Study of gas-phase reactions within the modified Marcus model. II. CH3OH + CH3 → CH2OH + CH4

Abstract

In the framework of the Marcus theoretical model, using the intramolecular reorganization (IMR) method, the kinetics of the CH3OH + CH3 → CH2OH + CH4 reaction was studied over the temperature range of 10–550 K. The electronic activation energy (Ea) was calculated at the UCCSD(T)/6-31+G**//B3LYP/6–31+G** level. The calculation of the reaction rate constant, k, defined as the integral over the distance Q(C⋯C), was carried out on the assumption that for a hydrogen atom in the activated complex (AC) two extreme types of vibrations are possible: isolated vibration in the reagent well (oscillatory model 1, in which all frequencies are identified as real) and vibration in the C⋯H⋯C structure (oscillatory model 2 corresponding to TST). In both cases, the temperature dependence of the rate constant includes nonlinear (80–550 K) and linear (10–80 K) sections. It is shown that the change in the dependence character is due to the presence of a minimum in the Q - Ea plot. Kinetic data were used to analyse the “temperature plateau” phenomenon, observed for the CH3OH + CH3 reaction in the solid phase. The course of the temperature dependence of this reaction at 10–150 K can be reproduced assuming that the energy of the reagents in the solid phase is close to the average for the effective interval Q of the enthalpy of activation of the gas-phase reaction, ΔHm∗, in a temperature range of 10–40 K. The value of the deuterium KIE for model 1 is ∼104, which is about an order of magnitude higher than the experimental value.