The influence of hindered rotations on recombination/dissociation kinetics

Abstract

AbstractThe variational canonical flexible transition state theory expressions for the thermal reaction rate constant are reviewed and then applied to the barrier‐less recombination reactions H + CCH, H + CF3, and CH + H2. The rate constant expressions can classically accommodate any description of the hindering potential governing relative orientation changes between the reactants at a given distance. The distance held fixed during the relative orientation changes can be fully optimized. The three applications display comparable and large variations between the harmonic oscillator and free rotor limits of the hindering potential. The actual hindering potentials are all derived from similar ab initio electronic structure calculations. With these potentials, the rate constant for H + CF3 is somewhat free‐rotor like, the rate constant for CH + H2 is mostly harmonic‐oscillator like, and the rate constant for H+CCH is largely in between these two limits.