Benzyne thermochemistry:: A benchmark ab initio study

The thermochemistry of the benzynes has been reinvestigated in a set of benchmark calculations including, e.g., multireference perturbation theory in combination with large basis sets up to correlation consistent polarized valence quadruple xi followed by basis set extrapolation procedures. The vibrational corrections have been deduced from multiconfiguration self-consistent field (MCSCF) calculations employing average atomic natural orbital basis sets. The quality of the isodesmic reactions has been investigated by analyzing the errors of the utilized methods in predicting the CH bond strengths and energies of the related molecules. It turns out that multireference third order perturbation theory, although occasionally better than second order, suffers from less systematic errors and thus is not as well suited for use in isodesmic reactions as the corresponding second order theory. The present extended calculations show that all the isodesmic reactions used in previous studies of the thermochemistry of the benzynes are adequate. Furthermore, it is demonstrated that multireference second order perturbation theory accurately reproduces the singlet-triplet energy splittings of the benzynes.