Experimental evidence for the 7,7,8,8-tetracyano- p -quinodimethane dianion in vacuoSteen Brøndsted Nielsen and Mogens Brøndsted Nielsen Citation: The Journal of Chemical Physics 119, 10069 (2003); doi: 10.1063/1.1618216 View online: http://dx.doi.org/10.1063/1.1618216 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/119/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in First-principles investigation on Rydberg and resonance excitations: A case study of the firefly luciferin anion J. Chem. Phys. 141, 044309 (2014); 10.1063/1.4890730 Communication: Stabilization of radical anions with weakly bound electron in condensed media: A case study ofdiacetonyl radical anion J. Chem. Phys. 135, 101103 (2011); 10.1063/1.3638690 Specific formation of negative ions from leucine and isoleucine molecules J. Chem. Phys. 132, 014301 (2010); 10.1063/1.3270154 Dianions of 7,7,8,8-tetracyano- p -quinodimethane and perfluorinated tetracyanoquinodimethane: Information onexcited states from lifetime measurements in an electrostatic storage ring and optical absorption spectroscopy J. Chem. Phys. 127, 124301 (2007); 10.1063/1.2771177 Nondissociative low-energy electron attachment to SF 6 , C 6 F 6 , C 10 F 8 , and c- C 7 F 14 : Negative ionlifetimes J. Chem. Phys. 117, 11222 (2002); 10.1063/1.1522713

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Experimental evidence for the 7,7,8,8-tetracyano- p -quinodimethane dianionin vacuo

Steen Brøndsted Nielsena)

Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark

Mogens Brøndsted Nielsenb)

Department of Chemistry, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark

~Received 5 June 2003; accepted 21 August 2003!

The existence of the unsolvated 7,7,8,8-tetracyano-p-quinodimethane~TCNQ! dianion is provedexperimentally. The dianion was formed in 50-keV collisions between TCNQ monoanions andsodium vapor. In the collision process, electron capture to the monoanion occurs with a cross sectionof about 1 Å2. The lifetime of the dianion is estimated to be in the order of microseconds or longeras the flight time from the collision cell to the detector is 5ms. The stability of the dianion waselucidated by density functional theory~DFT! and ab initio MP2 calculations. A selection ofdissociation reactions have been studied theoretically and compared with the experimentalresults. © 2003 American Institute of Physics.@DOI: 10.1063/1.1618216#

I. INTRODUCTION

Organic p-donors andp-acceptors play an importantrole in materials science in the quest for electrically conduct-ing materials, so-called organic metals.1 Tetrathiafulvalene~TTF! and 7,7,8,8-tetracyano-p-quinodimethane ~TCNQ!constitute a famous pair of donor and acceptor moleculesthat upon mixing afford a charge-transfer salt, TTF-TCNQ.This salt was known since 1973 as the first true organicmetal owing to its metallic conductivity down to 54 K.2,3

Moreover, the two reversible one-electron oxidations of TTFand the two reversible one-electron reductions of TCNQhave made these molecules and related derivatives attractiveas redox-active units in supramolecular chemistry.4–10

Recently, we reported upon the efficient formation ofTTF dicationsin vacuo by high-energy collisions betweenTTF1 radical cations and molecular oxygen.11 Electron strip-ping was favored significantly relative to fragmentation intotwo dithiole rings; this result contrasts the favored outcomefrom electron ionization experiments. Along this line, webecame interested in designing an experiment for generatingthe TCNQ dianionin vacuoassuming its possible existencewithout a solvent environment. In other words, the electronstripping experiment that was successfully designed for gen-erating TTF21 had to be reverted to an electron attachmentprocess in order to generate TCNQ22. The mere existence ofsmall molecular dianionsin vacuohas been the topic of sev-eral papers12–27 and is discussed comprehensively in recentreviews.17,18,26–28

One of us29,30 showed recently that electron capture bymonoanions occurred in high-energy collisions with sodiumvapor, e.g., the small Fe(CN)4

22 dianion was formed in col-lisions between Fe(CN)4

2 and Na. We reasoned that a similarexperiment on the radical anion of TCNQ, formed by elec-

trospray ionization, may provide a route to the TCNQ dian-ion. We note that Compton and Cooper31 have demonstratedpreviously that the TCNQ anion was formed in collisionsbetween TCNQ and a fast beam of cesium atoms (.1 eVcenter-of-mass energy!.

II. EXPERIMENT

A detailed description of the experimental setup is givenelsewhere.32,33Briefly, TCNQ anions were produced by elec-trospraying an acetonitril solution of TCNQ. The ions wereaccelerated to 50-keV kinetic energy and mass selected by amagnet. Collisions were performed with either neon or so-dium at a pressure of 4.731024 Torr. An electrostatic ana-lyzer scanned the product ions formed in the collisions, andthe ions were detected by a channeltron detector.

III. CALCULATIONAL DETAILS

Calculations were performed with theGAUSSIAN 9834 andGAUSSIAN 0335 program packages. Geometries and frequen-cies were calculated at the B3LYP/6-311G(d) level oftheory. The TCNQ neutral, monoanion, and dianion were alloptimized as planar structures (D2h point group!.36 All fre-quencies are positive, and the located stationary points arelocal minima on the potential energy surfaces. Single-pointenergies were calculated at both DFT~B3LYP! andab initio~MP2! models with different basis sets, 6-31111G(2d,p)and 6-31111G(3df,2pd) to test for basis set dependencies.A large basis set is necessary in order to properly describethe electronic structure of the anions and dianions.

IV. RESULTS AND DISCUSSION

A. Collision-induced dissociation versus collisionalelectron transfer

The spectra obtained for collisions between TCNQ an-ions (m/z 204! and neon and sodium are shown in Fig. 1. Inboth cases several fragment ions were obtained. Dominant

a!Electronic mail: sbn@phys.au.dkb!Electronic mail: mbn@chem.sdu.dk

JOURNAL OF CHEMICAL PHYSICS VOLUME 119, NUMBER 19 15 NOVEMBER 2003

100690021-9606/2003/119(19)/10069/4/$20.00 © 2003 American Institute of Physics

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fragmentation channels involve losses of N, CN, or HCN, orthe formation of CN2, C(CN)2

2 , and C(CN)32 ions. The

most significant difference between the two spectra is thepeak atm/z 102, half them/z of the parent ion. This peak isconsiderably larger in the sodium spectrum than in the neonspectrum. With neon as a collision gas, assumingly onlyfragmentation occurs~or possibly electron detachment!. Twosymmetric ring cleavage reactions would explain the peak atm/z 102. Yet, another possibility that cannot be excluded iselectron capture in collisions with rest gas O2 to afford the

TCNQ dianion. Whatever the exact origin of this peak in theneon spectrum, its abundance is enhanced considerably whenemploying instead sodium as a collision gas. It seems thatone specific reaction channel is now promoted, namely elec-tron capture to the monoanion generating the dianion. In an-other experiment, we selected as a parent ion the isotopomercontaining one13C (m/z 205! and found from a narrow scanexperiment that the daughter ion peak occurred indeed atm/z 102.5~Fig. 2! as expected for the doubly charged TCNQ13C-isotopomer. In contrast, ring cleavage would result inpeaks atm/z 102 and 103 of equal abundance, but thesepeaks are completely absent in the spectrum. This result un-equivocally confirms the assignment of them/z 102 peak tothe dianion in the sodium spectrum

Scheme 1. Sodium-promoted generation of the TCNQ dianion.

The cross section for electron capture is estimated to be1 Å2. The flight time from the collision region to the detectoris ca. 5 ms. Hence, the lifetime of the dianion is on the orderof microseconds or even longer.

B. Stability of the TCNQ dianion

To elucidate the electronic stability of the TCNQ dianionand the stability with respect to charge dissociation

Scheme 2. Dissociation energies for some charge dissociation reactions ofTCNQ22 ~cf. Table II!. Values in parentheses correspond to DFT singlepoint calculations (B3LYP/6-31111G(3df,2pd).

FIG. 1. High-energy collision spectra between TCNQ2 ~50-keV transla-tional energy! and either~a! Ne or ~b! Na.

FIG. 2. Narrow scans in them/z region of the TCNQ dianion. Two TCNQ2

isotopomers were selected by the magnet in two different collision experi-ments:@12C12H4N4#2, m/z 204~full line spectrum!, @12C11

13CH4N4#2, m/z205 ~broken line spectrum!. The dianion peaks occurred at 102.1 and 102.6,respectively. The absence of two equally abundant peaks atm/z 102 andm/z 103 for them/z 205 parent ion excludes the process of symmetricdissociation of@12C11

13CH4N4#2.

10070 J. Chem. Phys., Vol. 119, No. 19, 15 November 2003 S. B. Nielsen and M. B. Nielsen

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we performed density functional theory~DFT! structural op-timizations followed by single point calculations with differ-ent models. All energies were corrected for zero-point ener-gies from the B3LYP/6-311G(d) calculated frequencies.The results are listed in Table I. At the MP2/6-31111G(2d,p) level of theory, we obtain an adiabatic electronaffinity ~EA! for TCNQ of 2.83 eV. With the larger basis set,6-31111G(3df,2pd), the adiabatic EA is 2.93 eV. Verticalattachment of an electron to form the monoanion is favorableby 2.80 eV@6-31111G(2d,p) basis set# which is close tothe adiabatic value. Zakrzewskiet al.37 obtained a similarvertical EA of 2.74 eV viaab initio propagator calculations.Moreover, these values are in perfect agreement with experi-ment~2.8 eV, vertical!.31 We also tested the outcome of DFTsingle point calculations at the B3LYP/6-31111G(3df,2pd) level of theory, which gave an EA as high as3.76 eV in poor agreement with experiment.

DFT calculations predict the adiabatic EA of themonoanion to be negative by 0.4 eV. However, the moreconfident MP2/6-31111G(2d,p) calculation gives a posi-tive EA of 0.10 eV. With the larger basis set, the adiabatic EAis even higher: 0.19 eV, likely due to a better description ofthe electron correlation in the dianion. Also, our MP2 calcu-lations are in fair agreement with the value of 0.32 V calcu-lated by Zakrzewskiet al.37 We obtained a value close tozero for the vertical EA employing the MP2/6-31111G(2d,p) model but slightly negative (20.02 eV) whereasit becomes positive~0.08 eV! when using the larger basis set.Hence, we conclude that the TCNQ dianion is electronicallybound according to the MP2 model. However, Jonkmanet al.38 and Johansen39 reported negative EA values of themonoanion,21.79 eV and21.87 eV, respectively, based onab initio SCF-MO calculations. Thus, it is somewhat unclearwhether the dianion is actually stable or not with respect toelectron loss.

Even an electronically unbound dianion can have a longlifetime due to the Coulomb barrier against electron autode-tachment. A simple model often employed is that of an elec-tron in a square well potential at short range and a repulsiveCoulomb potential at long range.18–20,27At short range, theelectron feels the attractive potential of the nuclei and at longrange it is repelled by the negative charge. Such a model wasused successfully to describe several dianions and to predictthe existence of long-lived dianions which are unstable toelectron autodetachment.27 Indeed, Wang and coworkers21

were the first experimentally to show the existence of long-lived electronically unstable multiply charged dianions.

Dissociation energies for several fragmentation reactionsof TCNQ22 at 0 K are summarized in Scheme 2 and TableII. The values were obtained by MP2/6-31111G(2d,p)single point calculations on DFT-optimized structures. Ittranspires that the dissociation channels are endothermic, andwe conclude that the dianion is thermodynamically stableagainst charge dissociation. Loss of cyanide, CN2, is the lessendothermic reaction, only requiring 1.3 eV. Indeed, peaksassignable to the two fragments2 and CN2 are very abun-dant in the spectrum. The C(CN)2

2 ion ~5! is also formedrelatively easily in accordance with both experiment and cal-culations~DFT in this case!. However, these fragment ions–that are present in the neon spectrum also–may occur just aswell from the dissociation of vibrationally excited monoan-ions. All dissociation energies were also calculated at theB3LYP/6-31111G(3df,2pd) level, which in all cases gavesmaller values than obtained by MP2. For instance, at thislevel an endothermicity of 0.9 eV for forming21CN2 wasobtained.

Finally, we have studied the energetic requirements forsymmetric dissociation of the TCNQ anion into a neutral andan anionic fragment via two possible routes, generating ei-ther the pair1Õ7 or the pair 6Õ7 after two carbon carbondissections

Scheme 3. Dissociation energies for symmetric dissociations of TCNQ2

into neutral and anionic fragments~cf. Table II!. Values in parentheses cor-respond to DFT single point calculations@B3LYP/6-31111G(3df,2pd)#.

~Scheme 3, Table II!. Both channels are highly endothermic,the first reaction costs 8.8 eV and the second 7.5 eV, in niceagreement with the complete absence of the fragment ions1and6 in the narrow scan spectra depicted in Fig. 2. Similarhigh endothermicities~8.1 eV and 7.2 eV, respectively! wereobtained at the B3LYP/6-31111G(3df,2pd) level and all inall suggest that the peak atm/z 102 originates from

TABLE I. Summary of computational data.E5energy, ZPE5zero-point energy. EA5electron affinity ~adiabatic!. S5singlet, D5doublet. 1 Hartree527.2116 eV.

StateZPE

~Hartrees!Total Ea

~Hartrees!EAa

~eV!Total Eb

~Hartrees!EAb

~eV!Total Ec

~Hartrees!EAc

~eV!

TCNQ S 0.127719 2678.7997872 3.76 2677.0034274 2.83 2677.2544714 2.93TCNQ2 D 0.126538 2678.9368561 20.41 2677.1061492 0.10 2677.3610411 0.19TCNQ22 S 0.125623 2678.9208509 2677.1090267 2677.3670860

aB3LYP/6-31111G(3df,2pd)//B3LYP/6-311G(d).bMP2/6-31111G(2d,p)//B3LYP/6-311G(d).cMP2/6-31111G(3df,2pd)//B3LYP/6-311G(d).

10071J. Chem. Phys., Vol. 119, No. 19, 15 November 2003 The 7,7,8,8-tetracyano-p-quinodimethane dianion

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TCNQ22 in both investigated types of collisions@~i! rest gasO2 , ~ii ! Na# rather than from fragments1 and/or6.

V. CONCLUSIONS

We have demonstrated that gaseous TCNQ dianions areformed in 50-keV collisions between TCNQ monoanions andsodium vapor. To our knowledge, this is the first experimen-tal detection of TCNQ22 in vacuo. MP2 calculations predictthe TCNQ dianion to be electronically bound. Whether ornot this theoretical model correctly describes the electronicstructure of the dianion, the Coulomb barrier prevents thedianion from ‘‘instantaneous’’ electron detachment which inany case renders it long-lived enough to be detected on themicrosecond timescale. According to calculations, the dian-ion is stable towards charge dissociation processes.

ACKNOWLEDGMENTS

S.B.N. gratefully acknowledges a ‘‘Steno grant’’ fromthe Danish Natural Science Research Council~Grant No. 21-02-0129!. M.B.N. acknowledges a ‘‘Research project grant’’from the Danish Technical Research Council~Grant No. 26-01-0033!.

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TABLE II. A summary of computational data (MP2/6-31111G(2d,p)//B3LYP/6-311G(d) level of theory!. E5energy, ZPE5zero-point energy. S5singlet, D5doublet, T5triplet. 1 Hartree527.2116 eV.

StateZPE

~Hartrees!Total E

~Hartrees!

N2 T 254.457454CN2 S 0.004838 292.6556765

1 D 0.054062 2338.37418322 S 0.115935 2584.40068263 S 0.120002 2622.36884034 D 0.099862 a

5 D 0.019117 2223.25166086 D 0.055571 2338.4242157 S 0.059352 2338.3949069

aUnsuccessful termination of calculation.

10072 J. Chem. Phys., Vol. 119, No. 19, 15 November 2003 S. B. Nielsen and M. B. Nielsen

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