A direct transition state theory based study of methyl radical recombination kinetics

Multireference configuration interaction based quantum chemical estimates a re directly implemented in a variational transition state theory based anal ysis of the kinetics of methyl radical recombination. Separations ranging f rom 5.5 to 1.9 Angstrom are considered for two separate forms for the react ion coordinate. The a priori prediction for the high-pressure limit rate co nstant gradually decreases with increasing temperature, with a net decrease of a factor of 1.7 from 300 to 1700 K. Near room temperature, this theoret ical estimate is in quantitative agreement with the experimental data. At h igher temperatures, comparison between theory and experiment requires a mod el for the pressure dependence. Master equation calculations employing the exponential down energy transfer model suggest that the theoretical and exp erimental high-pressure limits gradually diverge with increasing temperatur e, with the former bring about 3 times greater than the latter at 1700 K. T he comparison with experiment also suggests that the energy transfer coeffi cient, [Delta E-down], increases with increasing temperature.