On the hydrogen abstraction reaction CH3 + CH3CN → CH4 + CH2CN. Importance of short-cut tunneling paths on the two-dimensional potential energy surface

The hydrogen atom abstraction reaction, CH3 + CH3CN → CH4 + CH2CN has been studied by means of both the ab initio MO and the reaction dynamics methods in order to shed light on the reaction mechanism at low temperature. The potential energy surface (PES [for the reaction was calculated by means of the MR-SD-CI method. By using the PES obtained, the canonical rate constants from high- to low-temperature regions were calculated by means of the convenient transition state theory along the transition state and the short-cut paths on two-dimensional (2D) PES. The dynamics calculations lead to the conclusion that the quantum mechanical tunneling effect strongly affects the reaction rate and that the short-cut path plays an important role in the reaction dynamics at low temperature. The deuterium abstraction reaction, CH3 + CH3CN → CH4 + CH2CN was also investigated for comparison. The deuterium substitution caused a large isotope effect on the reaction rate below 100 K. The ratio of the reaction rates (kHkD) was estimated to be ≈ 103 at 77 K. The present results were in good agreement with the experimental feature obtained at low temperature.