An ab initio quasi-classical direct dynamics investigation of the F + C₂H₄ → C₂H₃F + H product energy distributions

A direct dynamics technique, using energies, forces and second derivatives calculated at the UHF/6-31G* level of theory, was used to investigate product energy distributions of the F + C₂H₄ → C₂H₃F + H collision reaction. The shifting and broadening of the product translational energy distribution as the system moves from the exit-channel barrier to the products was studied. Since properties associated with the rupturing C···H bond are similar for the C₂H₅(+) and C₂H₄F(+) exit-channel barriers, and integration of the C₂H₅(+) → C₂H₄ ± H reaction is approximately 2.5 times faster than the C₂H₄F(+) → C₂H₃F + H reaction, trajectories of the former reaction were propagated to gain insight into the exit-channel dynamics. Ensemble averaged results for C₂H₅(+) dissociation are well described by a model based on isotropic exit-channel dynamics which assumes that the product relative translational distribution arises from the centrifugal potential and relative translational energy distributions at the exit-channel barrier plus the exit-channel potential release. The width of the product translational energy distribution is sensitive to overall rotational angular momentum and its partitioning between C₂H₄···H(+) orbital angular momentum and C₂H₄ rotational angular momentum. The simulated product translational energy distribution for the C₂H₄F(+) → C₂H₃F + H reaction is broadened by relative translation-vibrational couplings in the exit-channel and is similar to the distribution used to fit crossed molecular beam data. Approximately 50% of the available energy is in product relative translation, which also agrees with experiment. RRKM calculations indicate that a second reaction mechanism, involving 1-2 hydrogen migration prior to C-···H bond fission, does not significantly contribute to C₂H₃F + H product formation.