Unimolecular dissociation of methane: A trajectory study using Metropolis sampling

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Abstract
Classical trajectories have been used to compute the total and individual microcanonical rate coefficients as a function of energy for CH4→CH2+H2 and CH4→CH3+H. Reaction mechanisms and product translational energy distributions for both reaction channels have also been obtained as a function of the initial CH4 internal energy. A semiempirical valence‐force potential‐energy surface was used throughout. Averaging over the phase space available to CH4 was accomplished by a Metropolis‐type Monte Carlo procedure. Excellent first‐order decay plots are obtained by the integration of 103 or less trajectories. The branching ratio for the two open dissociation channels obtained from the decay plots is near unity over the energy range studied (6.0–7.25 eV). The total microcanonical rate coefficient for CH4 reaction exhibits an RRK‐type behavior as a function of energy. The computed distributions of CH2+H2 relative translational energy peak at energies well above zero as expected for a decomposition channel which has a back‐reaction barrier of about 0.345 eV. However, the corresponding distributions for the CH3+H product are also found to peak above zero even though there is no back‐reaction barrier. The results suggest that this effect is due to the fact that the total CH4 energy is well above the threshold for reaction. The mechanism for the three‐center elimination is found to be a ‘‘half‐reaction’’ of CH3+H.