Studies on the Kinetics of the CH + H 2 Reaction and Implications for the Reverse Reaction, 3 CH 2 + H.

The reaction of CH radicals with H 2 has been studied by the use of laser flash photolysis, probing CH decays under pseudo-first-order conditions using laser-induced fluorescence (LIF) over the temperature range 298-748 K at pressures of ∼5-100 Torr. Careful data analysis was required to separate the CH LIF signal at ∼428 nm from broad background fluorescence, and this interference increased with temperature. We believe that this interference may have been the source of anomalous pressure behavior reported previously in the literature (Brownsword, R. A.; J. Chem. Phys. 1997, 106, 7662-7677). The rate coefficient k 1 shows complex behavior: at low pressures, the main route for the CH 3 * formed from the insertion of CH into H 2 is the formation of 3 CH 2 + H, and as the pressure is increased, CH 3 * is increasingly stabilized to CH 3 . The kinetic data on CH + H 2 have been combined with experimental shock tube data on methyl decomposition and literature thermochemistry within a master equation program to precisely determine the rate coefficient of the reverse reaction, 3 CH 2 + H → CH + H 2 . The resulting parametrization is k CH 2 +H ( T ) = (1.69 ± 0.11) × 10 -10 × ( T /298 K) (0.05±0.010) cm 3 molecule -1 s -1 , where the errors are 1σ.