 Hydrogen bonded complexes
The importance of hydrogen bonding has led to continuing studies, both experimental and theoretical, aimed at quantifying this interaction. Hydrogen bonded complexes are of interest particularly in biochemical systems and also for a variety of other reasons; for example, they may be intermediates in intermolecular reactions. The molecular dynamics associated with hydrogen bonding plays a fundamental role in much of chemistry. Many studies of the associated rotational spectra have been made using jet spectroscopy. It is of significance that in the past 20 years, a detailed picture of hydrogen bonding has emerged from high-resolution microwave and infrared investigations of weakly bound complexes.
Consequently, many binary complexes such as those of HF, HzO and NH 3 , formed among themselves or with other small molecules, have been structurally characterized in an attempt to understand the nature of hydrogen bonding. The structure, and in several cases the dynamics, of such hydrogen bonded complexes can be well determined by analysing their microwave or radiofrequency spectra as was first done for the hydrogen fluoride dimer by Dyke et at. [7]. Homodimers are an essential starting point in the study of intermolecular dynamics because they frequently belong to a higher symmetry group and often make possible a precise group theoretical analysis of the tunnelling motions. HF [7,99-104], HCl [105], HCN [5, 106, 107], HCCH [108,109], HzO [61,110-112] and HzCO [113] are examples ofhomodimers for which interconversion tunnelling splittings have been observed in high-resolution spectroscopic studies. For the ammonia dimer, the effective ground state structure and measured dipole moments of(NH3 h and (ND3 h implied that there was no hydrogen bonding present [114-117]. Because the results ofthe different isotopic species were so similar an equilibrium structure not far from the effective structure was deduced. However, there is much controversy over whether the correct equilibrium structure is hydrogen bonded or not. This was discussed in the introduction of a paper by Tao and Klemperer [118] where a high level ab initio calculation on the equilibrium structure of the ammonia dimer was presented. Work is currently in progress to fully resolve this issue. It would be remiss not to mention that homodimers also occur between monomers not capable of forming hydrogen bonds, e.g. SOz [119].
This section on hydrogen bonded complexes has been divided into two parts: complexes involving and complexes not involving water. Within these two sections a selection of hydrogen bonded complexes will be mentioned with the focus being on important results for dimers with tunnelling splittings.
1.4.1 Complexes involving water
Water displays a wide range of bonding interactions; it accepts hydrogen bonds from HF [120, 121], HCI [122], HCN [123, 124] and HCCH [125]. However, water is also able to donate its hydrogen to form hydrogen bonds as seen in complexes with NH 3 [126], H2C=CH2 [127], and N 2 [128]. Two other water complexes, benzene"'water and pyrrole···water, are discussed in section 1.3.4. The dual nature of water interactions is especially apparent in the complex with formamide [129] where water both accepts a hydrogen from formamide and donates its hydrogen to form two hydrogen bonds.
The study of gas phase molecular complexes involving water by high resolution spectroscopic techniques provides accurate structural and dynamical data in the region of the potential minimum. This can serve as a useful guide in the modelling of water interactions in solution or in aqueous environments. From an examination of the gas phase complexes H20···HCCH [125] and H20···H 2C=CH2 [127], information on the important water-hydrocarbon interactions was provided which will be useful for testing model potentials used in liquid simulations of hydrophobic interactions.
One of the most important groups found in proteins is the peptide linkage which is often involved in hydrogen bonding within biological systems. Thus, the characterization of the hydrogen bonding interaction between water and formamide, which can be considered as a prototype for the peptide linkage, has been of considerable interest to theoreticians [130]. In 1988, the first experimental study of molecular complexes with an amide appeared when Lovas et al. [129] measured the microwave spectrum of the complex formed between water and formamide, as mentioned earlier, and also that of formamide and methanol. The water···formamide complex was considered as a prototype for the interaction of the peptide linkage with a single water molecule, and it was anticipated that the results of such studies could be used for the refinement ofmodels for peptide-H 20 interactions. The similarity in predicted structures for the formamide···water and formamide···methanol complexes provided motivation for the examination of the formamide···methanol rotational spectrum. In this case, though, the spectrum was complicated by the effects of the methyl top internal rotation. Results indicated an essentially planar, double hydrogen bonded structure for both species and that amides can form hydrogen bonds. The carbonyl hydrogen bond interaction in these formamide complexes is similar to that of H2CO···HF [131, 132] and H2CO···HCI [133], where the C=O···H angle was found to be 103.6° and 109° respectively. These hydrogen bonds deviated from linearity by 10-20°.
An analysis of the splitting in the formamide· ..methanol complex showed a substantial reduction of the methyl internal rotation barrier associated with the CH30H subunit. This substantial reduction of the barrier to internal rotation was induced by complexation as the barrier height was 36% smaller than that of methanol. Furthermore, it suggested a hydrogen bond interaction at the hydroxyl oxygen. Interestingly, many of the water dimer transitions were first observed during searches for the formamide···water or form amide···methanol species [129]. By choosing appropriate model systems, high resolution spectroscopy can furnish hydrogen bond lengths and force constants which characterize water-protein substituent interactions. With increasing interest in the study of protein folding and protein solvent interactions, it is important to have information on water interactions with the peptide linkage and with the various functional groups found in proteins.
Of all hydrogen bonded species, the water dimer is believed to be among the most intensively investigated. This is indicated in a 1991 review on the spectroscopy, structure and dynamics of the water dimer by Fraser [134] which shows that many attempts have been made, over a relatively short time span, to understand the intricacies associated with its internal dynamics. Despite its significance in many biological, chemical, physical and atmospheric processes, it was only 20 years ago that the first high resolution data relevant to the water dimer were recorded. Tunnelling between eight isoenergetic hydrogen bonded forms was found to complicate the rotational spectrum. One set of water dimer tunnelling motions may be envisaged as resulting from twofold rotations of either or both of the water units about their C2 axes. The other type of tunnelling motion is an interconversion tunnelling, similar to that occurring in (HFb whereby the two H20 subunits interchange proton-donor and proton-acceptor bonding roles. The water dimer potential energy surface, including the barrier heights of the different saddle points, is still not known in all details.
Although not a complex involving water, the formaldehyde dimer is mentioned in this section due to its similarities with the water dimer. Because other complexes involving formaldehyde displayed a wide variation in bonding this made a priori structure prediction for the dimer more difficult. In contrast to the heterodimers of formaldehyde with the acids HF [131, 132], HCI [133] and HCN [135] which were hydrogen bonded at the oxygen, the formaldehyde complex with acetylene [136], for example, exhibited a dual-bonded planar ring structure from which observed spectral doubling had been interpreted as arising from torsional motion of H2CO in the complex. Subsequently, in order to determine the structure and internal dynamics of the torsional motions of the formaldehyde dimer, Lovas et al. in 1990 [113] carried out a microwave study of(H 2CO)2 and (D2CO)2 in ajet.
Analogous to the water dimer, the microwave spectra of both species were found to be split by internal rotation of each monomer unit (exchanging hydrogen atoms) and an interchange tunnelling motion which exchanges the
donor-acceptor bonding roles of each unit. Tunnelling motions of the hydrogen exchange type were not only employed to assist in the interpretation of the water dimer [111, 61, 137] spectrum but also that of N 2··· H20 [128]. These interchange tunnelling motions had been well established in several other cases, e.g. (HF)2 [7] and (HCCHh [109]. The analysis of the tunnelling splittings in the formaldehyde dimer indicated that the overall symmetry must be Cs ' implying that a plane of symmetry is required. Furthermore, the total dipole moment in the complex was found to be substantially smaller than the value for the monomer indicating that the orientation of the H2CO monomer units is nearly antiparallel in the complex. Consequently, the geometry obtained has the orientation of the CO groups nearly antiparallel and the H-C-H planes perpendicular to each other. Between monomer units, the shortest carbon to oxygen distance, 298 pm, and hydrogen to oxygen distance, 218 pm, were indicative of a dual bond interaction to form a ring structure.
In 1990, Yaron et al. [138] used MBER and FTMW techniques to investigate the hydrogen bonded water···carbon monoxide complex. During this work, rotational transitions were observed for H20···CO, HDO···CO, D 20···CO, H 20··· 13CO, HDO··· 13CO and H/ 70···CO. In the H20···CO and D 20···CO complexes, a tunnelling motion which exchanged the free and bound hydrogens was found to give rise to two states, with measurably different rotational constants and dipole moments. For the equilibrium structure of the complex, the heavy atoms were approximately collinear and the water was hydrogen bonded to the carbon of CO. However, contrary to simple pictures of water hydrogen bonding in which the O-H bond of water is linearly directed towards a binding partner, the hydrogen bond in water···CO was found to be nonlinear by 11 S, i.e. the O-H bond of water made an angle of llS with a line connecting the oxygen of water to the centre of mass of CO. It was noted that the equilibrium tilt away from a linear hydrogen bond was in the direction opposite to the tunnelling path. This tunnelling was found to proceed through a saddle point, representing a C2v structure, with the hydrogens directed towards the CO subunit which, in turn, did not appear to be involved in the tunnelling and was not undergoing large amplitude motion. The barrier to exchange of these bound and free hydrogens was determined to be 210(20) cm - I. Because the water···carbon monoxide complex was undergoing tunnelling motion in the modes involving the light atoms, extracting the equilibrium structure required consideration of these vibrations.
1.4.2 Complexes not involving water
A large variety of hydrogen bonded complexes which do not involve water have been identified by making use of their rotational spectra in the gas phase. Parallel to the MBER measurements of the hydrogen fluoride dimer
and the water dimer, strongly hydrogen bonded complexes involving mostly HF and HCN were studied in the static gas with conventional microwave spectrometers [4-6]. The observation of the weakly hydrogen bonded Kr··· HCI complex [29, 32] opened the field for successful studies of hydrogen bonded complexes using pulsed nozzle FTMW spectroscopy.
Acetylene has been shown to form hydrogen bonds in a range of complexes, for example CZH4 ···HCCH [139], NH 3 ,··HCCH [140], HzO..·HCCH [141, 125] and CH3CN..·HCCH [142], but it is the acetylene dimer [109, 143] that has received substantial attention in recent years. In order to explain anomalies in the spectrum of this dimer, Fraser et al. [109] in 1988 invoked a model consisting of a T-shaped complex with interconversion tunnelling between four isoenergetic hydrogen bonded minima. Previously, T-shaped hydrogen bonded geometries had been reported for the related complexes ofacetylene with HF [144], HCI [145] and HCN [146] and such interconversion tunnelling had also been observed in other homodimers. This model of the acetylene dimer in a T-shaped configuration was verified by the observation of rotation-inversion transitions in the microwave spectrum in addition to the pure rotational transitions which had been measured by Prichard et al. [147]. From this work, the measured microwave splittings yielded a tunnelling frequency of 2.2 GHz, consistent with a "" 33.2 cm -1 barrier separating the four minima. However, to test the model, investigations of the tunnelling frequencies of isotopically substituted dimers were deemed necessary. In particular, dimers of the form (HCCDh would be of interest because the tunnelling potential would no longer have fourfold symmetry as two adjacent minima would correspond to deuterium bonding and the other two to hydrogen bonding.
Three years later, Matsumura et al. [143] measured the microwave spectra of all variations of deuterated acetylene dimers in which a deuterium participates in the hydrogen bond, i.e. the T-shaped complexes, (DCCDh, (DCCH}z, DCCD.. ·DCCH, DCCH .. ·DCCD, HCCH .. ·DCCD and HCCH.. ·DCCR. As in the case of the water dimer [Ill], deutrated acetylene dimers with a deuterium located only in the hydrogen bond could be detected. All, except the last two dimers in the previous list, showed evidence of an interconversion tunnelling motion like the tunnelling observed for (HCCH)z. The tunnelling potential of (DDCD)z was analysed using a one-coordinate model and the barrier height was determined to be 35.577 cm - 1, which is 2.371 cm -1 deeper than that of (HCCHh studied by Fraser et al. [109].
As mentioned in Section 1.1, the first spectroscopic investigation of complexes between carboxylic acids was carried out by Costain and Srivastava [1, 2] and later extended by Bellot and Wilson [3]. In the low resolution spectra, the absorption features were due to the superposition of a large number of transitions from vibrationally excited states of the van der Waals modes. It has only recently become possible to measure the rotational spectra of such large complexes with similar resolution and accuracy as obtainable
for the smaller complexes or monomers. Despite their large size and inherent flexibility, by using a cold supersonic molecular beam, the rotational spectra of the hydrogen bonded complexes CF3COOH···HCOOH and CF3COOH···CH3COOH [148], also termed bimolecules, have now been measured in the vibrational ground state. Even from the results of a large number of D, 13C and 180 substituted species of CF3COOH ... HCOOH, it was not possible to locate the hydrogens in the two hydrogen bonds accurately. These positions seemed to be extremely sensitive to vibrational effects. No problems were encountered for the location of the other nuclei in the HCOOH subunit. Assuming a reasonable length of the O-H bond, the observed anomalous differences between moments of inertia associated with the isotopes were rationalized as a very small increase ofthe centre-of-mass distance between the two subunits in the complex upon deuteration of each hydrogen atom in the hydrogen bond. This small increase was also found to be additive when both hydrogen bonds were deuterated simultaneously. Held and Pratt [149] recently came to the same conclusion during the analysis of the electronic spectra of the dimers of2-pyridone. Internal rotation splittings of the methyl group were observed in the rotational spectrum of CF3COOH···CH3COOH [148]. The internal rotation barrierof97.2(15)cm- 1
was determined to be significantly lower than that of 168.238(17) cm -1 [150] for the acetic acid monomer. It was noticed that the semirigid model for the internal rotation may no longer be accurate enough in these flexible bimolecules.
Several hydrogen bonded complexes composed of three monomers have had their rotational spectra observed and structures reported. A set of them consisted of the homotrimer (HCNh [15] and the heterotrimers containing the linear (HCNh dimer [5, 106, 107, 151]. They included X···(HCN)z with X = OC, N z, H3N and HzO [19], and (HCNh··· Y with Y = HF, HCI, HCF3 and COz [17]. These heterotrimers may be viewed as composites of two dimers, the (HCN)z with X··· HCN or HCN...Y. The hydrogen bond in the (HCNh dimer is relatively strong, (see Table IX of reference [152]) so the dimer acts largely as a subunit, enabling the larger complexes to be formed in readily detectable concentrations in a pulsed nozzle expansion [153].
In the heterotrimers characterized so far, the geometry is usually an overlap of that of the two dimers; for example, the linear (HCN)z plus the T-shaped HCN···CO z [154] gives aT-shaped (HCNh···COz [17]. An exception to this is the linear (HCN)z plus a quasilinear Ar···HCN [155] which gives a T-shaped Ar···(HCNh complex [14], with the Ar alongside a slightly perturbed (HCN)z. Although the geometry of the dimers is generally preserved, incorporation of them in a heterotrimer usually produces significant shrinkage of the hydrogen bonds involved.
In 1990, these studies were extended to two complexes, each with three different monomers [18]. With two of the monomers common to both complexes, these are best viewed as examples of X···HCN···HF with the HCN... HF complex as a subunit. In this particular study, the rotational
spectra for several isotopic species of the OC···HCN···HF and H3N .. ·HCN..·HF heterotrimers were investigated. Detection of these heterotrimers out of the many species possible required care in their generation; however, both were favoured by the strongly bonded HCN..·HF subunit (see Table IX of reference [152]). These heterotrimers were found to be effectively axially asymmetric with some shrinkage with respect to the distances in the dimers. Studies on the formation of dimers and trimers between Ar and HCN in supersonic jets have also been made by making use of the intensity of the rotational transitions [153].
1.5 Conclusion and outlook
We have seen from our survey of complexes involving an aromatic molecule that the delocalised n system present in the aromatic monomer offers a very attractive site for the binding of a second or third monomer. If a rare gas atom is a binding partner it has always been observed to bind to this system, in the location above the ring plane, and in those cases where there is a second rare gas atom, below as well. In the complexes involving benzene, the second monomer (not necessarily a rare gas atom), and the third if present, bind to the delocalized n system without exception. We have seen that if a binding partner contains electropositive hydrogens they form a hydrogen bond to the benzene n system. Symmetric-top or symmetric-top-like spectra also characterize complexes involving benzene. Sometimes transitions from excited internal rotation states of the monomer above benzene are also observed. Hetero-aromatic monomers present a second possible binding site to the binding partner; this leads to a choice between an in-plane structure or an above-plane structure. Presently, the preferred binding site is not always obvious. For example, when an electropositive hydrogen atom is on offer in pyrrole to CO, CO apparently prefers to bind to the delocalized n system, while in pyridine, which possesses an aromatic ring system similar to benzene above which CO internally rotates, CO preferentially binds in the plane of the ring but not along the C2 axis of pyridine. Conversely, a hydrogen bonded in-plane structure was found for pyrrole.. ·OH2 .
Aromatic systems are ubiquitous throughout nature with their intermolecular interactions in biology playing a crucial role. The microwave study of complexes involving aromatic systems leads to detailed information regarding the vibrationally averaged structure of these complexes which is governed by the intermolecular forces. Information regarding these intermolecular interactions is also accessible from these experiments. This information, however, is usually quite limited because the microwave measurements sample only the ground or very low lying vibrational states of the complex. It has been shown, in a few cases, that excited vibrational states which are not normally accessible in jet-cooled experiments can be sufficiently populated
to observe their microwave spectra by incorporating a pulsed glow discharge source into a pulsed nozzle FTMW spectrometer. Ar···HCI [156] and Kr··· HCI [52] have been observed using this technique. However, the observed spectroscopic constants refer to one quantum of HX stretching vibration, an intramolecular vibration.
Except where there exist low frequency vibrations in a monomer, the intramolecular vibrations can be treated as approximately adiabatically separable from the intermolecular vibrations. This is because of the large separation in the energies between intramolecular vibrations and intermolecular vibrations. Thus a different intermolecular potential energy surface is obtained for each vibrational level of the monomers. Detailed information on the intermolecular potential energy surfaces of small complexes further away from the minimum in this surface in the ground intramolecular state has been obtained from high resolution far-infrared studies. One such example is the Ar··· HCl complex [157]. This work has also included the dependence of the potential on the HCI monomer vibration. The 1992 review by Cohen and Saykally [158] discusses the far-infrared studies of small van der Waals complexes. Analogous studies of larger systems involving an aromatic monomer promises the same detailed understanding of their dynamical behaviour.
As well as the glow discharge being used to populate higher vibrational states in complexes, a discharge has also been applied in the observation of open-shell complexes, e.g. ArOH [48]. Other modifications to the pulsed nozzled FTMW spectrometer have been made to study short-lived complexes with interesting results. Three types of nozzles, fast-mixing, high-temperature or pyrolysis, were outlined and their application illustrated with examples by Legon [159]. A further example is the

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Jet Spectroscopy and Molecular Dynamics