Rate constants for reactions of ethylbenzene with hydroperoxyl radical

A central step in the low-temperature oxidation of hydrocarbons is the abstraction of H by the hydroperoxyl radical (HO₂). In this study, reaction rate constants are derived theoretically for H abstraction by HO₂ from the three distinct locations of H in ethylbenzene (primary, secondary and aromatic H, with H on the ortho carbon taken as an example of unreactive aromatic H) as well as for the addition of HO₂ at the four possible aromatic sites. Potential energy surface is mapped out based on the results of computations performed with the composite CBS-QB3 theoretical method. Rate constants are fitted to modified Arrhenius forms in the wide temperature range of 300-2000K. The dominant channel at all temperatures is found to be H abstraction from the secondary C of the ethyl chain with a rate constant expression of k=1.28×10-24T3.70exp-5100Tcm3molecule-1s-1. Abstraction from the primary C also contributes significantly at higher temperatures (>800K) with a rate constant expression of k=2.90×10-24T3.78exp-8400Tcm3molecule-1s-1. Reasonable agreement was obtained with the limited experimental data available in the literature. Addition at the four sites of the aromatic ring and abstraction of one of the C-H aromatic bonds are relatively unimportant over the temperature range studied. We also investigate the abstraction of H from the secondary C on the ethyl chain by triplet oxygen and report the associated rate expression. The results presented herein should be useful in modelling the oxidation of alkylbenzenes at lower temperatures.