Rate constants for H+CH4, CH3+H2, and CH4 dissociation at high temperature

The Laser Photolysis-Shock Tube technique coupled with H-atom atomic resonance absorption spectrometry has been used to study the reaction, H + CH4 CH3 + H-2, over the temperature range, 928-1697 K. Shock-tube studies on the reverse of this reaction, CH3 + H-2 --> H + CH4, using CH(3)l dissociation in the presence of H2 yielded H-atom formation rates and rate constants for the reverse process over the temperature range, 1269-1806 K. These results were transformed (using well-established equilibrium constants) to the forward direction, The combined results for H + CH4 can be represented by an experimental three parameter expression, k = 6.78 x 10(-21) T (3.156) exp(-4406 K/T) cm(3) molecule(-1) s(-1) (348-1950 K) that was evaluated from the present work and seven previous studies Using this evaluation, disagreements between previously reported values for the dissociation Of CH4 could be reconciled The thermal decomposition of CH4 was then studied in Kr bath gas. The dissociation results agreed with the earlier studies and were theoretically modeled with the Troe formalism. The energy transfer parameter necessary to explain both the present results and those of Kiefer and Kumaran (J Phys Chem 1993, 97,414) is, -< Delta E > (all)/cm(-1) = 0.3323 T-0.7. The low temperature data on the reverse reaction, H + CH3 (in He) from Brouard et al. (I Phys Chem 1989, 93, 4047) were also modeled with the Troe formalism. Lastly, the rate constant for H + CH4 was theoretically calculated using conventional transition state theory with Eckart tunneling corrections, The potential energy surface used was from Kraka et al. (J Chem Phys 1993, 99, 5306) and the derived T-dependence with this method agreed almost perfectly with the experimental evaluation. (C) 2001 John Wiley & Sons, Inc.