Pressure-Dependent Kinetics of O-Xylene Reaction with Oh Radical

OH-initiated oxidation reactions of o-xylene are widely concerned both in combustion and atmospheric chemistry. In this work, the kinetics of o-xylene reaction with OH radical has been studied systematically in a wide temperature range of 220-3000 K for high-pressure limit and several selected pressures from 1 torr to 500 atm using multi-structural variational transition state theory with the small-curvature tunneling approximation (MS-CVT/SCT) and the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) method. The calculations fully considered various factors which could affect the accuracy of calculated rate constants including anharmonicity of both low- and high-frequency modes and multiple low-energy conformers, variational effect, and tunneling. The results are in good agreement with available experimental data. The obtained overall rate constants exhibit a nonmonotonic temperature dependence due to the competition between the hydrogen abstraction reactions and addition reactions. At low temperatures, the addition channels are dominant reactions, but the abstraction reactions are also non-ignorable with a ~12% contribution to the overall rate constants at 298 K and 1 atm. Above 800 K, the abstraction reactions become dominant under all the pressure conditions. In addition, we observed a more significant pressure dependence of o-xylene plus OH reaction as compared to the similar toluene plus OH reaction, which is the effect of the additional methyl group. At T = 500-1000 K, the pressure can influence the total rate constants of o-xylene reaction by a factor of up to 2.5. These kinetics data provide us with comprehensive understanding of the mechanisms and pressure-dependence of kinetics for the o-xylene + OH reaction which is also beneficial to study of other similar aromatic hydrocarbon reactions