HYDROCARBON RADICAL REACTIONS WITH O-2 - COMPARISON OF ALLYL, FORMYL, AND VINYL TO ETHYL

The reactions of allyl, formyl, and vinyl radicals with molecular oxygen have been analyzed as addition reactions, in which the energized adduct has several pathways available for further reaction. Rate constants for each of the reaction channels are estimated using a chemical activation formalism based on the Quantum Rice-Ramsperger-Kassel theory, along with thermodynamically consistent input rate constants and falloff parameters. Results show good agreement with the limited experimental data available. The well depth of the initially formed adduct is shown to exert a major influence over the preferred reaction channels. In particular, the shallow (approximately 18 kcal/mol) well for the allyl addition results in very little apparent reaction, and the major channel is simply redissociation to the initial reactants. The deeper wells for formyl and vinyl addition to oxygen (approximately 40 kcal/mol) allow other reaction channels to open up even at low temperatures. Predictions for the vinyl addition indicate HCO and CH2O are major products at lower temperatures, while the vinoxy + O channel becomes more important at higher temperatures. Formyl addition is shown to produce CO + HO2 as the major reaction channel. Rate constants for the various reactions are presented over a wide range of temperature and pressure. The good agreement between these calculations and the experimental data support the hypothesis that the reactions between hydrocarbon radicals and oxygen proceed via chemically activated addition and that one does not need to invoke a direct hydrogen abstraction pathway.