A classical trajectory study of the dissociation and isomerization of C2H5.

Motivated by photodissociation experiments in which non-RRKM nanosecond lifetimes of the ethyl radical were reported, we have performed a classical trajectory study of the dissociation and isomerization of C2H5 over the energy range 100-150 kcal/mol. We used a customized version of the AIREBO semiempirical potential (Stuart, S. J.; et al. J. Chem. Phys. 2000, 112, 6472-6486) to more accurately describe the gas-phase decomposition of C2H5. This study constitutes one of the first gas-phase applications of this potential form. At each energy, 10,000 trajectories were run and all underwent dissociation in less than 100 ps. The calculated dissociation rate constants are consistent with RRKM models; no evidence was found for nanosecond lifetimes. An analytic kinetics model of isomerization/dissociation competition was developed that incorporated incomplete mode mixing through a postulated divided phase space. The fits of the model to the trajectory data are good and represent the trajectory results in detail through repeated isomerizations at all energies. The model correctly displays single exponential decay at lower energies, but at higher energies, multiexponential decay due to incomplete mode mixing becomes more apparent. At both ends of the energy range, we carried out similar trajectory studies on CD2CH3 to examine isotopic scrambling. The results largely support the assumption that a H or a D atom is equally likely to dissociate from the mixed-isotope methyl end of the molecule. The calculated fraction of products that have the D atom dissociation is ∼20%, twice the experimental value available at one energy within our range. The calculated degree of isotopic scrambling is non-monotonic with respect to energy due to a non-monotonic ratio of the isomerization to dissociation rate constants.