An ab initio study of the formation of alkoxy radicals by reactions of simple alkenes with the OH radical

The formation of ethoxy, propoxy and butoxy radicals in the reactions of ethene, propene, cis- and trans-2-butene with the OH radical has been modeled in the gaseous phase at the MP2/6-31+G(d) level. All the possible reaction pathways have been investigated, and the structures as well as the energetics have been determined. The reactants, prereaction complexes, transition states and products located along the alkene-OH radical reaction coordinates have been discussed thoroughly. The rate determining step for these reactions is the conversion of hydroxyalkyl radicals to alkoxy radicals. The reaction barriers and exothermicities for these small alkenes are more or less identical for the compounds studied. Nevertheless, addition of OH to the central carbon atom of propene is slightly favored kinetically and thermodynamically (1 kcal mol^-1) over the others.