A transition state wave packet study of the H+CH4 reaction.

Transition state wave packet calculations have been carried out to obtain the thermal rate constants for the H+CH(4) reaction on the Jordan-Gilbert potential energy surface. The eight-dimensional model for the X+YCZ(3) type of reaction was employed by restricting the nonreacting CZ(3) group under a C(3V) symmetry. We calculated the cumulative reaction probability for the total angular momentum J=0, from which the thermal rate constants were obtained for the temperature range between 250 and 500 K by employing the J-K shifting approximation. It is found that the eight-dimensional rate constants agree very well with the seven dimensional ones, in which the CH bond length in the nonreacting CH(3) group is fixed, suggesting that the additional mode for the symmetry stretching in CH(3) group does not have any important effect on the reaction within the temperature range considered here. The present transition state wave packet results agree well with rate constants obtained from the previous seven dimensional initial state selected wave packet study, indicating the consistence of the treatments involved in these two studies. On the other hand, it is found that the energy threshold for the cumulative reaction probability for J=0 from the present study is higher than that from the full dimensional multiconfiguration time-dependent Hartree study by about 0.06 eV, resulting in severe discrepancy between the present rate constants and the multiconfiguration time-dependent Hartree results.