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

Abstract

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.

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