Kinetic study of hydrogen abstraction reactions from n-propyl/ n-butylcyclohexane by hydrogen atom

Hydrogen abstraction reactions of alkylated cycloalkane by radicals play a fundamental role in combustion and atmospheric chemistry. In this work, we select two representative molecules in alkylated cycloalkane fuels, npropylcyclohexane (nPCH) and n-butylcyclohexane (nBCH), to investigate the kinetics of their hydrogen abstraction reactions with hydrogen atom. The rate constants over a broad temperature range of 298-2000 K were calculated by using canonical transition state theory coupled with the multistructural torsional anharmonicity effect (TST/MS-T). We stress the particular importance of considering the MS-T anharmonicity for the investigated species involved in the H-abstraction pathways. Due to the approximate cancelation of the anharmonicity factor between the transition-state species and the reactant, the anharmonicity factors for reactions are spread over a narrower range of 0.7-3.3. The anharmonicity will slightly accelerate the H-abstraction for the nBCH system over the whole temperature range. The current results show that the multistructural torsional anharmonicity will influence the accurate estimation of competing relationships. We found that the total Habstraction rate constants of the nBCH system by the TST/MS-T method are larger by factors of 1.2-1.3 than those for the nPCH system. The present data show that nPCH + H channels are less important by factors of 1.41-5.48 than currently estimated by the commonly used nPCH pyrolysis model. The H-abstraction ratios from the cyclic ring and side chain groups reveal that the rate rule for secondary carbon in chain alkanes does not represent the reactivity of all cycloalkanes well, e.g. the ratio between the cyclic ring and side chain group is high up to 2.8.

Automated summary

Hydrogen abstraction reactions of alkylated cycloalkane by radicals have a fundamental role in combustion and atmospheric chemistry. In this study, the kinetics of hydrogen abstraction reactions between npropylcyclohexane (nPCH) and n-butylcyclohexane (nBCH) molecules and hydrogen atom were investigated. The rate constants were calculated for a wide temperature range using canonical transition state theory coupled with multistructural torsional anharmonicity effect. The results show that the multistructural torsional anharmonicity influences competitive relationships and the reactivity of cycloalkanes, which challenges the commonly used pyrolysis model.