Ultraviolet Photoelectron Spectroscopy of the o-, m-, and p-Benzyne Negative Ions. Electron Affinities and Singlet-Triplet Splittings for o-, m-, and p-Benzyne

The 351 nm photoelectron spectra of the negative ions of o-, m-, and p-benzyne (1,2-, 1,3-, and 1,4-dehydrobenzene, respectively) and their perdeuterated isotopomers have been obtained. The o-benzyne ions were generated by the reaction of benzene and benzene-d6 with O-, while the mand p-benzyne ions were prepared by the gas-phase reaction between the corresponding 3and 4-(trimethylsilyl)phenyl anions and molecular fluorine, F2. The photoelectron spectra of the benzyne anions each contain two features, corresponding to formation of the singlet and triplet states of the biradicals. The electron affinities of oand p-benzyne are found to be 0.564 ( 0.007 and 1.265 ( 0.008 eV, respectively, while the electron affinities of deuterated oand p-benzyne are found to be 8 and 5 meV lower, respectively. The electron affinity of m-benzyne could not be determined from the photoelectron spectrum because the origin peak could not be assigned unequivocally. For oand p-benzyne, the singlet-triplet energy splittings can be obtained directly from the photoelectron spectrum, with values of 37.5 ( 0.3 and 3.8 ( 0.5 kcal/mol, respectively, obtained for the h4 species and 37.6 ( 0.3 and 3.9 ( 0.5 kcal/mol, respectively, obtained for the fully deuterated molecules. Using a previously reported value for the electron affinity of m-benzyne, the singlet-triplet splitting for this molecule is found to be 21.0 ( 0.3 kcal/mol. Vibrational frequencies are reported for the deuterated and nondeuterated forms of all three biradicals and for the corresponding negative ions. Using the measured electron affinities and previously reported heats of formation of o-, m-, and p-benzyne, the gas-phase acidities of the ortho, meta, and para positions of phenyl radical are calculated to be 377.4 ( 3.4, 386.8 ( 3.2, and 393.1 ( 3.0 kcal/mol, respectively, and the C-H bond energies at the ortho, meta, and para positions of phenyl anion are found to be 89.3 ( 3.3, 98.7 ( 3.1, and 105.0 ( 2.9 kcal/mol, respectively. The heats of formation of the singlet and triplet states of the benzynes are found to be in excellent agreement with the predictions derived from simple valence promotion energy models. Of the many different classes of organic biradicals, the benzynes have accumulated the longest history and most extensive record of experimental and theoretical investigation.1 Although “1,2-didehydrobenzenes” had been postulated as early as 18702 as intermediates in various reactions, it was the classic work of Wittig3 and Roberts4 in the 1940s and 1950s that firmly established the existence of o-benzyne 1 as a reactive intermediate in base-induced elimination reactions of halobenzenes. Since then, numerous methods for generating o-benzynes have been developed, such that these molecules have now become familiar reagents in organic and organometallic synthesis procedures.5-7 The other two benzyne isomers, m-benzyne 2, and p-benzyne 3, have been more elusive. The flash photolysis experiments with 1,3and 1,4-benzenediazonium carboxylates by Berry and co-workers8,9 and the mass spectrometric studies of diiodobenzenes by Fisher and Lossing10 provided some of the earliest evidence for these species. Attempts to generate 2 and 3 by base-induced elimination reactions were reported in the mid1970s;11,12 however, the nature of the intermediates formed in these reactions remained unclear. In 1972 Jones and Bergman13 demonstrated the intermediacy of p-benzyne in the pyrolysis † University of Colorado. ‡ Purdue University. § Texas Tech University. (1) Hoffman, R. W. Dehydrobenzene and Cycloalkynes; Academic Press: New York, 1967. (2) Dreher, E.; Otto, R. Leibigs Ann. Chem. 1870, 154, 93. (3) Wittig, G. Naturwiss 1942, 30, 696. (4) Roberts, J. D.; Simmons, H. E.; Carlsmith, L. A.; Vaughan, C. W. J. Am. Chem. Soc. 1953, 75, 3290. (5) Kauffman, T.; Wirthwein, R. Angew. Chem., Int. Ed. Engl. 1971, 10, 20. (6) Bryce, M. R.; Vernon, J. M. AdV. Heterocycl. Chem. 1981, 28, 183. (7) Bennett, M. A.; Schwemlein, H. P. Angew. Chem., Int. Ed. Engl. 1989, 28, 1296. (8) Berry, R. S.; Clardy, J.; Schafer, M. E. Tetrahedron Lett. 1965, 6, 1003. (9) Berry, R. S.; Clardy, J.; Schafer, M. E. Tetrahedron Lett. 1965, 6, 1011. (10) Fischer, I. P.; Lossing, F. P. J. Am. Chem. Soc. 1963, 85, 1018. (11) Washburn, W. N.; Zahler, R.; Chen, I. J. Am. Chem. Soc. 1978, 100, 5863. 5279 J. Am. Chem. Soc. 1998, 120, 5279-5290 S0002-7863(98)00335-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/15/1998 of cis-3-hexen-1,5-diyne through isotope-labeling and chemical trapping experiments. Cycloaromatization reactions of enediynes to p-benzynes, now known as “Bergman cyclizations”,14 have become an important current focus of anticancer drug design.15 This is due to the discovery of the endiyne antibiotics-potent antitumor agents that are believed to produce p-benzyne-type intermediates in the course of double-stranded DNA cleavage.16-18 Recent experimental work on the benzynes has produced a wealth of new physical data. Various gas-phase thermochemical properties of the benzynes have been determined, including absolute heats of formation,19-26 ionization potentials,27 electron affinities,28 and acidities.29 The infrared spectrum of 1 isolated in low-temperature matrixes has been measured by several groups,30-35 and the NMR properties36 of o-benzyne “guest” trapped in a hemicarcerand “host” have been reported. Recently, Sander and co-workers generated mand p-benzyne, 2 and 3, in argon matrixes by flash photolysis and measured their infrared spectra.37,38 Electronic structure calculations have played an important role in the evolving benzyne story.39,40 During the last three decades, more than 30 theoretical studies of the benzynes have been published, with half of these appearing in the last 10 years. Virtually every style of ab initio MO, semiempirical MO, and density functional calculation has been used to investigate the geometries, electronic structures, energetics, and spectroscopic properties of the benzynes. The consensus from high-level calculations is that all three benzynes have singlet ground states and that 1 is the most stable benzyne isomer while 3 is the least stable isomer. These conclusions are consistent with indirect evidence derived from chemical trapping experiments41 and with the measured heats of formation for the three benzynes.25 A key property of the benzynes, one that provides an essential link between their thermochemistry, reactivity, and electronic structures, is the energy difference between the singlet ground state and the lowest-lying triplet state, i.e., the “singlet-triplet gap”, ∆EST. The magnitude of ∆EST for singlet biradicals provides a direct measure of the extent of interaction between the nominally nonbonding orbitals. This, in turn, gives valuable insights regarding the balance of through-bond and throughspace components of the interaction.42,43 Simple valence-bond models have been proposed by Chen and co-workers that equate the singlet-triplet gaps for singlet biradicals with the reduction in their heats of formation compared to bond energy additivity estimates27 and with the increase in the activation energies for H-atom abstraction reactions relative to monoradicals.44,45 While accurate singlet-triplet splittings are essential for understanding biradicals, they are difficult to measure, and, for some systems, they can be extremely hard to calculate accurately by ab initio methods.40,46 One of the best experimental methods for determining singlet-triplet splittings in biradicals, carbenes, and other open-shell organic species is negative ion photoelectron spectroscopy (NIPES).47 In this experiment, the output of a UV or visible laser is crossed with a mass-selected beam of negative ions corresponding to the biradical or carbene of interest, and the energy spectrum of the resulting photoelectrons is measured. The adiabatic electron affinity, electronic state term energies, and vibrational frequencies for the neutral photoproduct can be derived from the energy differences between assigned features in the spectrum. The 488 nm photoelectron spectrum of the negative ion of o-benzyne, 1-, was reported more than 10 years ago by Leopold, StevensMiller, and Lineberger (LSL).48 Photodetachment to two different electronic states was apparent from the spectrum, and careful assignment of the two electronic band origins provided values for the electron affinity (EA) and ∆EST of o-benzyne of 0.560 ( 0.010 eV and 37.7 ( 0.6 kcal/mol, respectively. Analysis of the vibrational structure in the singlet feature also helped resolve the long-standing controversy regarding the CtC stretching frequency in o-benzyne. The value obtained, 1860 cm-1, was much lower than those derived from low-temperature matrix IR experiments but was in good agreement with theoretical predictions. The most recent measurement of the matrix IR spectrum of o-benzyne by Radziszewski and coworkers35 gives a CtC stretching frequency that is in good agreement with the NIPES results of LSL. One of the greatest challenges in obtaining photoelectron spectra for negative ions of systems such as the benzynes is the synthesis of the gaseous negative ions. In the LSL study, o-benzyne negative ion, 1-, was generated by the reaction of benzene with O(eq 1). Deuterium labeling experiments (12) Billups, W. E.; Buynak, J. D.; Butler, D. J. Org. Chem. 1979, 44, 4218. (13) Jones, R. R.; Bergman, R. G. J. Am. Chem. Soc. 1972, 94, 660. (14) Bergman, R. G. Acc. Chem. Res. 1973, 6, 25. (15) Enediyne Antibiotics as Antitumor Agents; Borders, D.; Doyle, T. W., Eds.; Marcel Dekker: New York, 1995. (16) Zein, N.; Sinha, A. M.; McGahren, W. J.; Ellestad, G. A. Science 1988, 240, 1198. (17) Nicolau, K. C.; Dai, W.-M. Angew. Chem., Int. Ed., Engl. 1991, 30, 1387. (18) Nicolau, K. C.; Smith, A. L. Acc. Chem. Res. 1992, 25, 497. (19) Rosenstock, H. M.; Stockbauer, R.; Parr, A. C. J. Chim. Phys. 1980, 77, 745. (20) Pollack, S. K.; Hehre, W. J. Tetrahedron Lett. 1980, 21, 2483. (21) Maccoll, A.; Mathur, D. Org. Mass Spectrom. 1981, 16, 261. (22) Moini, M.; Leroi, G. E. J. Phys. Chem. 1986, 90, 4002. (23) Riveros, J. M.; Ingemann, S.; Nibbering, N. M. M. J. Am. Chem. Soc. 1991, 113, 1053. (24) Guo, Y.; Grabowski, J. J. J. Am. Chem. Soc. 1991, 113, 5923. (25) Wenthold, P. G.; Squires, R. R. J. Am. Chem. Soc. 1994, 116, 6401. (26) Matimba, H. E. K.; Crabbendam, A. M.; Ingemann, S.; Nibbering, N. M. M. J. Chem. Soc., Chem. Commun. 1991, 644. (27) Zhang, X.; Chen, P. J. Am. Chem. Soc. 1992, 114, 3147. (28) Wenthold, P. G.; Hu, J.; Squires, R. R. J. Am. Chem. Soc. 1996, 118, 11865. (29) Gronert, S.; DePuy, C. H. J. Am. Chem. Soc. 1989, 111, 9253. (30) Chapman, O. L.; Mattes, K.; McIntosh, C. L.; Pecansky, J.; Calder, G. V.; Orr, G. J. Am. Chem. Soc. 1973, 95, 6134. (31) Chapman, O. L.; Chang, C.-C.; Kolc, J.; Rosenquist, N. R.; Tomioka, H. J. Am. Chem. Soc. 1975, 97, 6586. (32) Dunkin, I. R.; MacDonald, J. G. J. Chem. Soc., Chem. Commun. 1979, 772. (33) Nam, H.-H.; Leroi, G. E. J. Mol. Struct. 1987, 157, 301. (34) Wentrup, C.; Blanch, R.; Briehl, H.; Gross, G. J. Am. Chem. Soc. 1988, 110, 1874. (35) Radziszewski, J. G.; Hess, B. A., Jr.; Zahradnik, R. J. Am. Chem. Soc. 1992, 114, 52. (36) Warmuth, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1347. (37) Marquardt, R.; Sander, W.; Kraka, E. Angew. Chem., Int. Ed. Engl. 1996, 35, 746. (38) Marquardt, R.; Balster, A.; Sander, W.; Kraka, E.; Cremer, D.; Radzizewski, J. G. Angew. Chem. Int. Ed. Engl. 1998, 37, 955. (39) A listing of theoretical studies of the benzynes is given by Nash and Squires: Nash, J. J.; Squires, R. R. J. Am. Chem. Soc. 1996, 118, 11872. (40) Cramer, C. J.; Nash, J. J.; Squires, R. R. Chem. Phys. Lett. 1997, 277, 311. (41) Lockhart, T. P.; Bergman, R. G. J. Am. Chem. Soc. 1981, 103, 4091. (42) Hoffman, R.; Imamura, A.; Hehre, W. J. J. Am. Chem. Soc. 1968, 90, 1499. (43) Hoffman, R. Acc. Chem. Res. 1971, 4, 1. (44) Logan, C. F.; Chen, P. J. Am. Chem. Soc. 1996, 118, 2113. (45) Schottelius, M. J.; Chen, P. J. Am. Chem. Soc. 1996, 118, 4896. (46) Wierschke, S. G.; Nash, J. J.; Squires, R. R. J. Am. Chem. Soc. 1993, 115, 11958. (47) Ervin, K. M.; Lineberger, W. C. In AdVances in Gas-Phase Ion Chemistry; Adams, N. G., Babcock, L. M., Eds.; JAI Press: Greenwich, 1992; Vol. 1; p 121. (48) Leopold, D. G.; Stevens-Miller, A. E.; Lineberger, W. C. J. Am. Chem. Soc. 1986, 108, 1379. 5280 J. Am. Chem. Soc., Vol. 120, No. 21, 1998 Wenthold et al.