Comparisons of statistical and nonstatistical behavior for bond fission reactions in 1,2-difluoroethane, disilane, and the 2-chloroethyl radical

The unimolecular dissociation reactions of the 2-chloroethyl radical involving C–H and C–Cl 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. Comparison of the 2-chloroethyl radical, 1,2-difluoroethane, and disilane results again demonstrates that the existence of an energy decay rate out of a given bond that is fast relative to the unimolecular reaction rate is not a sufficient condition to guarantee statistical dynamics. It is found that the statistical behavior observed for 2-chloroethyl is due, in large part, to an increase in the potential coupling between the dissociating atom and the beta-carbon that occurs as the bond breaks. This coupling is associated with the conversion of the C–C single bond to a C■C double bond upon C–Cl or C–H bond fission in 2-chloroethyl. It is concluded that unimolecular reactions will tend to exhibit nonstatistical dynamics if (1) the internal energy is close to the dissociation threshold, (2) motion along the reaction coordinate does not produce large energetic changes in one of more bonds in the remainder of the molecule, and (3) there exists a formation coordinate for the activated reactant that is strongly coupled to the dissociation coordinate but only weakly coupled to the other internal coordinates of the molecule.