Thermochemistry, Reaction Paths, and Kinetics on the Hydroperoxy-Ethyl Radical Reaction with O₂:  New Chain Branching Reactions in Hydrocarbon Oxidation

Ab initio and density functional calculations are performed to determine thermochemical and kinetic parameters in analysis of the 2 hydroperoxy-ethyl radical association with O2. The system serves as an initial model for O2 association with higher molecular weight alkyl-hydroperoxide radicals and is an important component in the well-studied ethyl radical plus O2 reaction system. The CBS-Q//B3LYP/6-31G(d,p) and G3(MP2) composite methods are utilized to calculate energies. The well depth is determined as 35 kcal/mol and transition state results show two low energy paths (barriers below the entrance channel) for reaction to new products:  (i) a HO2 molecular elimination and (ii) a hydrogen shift path. Intramolecular hydrogen transfer (five-member ring) leads to 2 hydroperoxide acetadehyde + OH, where the barrier is ca. 7 kcal/mol lower than previously estimated. The HOOCH2CH(O) formed here is chemically activated and a significant fraction dissociates to OH + formyl-methoxy radical, before stabilization. Th...