Monte Carlo transition-state study of angular momentum effects on the unimolecular dissociation of CH4 on the Duchovic–Hase–Schlegel ab initio surface

A previously formulated Monte Carlo transition-state theory approach to unimolecular reactions has been extended to the calculation of microcanonical rate coefficients for specific angular momentum states. The method is applied to a study of two-center dissociation of CH4 on the Duchovic–Hase–Schlegel (DHS) ab initio potential-energy surface. The rotationally averaged microcanonical rate coefficients on the DHS surface are found to be consistently less than those previously calculated on a semiempirical surface due principally to the larger C–H bond dissciation energy on the DHS surface. The rate coefficients are found to be relatively insensitive to the other topographical features of the potential-energy surface. Angular momentum is found to reduce k(E;J) by an amount in excess of the centrifugal effect. The magnitude of the angular momentum effect is found to decrease as the ratio of the rotational energy to the energy in excess of threshold decreases. Overall, the MCTST procedure is found to be computationally efficient for such calculations.