Theoretical Study on Kinetics of Hydrogen Abstraction Reactions for Alkylcyclohexane

In the domains of combustion and atmospheric chemistry, radicals play a crucial role in hydrogen abstraction reactions involving alkylated cycloalkanes and these reactions are fundamentally significant. In this study, we choose two typical molecules from short-chain alkylcyclohexane fuels, namely, methylcyclohexane (MCH) and ethylcyclohexane (ECH), to explore the kinetics of H-abstractions with a hydrogen atom. Multistructural canonical variational transition state theory (MS-VTST) combined with the multidimensional tunneling method was employed to calculate the rate constants across a wide temperature range from 298 to 2000 K. The fastest reaction rate of hydrogen abstraction occurs at the tertiary carbon site. The impact of the MS-T anharmonicity, the tunneling effect, and the variational effect on these site-specific hydrogen-abstraction pathways will be elucidated. Specifically, the tunneling effect becomes more pronounced when the temperature is below 450 K. The variational effect has a minor effect on the rate. At the same carbon position, the rate of H abstractions is greatly influenced by the MS-T anharmonicity. Specifically, of all carbon positions, the primary carbon position on the side chain is subject to the most significant impact. We also compared this with H-abstractions for <i>n</i>-propylcyclohexane and <i>n</i>-butylcyclohexane, examining the differences in the multistructural torsional anharmonicity and reaction rates. In this study, we obtain some systematic conclusions and site-specific kinetic data. The aim of this is to improve the precision of the chemical kinetic model regarding the pyrolysis of long-chain cycloalkanes.