Canonical variational transition state theory for a radical combination reaction on <i>a</i> <i>b</i> <i>i</i> <i>n</i> <i>i</i> <i>t</i> <i>i</i> <i>o</i> potential energy surfaces: H+CH3

Abstract

Canonical variational transition state theory calculations have been performed for the reaction H+CH3→CH4 on potential energy surfaces based on ab initio calculations. Most vibrations were treated as harmonic. The resulting energy levels and partition functions were compared to empirical rules. For the two rotational degrees of freedom (χ) of CH3 which become bending vibrations in CH4, changing from a harmonic oscillator treatment to a hindered rotor treatment changed the partition functions by an order of magnitude or more for C ⋅ ⋅ ⋅ H distances, R, greater than 0.3 nm. The variation of potential energy with R was taken as a standard Morse function, as a stiff Morse function with a variable parameter β or as a Lippincott function. The value of R for which the rate was minimum was found to vary between 0.25 and 0.5 nm, depending on the temperature and the assumed variation of potential energy with R and χ. Provided the χ bending modes were treated as hindered rotations for large values of R, the limiting values of the rate coefficients were similar to the results of experiments, of classical trajectory calculations, and of a modified version of simple collision theory.