High-level theoretical characterization of the vinoxy radical (^•CH₂CHO) + O₂ reaction

Numerous processes in atmospheric and combustion chemistry produce the vinoxy radical (^•CH₂CHO). To understand the fate of this radical and to provide reliable energies needed for kinetic modeling of such processes, we have examined its reaction with O₂ using highly reliable theoretical methods. Utilizing the focal point approach, the energetics of this reaction and subsequent reactions were obtained using coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] extrapolated to the complete basis set limit. These extrapolated energies were appended with several corrections including a treatment of full triples and connected quadruple excitations, i.e., CCSDT(Q). In addition, this study models the initial vinoxy radical + O₂ reaction for the first time with multireference methods. We predict a barrier for this reaction of approximately 0.4 kcal mol^-1. This result agrees with experimental findings but is in disagreement with previous theoretical studies. The vinoxy radical + O₂ reaction produces a 2-oxoethylperoxy radical which can undergo a number of unimolecular reactions. Abstraction of a β-hydrogen (a 1,4-hydrogen shift) and dissociation back to reactants are predicted to be competitive to each other due to their similar barriers of 21.2 and 22.3 kcal mol^-1, respectively. The minimum-energy β-hydrogen abstraction pathway produces a hydroperoxy radical (QOOH) that eventually decomposes to formaldehyde, CO, and ^•OH. Two other unimolecular reactions of the peroxy radical are α-hydrogen abstraction (38.7 kcal mol^-1 barrier) and HO₂^• elimination (43.5 kcal mol^-1 barrier). These pathways lead to glyoxal + ^•OH and ketene + HO₂^• formation, respectively, but they are expected to be uncompetitive due to their high barriers.