The hydrogen abstraction reaction H + C2H6 → H2(v,j) + C2H5. Part II. Theoretical kinetics and dynamics study

Abstract

Two important issues motivated the present study: the role of the tunnelling contribution at low temperatures and the role of the alkyl fragment in the dynamics. Using a recently developed full-dimensional analytical potential energy surface (PES), named PES-2018 (Part I), kinetics and dynamics studies were performed. The kinetics study was performed using the variational transition-state theory with multidimensional tunnelling over the temperature range of 200–2000 K. At high temperatures, T ≥ 400 K, the calculated thermal rate constants reproduce the experimental evidence, with differences of 25%, with respect to experimental measures, while at low temperatures, T ≤ 300 K, the tunnelling effect plays an important role although, unfortunately, no experimental information is available for comparison. We found that the tunnelling contribution is strongly dependent on the theoretical approach used to calculate it, with differences of a factor of about ∼30. For the dynamics, quasi-classical trajectory calculations were performed at different collision energies in the range of 10–50 kcal mol⁻¹, taking into account the zero-point energy violation problem in the final analysis. Excitation function increased with collision energy, reproducing the experimental values, and the H₂(v,j) product showed cold vibrational and rotational distributions, thus again simulating experiments. We found that the ethyl radical product presents small internal energy, similar to the methyl radical product in the H + CH₄ reaction, indicating that a priori the size of the alkyl radical does not play an important role in the dynamics.