High accuracy ab initio quantum chemistry in combustion studies

The thermochemistry and spectral properties of intermediates and transition structures, found in combustion environments, are computed using rigorous computational techniques. The highly accurate coupled cluster family of methods, extending all the way up to the quasiperturbative treatment of quadruple excitations [CCSDT(Q)] are utilized to obtain accurate energetics within the focal point approach. Anharmonic vibrational frequencies are extracted from accurate quartic force fields using vibrational perturbation theory. The unimolecular dissociation of ortho-benzyne is characterized as a retro-Diels-Alder process with a barrier height of 88.0 0.5 kcal mol and an enthalpy of 52.4 0.5 kcal mol. Related to this work, the enthalpy of formation for Diacetylene "H = 109.4 0.3 kcal mol is computed from the corresponding quantity for acetylene and high level computational methods, explicitly accounting for vibrational anharmonicity. The characteristic vibrational frequencies of the NCCO radical are computed to facilitate its monitoring in kinetics studies. Our computations yield (1, 2) = (2171, 1898) cm, in contrast to the experimentally derived values (2093, 1774) cm, at which no absorption could be observed in subsequent experiments.