Characterization Of The 2-Methylvinoxy Radical + O-2 Reaction: A Focal Point Analysis And Composite Multireference Study

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 O-2. The reactions of 1-methylvinoxy radical and 2-methylvinoxy radical with molecular oxygen have experimental high pressure limiting rate constants, k(infinity), 5-7 times higher than that of the vinoxy plus O-2 reaction. In this work, high-level ab initio quantum chemical computations are applied to the 2-methylvinoxy radical plus O-2 system, namely, the formation and isomerization of the 1-oxo-2-propylperoxy radical, the immediate product of O-2 addition to the 2-methylvinoxy radical. Multireference methods were applied to the entrance channel. No barrier to O-2 addition could be located, and more sophisticated treatment of dynamic electron correlation shows that the principal difference between O-2 addition to the vinoxy and 2-methylvinoxy radicals is a larger steric factor for 2-methylvinoxy + O-2. This is attributed to the favorable interaction between the incoming O-2 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)-O-center dot as the final products.