Thermochemistry, Reaction Paths, and Kinetics on the Hydroperoxy-Ethyl Radical Reaction with O_2: New Chain Branching Reactions in Hydrocarbon Oxidation

摘要

Ab initio and density functional calculations are performed to determine thermochemical and kinetic parameters in analysis of the 2 hydroperoxy-ethyl radical association with O_2. The system serves as an initial model for O_2 association with higher molecular weight alkyl-hydroperoxide radicals and is an important component in the well-studied ethyl radical plus O_2 reaction system. The CBS-1//B2LYP/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 HO_2 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 HOOCH_2CH(=O) formed here is chemically activated and a significant fraction dissociates to OH + formyl-methoxy radical, before stabilization. The barrier for hydrogen transfer is several kcal/mole lower than the corresponding reaction in a conventional hydrocarbon for this five-member ring transition state because the weak C-H bond on the hydroperoxide carbon. The second path is unimolecular HO_2 elimination leading to a vinyl hydroperoxide + HO_2. The vinyl hydroperoxide has a weak (22.5 kcal/mol) CH_2=CHO-OH bond and rapidly dissociates to formyl methyl plus OH radicals; a second low energy chain branching path in low-temperature HC oxidation. Kinetic analysis with falloff on chemical activation and unimoleculardissociation, illustrate that both low energy paths are competing. Results also show significant formation of a diradical, ~·OCH_2CH_2OO~· + OH, an additional new path to chain branching, which results from the chemical activation reaction. The HO_2 molecular elimination plus vinyl hydroperoxide dominates the H transfer by a factor of 1.8 at low temperatures, a result of its small entropy advantage. At high temperatures, dissociation to the higher energy, but loose transition state, hydroperoxide ethyl radical + O_2 (back to reactants) is the dominant path.