Multi-structural variational kinetics study on hydrogen abstraction reactions of cyclopentanol and cyclopentane by hydroperoxyl radical with anharmonicity, recrossing and tunneling effects.

To date, it is still a challenge to accurately predict the reaction kinetics of H-atom abstractions by the HO radical in cycloalkanes and cyclic alcohols, which plays a fundamental role in both atmospheric and combustion chemistry. Cyclopentanol (CPL) is a novel alternative fuel derived from lignocellulosic biomass, while cyclopentane (CPT) is a representative component in conventional fossil fuels. Both are promising gasoline additives due to their high-octane and knock-resistant features, and therefore selected as our target molecules for detailed theoretical investigation in this work. The rate constants of H-abstraction by HO over a wide temperature range of 200-2000 K were calculated using multi-structural variational transition state theory (MS-CVT) in multi-dimensional small-curvature tunneling approximation (SCT) including multiple structural and torsional potential anharmonicity (MS-T), recrossing and tunneling effects. For comparison, the "single-structural rigid-rotor quasiharmonic oscillator" (SS-QH) rate constants corrected by multi-structural local harmonic approximation (MS-LH) and different quantum tunneling methods including one-dimensional Eckart and zero-curvature tunneling (ZCT) were also obtained in this work. The importance of considering the anharmonicity, recrossing and multi-dimensional tunneling effects was stressed by analyzing the MS-T and MS-LH factors and transmission coefficients for each studied reaction. In general, the MS-T anharmonicity was found to increase the rate constants by certain values especially at high temperatures; the multi-dimensional tunneling effect significantly increased the rate constants at low temperatures as expected; and the recrossing effect decreased the rate constants but it was only remarkable for the α and β carbon sites in CPL and the secondary carbon site in CPT. The comparison of the results from the different theoretical kinetic corrections in this work or empirically estimated methods from the literature showed significant deviations in the site-specific rate constants, branching ratios (competition of different channels) and Arrhenius activation energies with pronounced temperature dependence.