Singlet benzyne thermochemistry: a CASPT2 study of the enthalpies of formation

The enthalpies of formation for singlet benzynes were examined at the CASPT2 (second order perturbation theory, based on a complete active space SCF reference function) level of theory. The study employed isodesmic reactions in order to minimize the influence of systematic errors of the method. The results obtained for m- and p-benzyne were found to dispute earlier experimentally deduced values; a discrepancy of about 5 kcal/mol was observed. As a consequence, the enthalpy of reaction of the so called Bergman reaction, deduced previously from the experimental enthalpy of formation of p-benzyne, seems to be overestimated by about 5 kcal/mol. Furthermore, it was observed that the CCSD(T) method (coupled cluster singles and doubles with a perturbatative estimate of the triples contribution) is inappropriate for a balanced treatment of the Bergman reaction, due to the extensive differences in the electronic structures of the two reactants.