Characterization of the 2-methylvinoxy radical + O₂ reaction: A focal point analysis and composite multireference...

Vinoxy radicals are involved in numerous atmospheric and combustion mechanisms. High-level theoretical methods have recently shed new light on the reaction of the unsubstituted vinoxy radical with O₂. The reactions of 1-methylvinoxy radical and 2-methylvinoxy radical with molecular oxygen have experimental high pressure limiting rate constants, k_∞, 5-7 times higher than that of the vinoxy plus O₂ reaction. In this work, high-level ab initio quantum chemical computations are applied to the 2-methylvinoxy radical plus O₂ system, namely, the formation and isomerization of the 1-oxo-2-propylperoxy radical, the immediate product of O₂ addition to the 2-methylvinoxy radical. Multireference methods were applied to the entrance channel. No barrier to O₂ addition could be located, and more sophisticated treatment of dynamic electron correlation shows that the principal difference between O₂ addition to the vinoxy and 2-methylvinoxy radicals is a larger steric factor for 2-methylvinoxy + O₂. This is attributed to the favorable interaction between the incoming O₂ molecule and the methyl group of the 2-methylvinoxy radical. Via the focal point approach, energetics for this reaction were determined, in most cases, to chemical accuracy. The coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] correlation energy and Hartree-Fock energies were independently extrapolated to the complete basis set limit. A correction for the effect of higher excitations was computed at the CCSDT(Q)/6-31G level. Corrections for the frozen-core approximation, the Born-Oppenheimer approximation, the nonrelativistic approximation, and the zero-point vibrational energy were included. From the 1-oxo-2-propylperoxy radical, dissociation to reactants is competitive with the lowest energy isomerization pathway. The lowest energy isomerization pathway ultimately forms acetaldehyde, CO, and ^·OH as the final products.