Temperature and Isotope Dependence of the Reaction of Methyl Radicals with Deuterium Atoms

The reactions of methyl isotopomers (CH3, CH2D, and CHD2) with excess deuterium atoms have been studied using discharge flow/mass spectrometry at 298 K and at pressures of ∼1 Torr (helium). At these low pressures the initially formed methane complex is not stabilized. However, zero-point energy differences between methyl isotopomers mean that ejection of H from energized methane is favored. In consequence, regeneration of the reactant isotopomer is inefficient and values of k 1a - c may be extracted from the appropriate methyl radical decay. The experimental values can be used to calculate the high-pressure values for each isotopic reaction: (1a) CH3 + D → CH2D + H, = (2.3 ± 0.6) × 10-10 cm3 molecule-1 s-1; (1b) CH2D + D → CHD2 + H, = (2.1 ± 0.5) × 10-10 cm3 molecule-1 s-1; (1c) CHD2 + D → CD3 + H, = (1.9 ± 0.5) × 10-10 cm3 molecule-1 s-1. These, in turn, can be corrected for isotopic substitution and averaged to give a value of (2.9 ± 0.7) × 10-10 cm3 molecule-1 s-1 for the limiting high-pressure recombination rate coefficient of CH3 and H. The errors of ∼25% are estimates of both the statistical and systematic errors in the measurements and calculations. The results are in agreement with an earlier direct determination of reaction 1a and recent theoretical calculations. The previous direct studies of CH3 + H in the fall off region have been reanalyzed using master equation techniques and are now shown to be in good agreement with current experimental and theoretical calculations. Reaction 1c was also studied at 200 K, with k 1c falling by approximately 35% from its room-temperature value, confirming theoretical predictions of a positive temperature dependence for the high-pressure limiting rate coefficient for the reaction CH3 + H + M → CH4 + M.