Implementation of a variational code for the calculation of rate constants and application to barrierless dissociation and radical recombination reactions: CH 3OH = CH 3 + OH

Abstract

Here, a new implementation for calculating canonical variational rate constants is introduced and tested against methanol dissociation rate constants and the (high pressure) radical recombination reaction, CH 3 + OH → CH 3OH. Reaction paths are described at the CASSCF level, with energy corrections at MRMP2. Molecular properties for both stationary and non-stationary points along the reaction path are used for the calculation of rate constants. Different models for the low vibrational frequencies are discussed: harmonic oscillator and free rotor models. The CH 3 + OH → CH 3OH recombination rate constants are determined from dissociation rate constants and equilibrium constants. Our calculated rate constants agree with previously reported data for these reactions, suggesting the good performance of our implemented code in calculating canonical variational rate constants. The rate expressions are suggested: as: k dis (T) = 8.42 × 10 +16 × e -96.18/RT and k rec (T) = 2.05 × 10 -10 × (T/298) -0.24 × e -0.30/RT, in units s -1 and cm 3 molecule -1 s -1, respectively, and over the temperature range 300-2000 K, with activation energies in kcal/mol. © 2012 Wiley Periodicals, Inc. A new implementation for calculating canonical variational rate constants is introduced and tested against methanol dissociation rate constants and the (high pressure) radical recombination reaction, CH 3 + OH → CH 3OH. Different models for treating low vibrational frequencies in the CVTST calculations are presented and discussed. The rate expressions are suggested as: k dis (T) = 8.42 × 10 +16 × e -96.18/RT and k rec (T) = 2.05 × 10 -10 × (T/298) -0.24 × e -0.30/RT, in units s -1 and cm 3 molecule -1 s -1, respectively, and over the temperature range 300-2000 K, with activation energies in kcal/mol. Copyright © 2012 Wiley Periodicals, Inc.