Ab initio and RRKM calculations for multichannel rate constants of the C2H3+O-2 reaction

A potential energy surface for the reaction of vinyl radical with molecular oxygen has been studied using the ab initio G2M(RCC,MP2) method. The most favorable reaction pathway leading to the major CHO + CH2O products is the following: C2H3 + O-2 --> vinylperoxy radical 1 or 1' --> TS 8 --> dioxiranylmethyl radical 3 --> TS 9' oxiranyloxy radical 10 --> TS 11 --> formyloxymethyl radical 12' --> TS 13' --> CHO + CH2O, where the rate-determining step is oxygen migration to the CC bridging position via TS 9', lying below the reactants by 14.3 kcal/mol. The C2H3O + O products can be formed by elimination of the oxygen atom from C2H3OO via TS 23, which is by 7.8 kcal/mol lower in energy than the reactants, but by 6.5 kcal/mol higher than TS 9'. The hydrogen migration in 1' gives rise to another significant product channel. C2H3 + O-2 --> 1' --> TS 25' --> C2H2 + O2H, with TS 25' lying below C2H3 + O-2 by 3.5 kcal/mol. Multichannel RRKM calculations have been carried out for the total and individual rate constants for various channels using the G2M(RCC,MP2) energetics and molecular parameters of the intermediates and transition states. The computed low pressure reaction rate constant is in quantitative agreement with experiment. At atmospheric pressure, the title reaction is dominated by the stabilization of vinylperoxy radical C2H3OO at room temperature. In the 500-900 K temperature range, the CHO + CH2O channel has the highest rate constant, and at T greater than or equal to 900 K, C2H3O + O are the major products. At very high temperatures, the channel producing C2H2 + O2H becomes competitive.