2008 Volume 50 Issue 151 Pages 29-38
Reactions of alkyl radicals, R, with molecular oxygen (O2) are critical components in chemical models of autoignition phenomena. The fundamental kinetics of the R + O2 reactions is governed by a complex web of interrelated elementary physical chemistry processes. At low temperatures and moderate pressures the reactions form stabilized alkylperoxy radicals, RO2, but at higher temperatures thermal dissociation of the alkylperoxy radicals becomes more rapid, and formation of hydroperoxyl radicals (HO2) and the conjugate alkenes begins to dominate the reaction. Crucially for ignition chemistry, internal isomerization of the RO2 radicals produces hydroperoxyalkyl radicals, often denoted by QOOH, reactions of which are the key to low-temperature chain branching. Over the last decade the understanding of these important reactions has changed greatly, in part because of the combination of high-level theoretical studies with pulsed-photolytic kinetics experiments that probe HO2 or OH product formation. For example, it is now clear that HO2 elimination occurs directly from an alkylperoxy radical without first isomerizing to QOOH. In addition, the importance of including formally direct chemical activation pathways, especially for formation of products but also for formation of the QOOH species, in kinetic modeling of R + O2 chemistry has been demonstrated. Furthermore, it appears that the crucial rate coefficients for the isomerization of RO2 radicals to QOOH may be significantly larger than previously thought. This article reviews the status of general models of alkyl + O2 reactions in the context of some of the most recent pulsed-photolysis experiments.
