RATE-CONSTANT RATIO FOR THE REACTIONS OF OH WITH CH3D AND CH4

The rate constant ratio and temperature dependence have been determined for the reactions of OH with CH3D and CH4. The compounds HFC-134a (CF3CFH2) and HCFC-141b(CH3CCl2F) were used as transfer standards, whereby the ratios k(CH3D)/k(134a) and k(CH3D)/k(141b) were measured as a function of temperature in the range 298-360 K. Combined with previous ratio measurements involving CH4, HFC-134a, and HCFC-141b, the results yield the (preferred) value k(CH3D)/k(CH4) = 1.10 exp(-75/T). The ratio at 298 K is 0.86. At 277 K, which is typical of the troposphere, the ratio is 0.84. Previous laboratory measurements of Gordon and Mulac,2 which seemed to suggest a much smaller ratio, are shown to be reconcilable with the present results by an alternative treatment of their data. The results of the present work are compatible with predictions based on correlations of OH abstraction rate constants with C-H and C-D bond energies and reaction exothermicities.