The temperature and pressure dependence of the rate constant of the methyl-methyl recombination reaction with He bath gas has been studied using time-resolved time-of-flight mass spectrometry. Methyl radicals were produced by the 193 nm laser photolysis of acetone. In the observed temperature (300-700 K) and pressure (0.6-10 Torr) range, the rate constant exhibits a negative temperature dependence and falloff behavior typical for recombination reactions. The integrity of the measurements has been validated by determining the recombination rate constant with Ar (1 Torr) as the bath gas at room temperature and by analyzing the yield of the reaction product, ethane. In addition, rate constants were calculated theoretically using variable reaction coordinate transition state theory in a manner that improves upon the previous treatment of Wagner and Wardlaw by incorporating high-level ab initio results. The calculated high-pressure rate constant can be expressed as k ∞ theory(T) = 7.42 × 10 11 (T/298 K)- 0.69 e -88K/T cm 3 molecule -1 s -1. With reasonable downward energy transfer parameters, the experimentally observed pressure dependence of the rate constants for Ar, He, and H 2 bath gases were reproduced very well using master equation analysis. Troe's equation, describing the T and P dependence of the recombination rate constant, was fit to a set of data for He as bath gas comprised of rate constants from this work and taken from the literature. With k ∞(T) set to be the high-pressure limit rate constant calculated here, the other remaining parameters can be given by k 0(T) = 1.17 × 10 -25 (T/298 K) -3.75 e -494K/T cm 6 molecule 2 s -1 and F cent(T) = e -r/570K.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry