Reaction path Hamiltonian for the association reactions CH3 + H and CH3 + D

Coupling coefficients and normal mode frequencies appearing in the reaction path Hamiltonian formulated by Miller, Handy, and Adams have been calculated for the title reactions as a function of distance along the reaction path. The calculation of the coupling coefficients requires the use of symmetry coordinates which are described herein. It is found that the carbon–hydrogen separation provides an excellent representation of the reaction path coordinate, being linearly related to it. The coupling coefficients for CH3 + H and CH3 + D are approximately the same in the region of variational transition states and do not support the suggestion that an apparent isotope anomaly in the experimentally derived rates of these reactions might be attributable to different dynamics along the reaction path. The relative magnitudes of coupling coefficients for CH3 + H are used to assess some of the usual assumptions in variational transition state theory concerning separability of different types of motion in the transition state region.