Pressure-Dependent OH Yields in Alkene plus HO2 Reactions: A Theoretical Study

The major bimolecular product of alkyl + O-2 reactions is alkene + hydroperoxyl radical (HO2), but in the reverse direction, the reactants are reformed to a very limited extent only. The most important products of the alkene + HO2 reactions are alkylperoxy radical (ROO center dot), hydroxyl radical (OH) + cyclic ether, and the corresponding hydroperoxyalkyl ((center dot)QOOH) species. Moreover, abstraction of allylic hydrogens can compete with the addition, further complicating the possible outcome of this reaction type and its effect on low-temperature combustion chemistry. In this paper, six alkene + HO2 reactions and the reaction between an unsaturated oxygenate and HO2 are studied based on previously established potential energy surfaces. The studied unsaturated compounds are ethene, propene, 1-butene, trans-2-butene, isobutene, cyclohexene, and vinyl alcohol. Using multiwell master equations, temperature-(300-1200 K) and pressure-dependent rate coefficients and branching fractions are calculated for these reactions. The importance of this reaction type for the combustion of unsaturated compounds is also assessed, and we show that, to get reliable results, it is important to include the pressure-dependence of the rate coefficients in the calculations.