Multiple transition states in unimolecular reactions

Abstract

We report the results of a variational TST calculation on the reaction CH+4 → CH+3+H in the zero angular momentum limit using a simple model to account for the smooth conversion of the transitional modes from vibrations of the reagent to rotational-orbital modes of the products as a function of energy and position along the reaction coordinate. The calculations show that the simultaneous existence of multiple transition states depends strongly on the behavior of the transitional modes. Specifically, if the transitional modes loosen early in the decomposition process the orbiting transition state is the dominant transition state, and in extreme cases the only transition state, until energies well above threshold are reached. If, on the other hand, the loosening occurs late in the decomposition, the oribiting transition state, although present, may always be dominated by the ‘‘free energy bottleneck’’ tight transition state. Since the long range potential only mildly affects the orbiting transition state in the zero angular momentum limit, our results suggest that multiple transition states can also arise in neutral systems.