Kinetics of the hydrogen abstraction alkane + O2 → alkyl + HO2 reaction class: an application of the reaction class transition state theory

Abstract

In this work, the kinetics of reaction class of hydrogen abstraction from saturated hydrocarbons by O2 molecules has been studied. The high-pressure reaction rate constants were determined using reaction class transition state theory/structure-activity relationship (RC-TST/SAR) methodology, augmented by linear energy relationship (LER) and/or barrier height grouping (BHG) approximations for evaluation of the reaction barrier heights. The parameters needed have been derived from DFT calculations at M06-2X/aug-cc-pVTZ level for a training set of 23 reactions, involving hydrogen abstraction by O2 molecule at primary, secondary, and tertiary carbon sites. The reference reaction rate constant C2H6 + O2 → C2H5 + HO2 was obtained by extrapolation of the simplest reaction within the title family CH4 + O2 → CH3 + HO2. Kinetic parameters of the later one, calculated from canonical variational transition state theory (CVT), were taken from literature. The influence of low-frequency internal rotations has been investigated in details. The error analysis shows that the average systematic error of RC-TST/SAR-derived rate constants at low temperatures is within 25% compared to the explicit RC-TST results and diminishes at higher temperatures. This suggests that the proposed methodology can be effectively implemented in the automated mechanism generation codes to create the fuel combustion mechanisms.