Comparisons of Statistical and Nonstatistical Behavior for Bond Fission Reactionsin 1,2-Difluoroethane, Disilane, and the 2-Chloroethyl Radical

摘要

The unimolecular dissociation reactions of the 2-chloroethyl radical involving C-H and C-C1 bond fissions are investigated using classical trajectories and two variational transition-state theory methods on the same potential-energy surface. The transition-state theory methods employed are the efficient microcanonical sampling-transition state theory method, previously used to study the decomposition dynamics of disilane and 1,2-difluoroethane, and a J-conserving variant of this method that introduces constraining equations in the efficient microcanonical sampling procedure, such that the sampling is restricted to phase-space points associated with both a constant value of the system energy and total angular momentum. The results demonstrate that the unimolecular dissociation of the 2-chloroethyl radical is well described by statistical theories that assume an equal weight for all energetically accessible phase-space points. The results obtained from the statistical calculations form upper bounds to the trajectory-computed rate coefficients as expected for a statistical system. In addition, there is no evidence of mode-specific dynamics present in the trajectory results. The statistical behavior of the 2-chloroethyl radical stands in sharp contrast to the dissociation dynamics of disilane and 1,2-difluoroethane which have previously been shown to exhibit pronounced nonstatistical effects. It is shown that the existence of nonstatistical behavior cannot, in general, be qualitatively predicted from energy considerations alone.