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

The structures, total energies, singlet-triplet splittings, and absolute heats of formation of o-, m-, and p-benzyne have been calculated with 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-triplet energy difference and absolute heat of formation for o-benzyne with the experimentally-determined values. Correlation-consistent CI (CCCI) methods, when used in conjunction with relatively large basis sets and molecular structures derived from MCSCF-based geometry optimizations, are found to give the best agreement, although the performance of larger-scale CI (e.g., CISD) calculations is comparable. All three benzyne isomers are found to have singlet biradical ground states at each of the levels of theory used. The most probable values for the singlet-triplet splittings in o-, m-, and p-benzyne derived from the CI calculations are 36 +/- 2, 17 +/- 1, and 2.2 +/- 0.5 kcal/mol, respectively. The energetics of the hypothetical isodesmic reaction between each of the benzynes and benzene to produce two phenyl radicals have been evaluated. These energy changes are discussed in terms of the stabilization or destabilization of the singlet and triplet states of each biradical with respect to simple bond-strength additivity models. The calculated energy differences have also been combined with the experimentally-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 excellent agreement with the experimental value; the predicted heats of formation for the singlet ground states of o-, m-, and p-benzyne are 107, 125, and 138 kcal/mol, respectively. A comparison of the theoretically-predicted heats of formation with the experimental values recently determined in this laboratory (Wenthold, P. G.; Paulino, J. A.; Squires, R. R. J. Am. Chem. Soc. 1991, 113, 7414) suggests that the measured heats of formation for m- and p-benzyne are both too low by 9-10 kcal/mol.