Simplified statistical adiabatic channel model calculations of the reaction H+CH3→CH4 on a b i n i t i o potential energy surfaces

Abstract

The simplified version of the statistical adiabatic channel model has been applied to calculate limiting high pressure rate coefficients for the title recombination reaction between 200–2000 K. The energy pattern of the channel threshold energies was analyzed in terms of recently reported ab initio potential energy surfaces. The calculated rate coefficients are in very good agreement with quasiclassical trajectory, variational transition state theory, and canonical variational transition state theory calculations performed on the same surfaces. An analysis with a factorized formalism allows one to study separately the role that the motion on the reaction coordinate and the degrees of freedom orthogonal to them, play on the rate coefficients. A recent experimental rate coefficient lies between the values calculated with the ab initio surfaces constructed with data of Duchovic, Hase, and Schlegel–Brown and Truhlar, and with similar information from Hirst. In addition, translational energy dependent thermal averaged cross sections were computed and compared with quasiclassical trajectory results. Based on an experimental limiting low pressure rate coefficient, the average energy transferred per each He–CH4 collision was calculated using the low pressure range unimolecular rate theory with explicit account of the potential energy surface features. Confrontation with direct measurement of collisional energy transfer shows that the abovementioned surfaces also give the best agreement between the experimental and calculated values.