Ab initio kinetics on low temperature oxidation of iso-pentane: The first oxygen addition

The chemistry of R + O2 reaction in the low temperature oxidation of iso-pentane has been investigated by using quantum chemical calculations coupled with RRKM/master-equation simulations. All the four independent C5H11 radicals of the iso-pentane molecule were included in the investigation, i.e., 2-methylbutyl, 3-methylbutyl, 1,2-dimethylpropyl and 1,1-dimethylpropyl. The major reaction channels were explored in great detail, and the rate constants of key reactions were provided at high pressure limit and the falloff region. The QOOH radicals generated in R + O2 reactions, which are of particular importance to subsequent chain-branching, are explicitly identified. For these QOOH radicals, the buildup of concentration is typically attributed to the combined effect of the rapid equilibration with preceding alkylperoxy radical and the absence of fast dissociation channels. In addition, these QOOH radicals are all formed from isomerization of RO2 via six-member ring transition states, demonstrating the great significance of 1,5 H-transfers in the low temperature oxidation of alkanes. This work provides useful data for kinetic modeling of low temperature combustion for surrogate fuels and can be used as a starting point for the study about the reaction kinetics of the second oxygen addition in higher alkanes.