A Direct Transition State Theory Based Study of Methyl Radical Recombination Kinetics

Multireference configuration interaction based quantum chemical estimates are directly implemented in a variational transition state theory based analysis of the kinetics of methyl radical recombination. Separations ranging from 5.5 to 1.9 Å are considered for two separate forms for the reaction coordinate. The a priori prediction for the high-pressure limit rate constant gradually decreases with increasing temperature, with a net decrease of a factor of 1.7 from 300 to 1700 K. Near room temperature, this theoretical estimate is in quantitative agreement with the experimental data. At higher temperatures, comparison between theory and experiment requires a model for the pressure dependence. Master equation calculations employing the exponential down energy transfer model suggest that the theoretical and experimental high-pressure limits gradually diverge with increasing temperature, with the former being about 3 times greater than the latter at 1700 K. The comparison with experiment also suggests that the energy transfer coefficient, 〈ΔEdown〉, increases with increasing temperature.