Trajectory studies of model H–C–C→H+C = C dissociation. II. Angular momenta and energy partitioning and their relation to non-RRKM dynamics

Abstract

The model alkyl dissociation reaction H–C–C→H+C = C has been studied on a potential energy surface derived from an analytic potential energy function for ethyl radical dissociation. Nonrandom excitation of H–C–C is simulated by the chemical activation reaction H+C = C→H–C–C, and different initial relative translational, rotational, and vibrational energies are investigated. Comparisons are made between the unimolecular dynamics of nonrandomly excited H–C–C radicals and those excited randomly. These two types of excitation yield strikingly different unimolecular lifetime distributions, each non-RRKM. However, if angular momentum constraints are propertly included, the partitioning of product energies is independent of the excitation process. For total energies slightly in excess of the dissociation energy the energy distributions at the dissociation barrier are in excellent agreement with the RRKM predictions, and the nonstatistical product energies arise from the preferential release of potential energy in the exit channel to relative translation. As the total energy is increased, the relative translational energy at the dissociation barrier exceeds the RRKM prediction. This results from the system remaining on the same effective potential energy curve before and after crossing the dissociation barrier, which nonstatistically apportions energy to relative translation. A dynamical model based upon angular momentum constraints and conservations yields a quantitative fit to the partitioning of angular momenta and energy in the products.