Photoelectron Spectroscopy of m-Xylylene Anion

The 351-nm photoelectron spectrum of the negative ion of 1,3-benzoquinodimethane (m-xylylene) is reported. Features are observed in the photoelectron spectrum corresponding to formation of the B2, A1, and B2 states of m-xylylene. The electron affinity of the triplet ground state is found to be 0.919 ( 0.008 eV, and vibrational frequencies of 290, 540, and 1500 cm-1 are obtained. The active modes are assigned to R-carbon bending, ring deformation, and methylene bending, respectively. The A1 state is found to lie 9.6 ( 0.2 kcal/mol higher in energy than the ground state, in good agreement with theoretical predictions. Vibrational frequencies of 265, 1000, and 1265 cm-1 are found for this state. The B2 is estimated to be <21.5 kcal/mol higher in energy than the ground state. Density functional calculations have been carried out on the negative ion, indicating that the B1 ion is a minimum on the potential energy surface, lying 2.9 kcal/mol lower in energy than the A2 ion, which is a transition state. Recent approaches to the design and construction of magnetic organic compounds (organoferromagnets) have utilized biradicals, such as trimethylenemethane (TMM) derivatives or mbenzoquinodimethanes (m-xylylenes), as “ferromagnetic coupling units.1 ” These types of linkages are used because the parent biradicals, TMM (1) and m-xylylene (2), are known to possess triplet ground states,2,3 and thus serve as high spin linkages. Moreover, the singlet-triplet energy splittings for these biradicals (∆EST) are believed to be large (ca. 0.5 eV),4-7 so small perturbations in their structure will not change the nature of the ground state. The high spin preference in these molecules is predicted by simple qualitative theory,8 and is a result of electron repulsion that destabilizes the singlet states. The physical properties of these high-spin biradicals have been of considerable interest. In particular, TMM (1) has been the subject of experimental studies for 30 years, and of theoretical studies nearly 20 years longer. EPR measurements by Dowd showed that the biradical had a triplet ground state,2,9 and in subsequent experiments, the proton hyperfine coupling10,11 and rate of ring closure of the ground state12,13 were determined. More recently, Maier and co-workers14,15 have obtained the infrared spectrum of the ground state of 1. Assignments of the vibrational modes were readily made on the basis of results from ab initio calculations. Additional vibrational information for the triplet and planar singlet states of TMM has come from the recently reported photoelectron spectrum of the TMM negative ion.4 Moreover, using a simple thermochemical cycle, the heat of formation of TMM could be determined from the measured electron affinity. Altogether, the experimental studies listed here, along with more than a dozen computational studies,16 have led to a substantial understanding of the properties and electronic structure of this biradical. In contrast, little is known experimentally or theoretically about m-xylylene, 2. Elegant ESR studies by Wright and Platz3 demonstrated that 2 has a triplet ground state, as predicted by theory. For these experiments, the biradical was generated from the double hydrogen atom transfer reaction of the corresponding (bis)carbene, 2a, with the ethanol matrix utilized for the study (eq 1). The (bis)carbene 2a was prepared by photolysis of the corresponding bis(diazo) compound.17 Migirdicyan and Baudet18 reported the electronic spectrum of 2, generated by UV photolysis of m-xylene in a hydrocarbon matrix at 77 K. From the vibrational structure observed in the fluorescence spectrum, vibrational frequencies of 530 and 988 cm-1 were obtained. These were assigned to ring deformation and ring-breathing modes of the ground state biradical.18 Finally, a lower limit of 76 kcal/mol for the heat of formation of 2 was determined by X Abstract published in AdVance ACS Abstracts, February 1, 1997. (1) Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88. (2) Dowd, P. J. Am. Chem. Soc. 1966, 88, 2587. (3) Wright, B. B.; Platz, M. S. J. Am. Chem. Soc. 1983, 105, 628. (4) Wenthold, P. G.; Hu, J.; Squires, R. R.; Lineberger, W. C. J. Am. Chem. Soc. 1996, 118, 475. (5) Fort, R. C., Jr.; Getty, S. J.; Hrovat, D. A.; Lahti, P. M.; Borden, W. T. J. Am. Chem. Soc. 1992, 114, 7549. (6) Kato, S.; Morokuma, K.; Feller, D.; Davidson, E. R.; Borden, W. T. J. Am. Chem. Soc. 1983, 105, 1791. (7) Lahti, P. M.; Rossi, A. R.; Berson, J. A. J. Am. Chem. Soc. 1985, 107, 2273. (8) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587. (9) Baseman, R. J.; Pratt, D. W.; Chow, M.; Dowd, P. J. Am. Chem. Soc. 1976, 98, 5726. (10) Dowd, P.; Sachdev, K. J. Am. Chem. Soc. 1967, 89, 715. (11) Dowd, P.; Gold, A.; Sachdev, K. J. Am. Chem. Soc. 1968, 90, 2715. (12) Dowd, P.; Chow, M. J. Am. Chem. Soc. 1977, 99, 6438. (13) Dowd, P.; Chow, M. Tetrahedron 1982, 38, 799. (14) Maier, G.; Reisenauer, H. P.; Lanz, K.; Tross, R.; Jürgen, D.; Hess, B. A., Jr.; Schaad, L. J. Angew. Chem., Int. Ed. Engl. 1993, 32, 74. (15) Maier, G.; Jürgen, D.; Tross, R.; Reisenauer, H. P.; Hess, B. A., Jr.; Schaad, L. J. Chem. Phys. 1994, 189, 367. (16) Cramer, C. J.; Smith, B. A. J. Phys. Chem. 1996, 100, 9664 and references therein. (17) Trozzolo, A. M.; Murray, R. W.; Smolinsky, G.; Yager, W. A.; Wasserman, E. J. Am. Chem. Soc. 1963, 85, 2526. (18) Migirdicyan, E.; Baudet, J. J. Am. Chem. Soc. 1975, 97, 7400. 1354 J. Am. Chem. Soc. 1997, 119, 1354-1359 S0002-7863(96)02383-9 CCC: $14.00 © 1997 American Chemical Society Pollack et al.,19 who found that they could not deprotonate the 3-methylbenzyl cation in the gas phase. Detailed studies of the reactivity of 2 have been carried out by Goodman and Berson.20 It was found that 2, prepared from the bis-methylenic hydrocarbon 4 (eq 2),21,22 can be trapped by conjugated dienes to form vinyl indanes andm-cyclophenes.21-23 Product distributions obtained from trapping deuterium labeled reagents indicated a symmetric intermediate;22 however, the spin state of the reacting species could not be determined. The reactivity of several derivatives of 2 has also been studied. Gajewski et al.24 have examined the reactions of alkylated m-xylylenes in solution. They found that the tetramethyl derivative of 2 undergoes rapid dimerization at room temperature, and that the formation of the dimer was inhibited by the presence of oxygen. In addition, a cyclopropylsubstituted m-xylylene was studied. It was found that the cyclopropyl group remained intact during the dimerization, suggesting that the reacting species was a triplet biradical. Similar experiments carried out previously25 with cyclopropylsubstituted TMM found that some of the product ring-opened, which was interpreted to mean that a singlet biradical was generated. Perhaps the oldest and most famous derivative of m-xylylene is Schlenk’s hydrocarbon, 3.26 This ground-state triplet biradical is well characterized.27,28 A summary of the properties of other m-xylenes, including naptho and oxo derivatives, is provided elsewhere.29 Semiempirical methods7,30-32 and ab initio molecular orbital calculations5,6 have been carried out to examine the electronic structure of 2. The most instructive of these is the multiconfigurational SCF study by Kato et al.,6 in which a clear description of the electronic structure of 2 is provided. The two Hückel non-bonding molecular orbitals (NBMO’s) for m-xylylene are shown in Figure 1. The 3b1 orbital resembles that for a pentadienyl system, while the 2a2 orbital resembles a heptatrienyl system. Three low-lying electronic states, one triplet and two singlets, can be formed by placing two electrons in the two NBMO’s. The B2 and B2 states are formed by placing one electron in each of the orbitals, and are open shell states. The wave functions for these states are shown in eqs 3 and 4, respectively. A closed-shell A1 state is formed by using a two-configuration wave function, as shown in eq 5, The weighted sums and differences of the 3b1 and 2a2 orbitals shown in eq 5 are the generalized valence bond (GVB) orbitals of m-xylylene. The GVB orbitals are best represented as benzylic orbitals, as shown in Figure 2. As discussed by Kato et al.,6 electron repulsion within the singlet states will distort the NBMO’s from the pure Hückel orbitals. Thus, the bonding in the singlet states of m-xylylene will be more localized than that in the triplet. A schematic picture of the bonding in the B2 and A1 states is shown in Figure 3. The calculations that have been carried out on m-xylylene have focused for the most part on the relative energies of the three electronic states. Ab initio calculations that include π correlation5 predict the B2 state to be the lowest energy state, with the A1 and B2 states lying ca. 10 and 23 kcal/mol (0.4 and 1 eV) higher in energy, respectively. Here we report the 351-nm photoelectron spectrum of the negative ion of m-xylylene, 2•-. Electron detachment to form three electronic states of 2 is observed. The relative energies of the three states are obtained, and are in good agreement with theoretical predictions. Experimental vibrational frequencies are obtained for the B2 and A1 states of m-xylylene, and are readily assigned by comparison with the frequencies of m-xylene and with the aid of ab initio calculations. (19) Pollack, S. K.; Raine, B. C.; Hehre, W. J. J. Am. Chem. Soc. 1981, 103, 6308. (20) Berson, J. A. In The Chemistry of Quinonoid Compounds; Patai, S., Rappoprt, Z., Eds.; Wiley: New York, 1988; Vol. II, Chapter 10. (21) Goodman, J. L.; Berson, J. A. J. Am. Chem. Soc. 1984, 106, 1867. (22) Goodman, J. L.; Berson, J. A. J. Am. Chem. Soc. 1985, 107, 5409. (23) Goodman, J. L.; Berson, J. A. J. Am. Chem. Soc. 1985, 107, 5424. (24) Gajewski, J. J.; Paul, G. C.; Chang, M. J.; Gortva, A. M. J. Am. Chem. Soc. 1994, 116, 5150. (25) Adam, W.; Finzel, R. J. Am. Chem. Soc. 1992, 114, 4563. (26) Schlenk, W.; Brauns, M. Ber. Dtsch. Chem. Ges. 1915, 48, 661, 716. (27) Kothe, G.; Denkel, K.-H.; Summermann, W. Angew. Chem., Int. Ed. Eng. 1970, 9, 906. (28) Luckhurst, G. R.; Pedulli, G. F.; Tiecco, M. J. Chem. Soc. B 1971, 329. (29) Platz, M. In Diradicals; Borden, W. T., Ed.; John Wiley & Sons: New York, 1982; p 195. (30) Baudet, J. J. Chim. Phys. Phys.-Chim. Biol. 1971, 68, 191. (31) Flynn, C. R.; Michl, J. J. Am. Chem. Soc. 1974, 96, 3280. (32) Döhnert, D.; Koutecky, J. J. Am. Chem. Soc. 1980, 102, 1789. Figure 1. Schematic representation of the Hückel non-bonding MO’s in m-xylylene. Figure 2. Schematic representation of the GVB orbitals of m-xylylene, constructed from the linear combinations of the non-bonding molecular