A MULTICONFIGURATIONAL SCF AND CORRELATION-CONSISTENT CI STUDY OF THESTRUCTURES, STABILITIES, AND SINGLET-TRIPLET SPLITTINGS OF O-BENZYNE,M-BENZYNE, AND P-BENZYNE

The structures, total energies, singlet-triplet splittings, and absolu te heats of formation of o-, m-, and p-benzyne have been calculated wi th the use of a variety of multiconfigurational self-consistent field (MCSCF) and configuration-interaction (CI) methods. The performance of each method is evaluated by comparison of the calculated singlet-trip let energy difference and absolute heat of formation for o-benzyne wit h the experimentally-determined values. Correlation-consistent CI (CCC I) methods, when used in conjunction with relatively large basis sets and molecular structures derived from MCSCF-based geometry optimizatio ns, are found to give the best agreement, although the performance of larger-scale CI (e.g., CISD) calculations is comparable. All three ben zyne isomers are found to have singlet biradical ground states at each of the levels of theory used. The most probable values for the single t-triplet splittings in o-, m-, and p-benzyne derived from the CI calc ulations are 36 +/- 2, 17 +/- 1, and 2.2 +/- 0.5 kcal/mol, respectivel y. The energetics of the hypothetical isodesmic reaction between each of the benzynes and benzene to produce two phenyl radicals have been e valuated. These energy changes are discussed in terms of the stabiliza tion or destabilization of the singlet and triplet states of each bira dical with respect to simple bond-strength additivity models. The calc ulated energy differences have also been combined with the experimenta lly-determined heats of formation of benzene and the phenyl radical in order to derive the absolute heats of formation for the three benzyne isomers. The value obtained for o-benzyne using CCCI methods is in ex cellent agreement with the experimental value; the predicted heats of formation for the singlet ground states of o-, m-, and p-benzyne are 1 07, 125, and 138 kcal/mol, respectively. A comparison of the theoretic ally-predicted heats of formation with the experimental values recentl y determined in this laboratory (Wenthold, P. G.; Paulino, J. A.; Squi res, R. R. J. Am. Chem. Soc. 1991, 113, 7414) suggests that the measur ed heats of formation for m- and p-benzyne are both too low by 9-10 kc al/mol.