The reactions of methane and hydrofluoromethanes (CH4-nFn, n = 0, 1, 2, or 3) with a hydroxyl radical have been investigated by modified GAUSSIAN-2 (G2M) method. Reaction enthalpies have been compared to those obtained by original G2 and G2MP2 schemes. The average absolute error for the reaction enthalpies calculated with the G2M method was smaller than those calculated with the G2 and G2MP2 schemes. G2M reaction enthalpies were of chemical accuracy. Moreover, the G2M method and selected MP2 schemes (those with the smallest average errors in the calculations for the reaction enthalpies) were used to compute the classical barrier heights. According to the transition state theory, the reaction rates were computed in the temperature range of 280°420 K and then the activation energies were obtained by the least-squares fitting to the Arrhenius equation. The activation energies calculated by using G2M barrier heights showed the best agreement with the experimentally derived values. The influence of the fluorine substitution effect on the activation energy was correctly described only by G2M barrier heights. |
