An apparent isotope anomaly in the reaction CH 3 + H/D

Abstract

The reaction between CH 3 and D has been studied by laser flash photolysis, using absorption and resonance fluorescence to monitor decay of CH 3 and D, with [CH 3] ⪢ [D]. A rate constant k 2 = (1.75 ± 0.045) × 10 −10 cm 3 molecule −1 s −1 was obtained for 50 ⩽ P ⩽ 600 Torr, 289 ⩽ T ⩽ 401 K. Absence of a pressure dependence in k 2 demonstrates that the reaction is at its high-pressure limit, because of preferential fragmentation of CH 3D ★ into CH 2D + H. Decay of D is shown to be free from complications at 300 K, but at 400 K regeneration of D by reaction between OD and D 2 has to be included explicitly. Results were analysed by a numerical method according to a scheme which includes this and other, less important, reactions, k 2 shows no dependence on [D 2] over a tenfold range. The value calculated from k 2 for the limiting high pressure rate constant for CH 3 + H is at least a factor of two lower than that obtained by extrapolation of rate data from similar experiments on CH 3 + H.