Hydrogen Abstraction from Methane and Hydrofluoromethanes by ^•OH Radical:  Modified GAUSSIAN-2 Study

Abstract

The reactions of methane and hydrofluoromethanes (CH4 - n F n , n = 0, 1, 2, or 3) with a hydroxyl radical have been investigated by a 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.

Ab initio study of reactions of hydroxyl radicals with chloro‐ and fluoro‐substituted methanes

“…A measure of these quantities are the troposphere lifetimes of these compounds, which can be determined primarily by estimating the rates of their reactions with ⋅ OH radicals. Given the importance of estimating the atmospheric lifetimes of these species, it is no surprise that the kinetics and energetics of their reactions with ⋅ OH radicals have been the subject of numerous experimental 3–14 and theoretical 15–23 investigations.…”

“…The reaction of methane with ⋅ OH radical has extensively been studied at different levels of ab initio theory 20–34 since it can be considered as a simple model for the key industry reactions of hydrocarbon combustion.…”

“…The third study 22 calculated the temperature dependence of rate constants for the reactions of ⋅ OH radicals with hydrofluoromethanes at the HF/6‐31G(d) and MP2/6‐31G(d) levels. The last study 23 used a modified Gaussian‐2 (G2M) method to describe the kinetics and energetics of the ⋅ OH/hydrofluoromethane systems. It was concluded that the G2M reaction enthalpies and activation energies are accurate and well compared with experimentally derived values.…”

Kinetic parameters for the reaction of hydroxyl radical with CH3OCH2F (HFE‐161) in the temperature range of 200–400 K: Transition state theory and Ab initio calculations

“…In Wigner's method, the transmission coefficient for tunneling is calculated using the expression, where v* is the imaginary frequency associated with the TS. This method is very widely used for the abstraction reactions by various research groups [27][28][29][30][31][32]. The final rate coefficients were calculated using the equation,…”

Computational study on OH radical reaction with CHF2CHFCHF2(HFC‐245ea) between 200 and 400 K

“…The value of (T ) was calculated by using two methods, namely Wigner's empirical method [34] and Eckart's method [35,36]. The conventional transition state theory coupled with Wigner's and Eckart tunneling method was used by various research groups [7,12,[17][18][19][37][38][39][40] to compute the rate coefficients of the abstraction reactions especially in the lower temperatures, relevant to the earth's atmosphere.…”