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 produ ct energy distributions of the F + C2H4 --> C2H3F + H collision reaction. T he shifting and broadening of the product translational energy distribution as the system moves from the exit-channel barrier to the products was stud ied. Since properties associated with the rupturing C ... H bond are simila r for the C(2)H(5)double dagger and C2H4F double dagger exit-channel barrie rs, and integration of the C(2)H(5)double dagger --> C2H4 + H reaction is a pproximately 2.5 times faster than the C2H4F double dagger --> C2H3F + H re action, trajectories of the former reaction were propagated to gain insight into the exit-channel dynamics. Ensemble averaged results for C(2)H(5)doub le dagger dissociation are well described by a model based on isotropic exi t-channel dynamics which assumes that the product relative translational di stribution arises from the centrifugal potential and relative translational energy distributions at the exit-channel barrier plus the exit-channel pot ential release. The width of the product translational energy distribution is sensitive to overall rotational angular momentum and its partitioning be tween C2H4... H double dagger orbital angular momentum and C(2)H(4)double d agger rotational angular momentum. The simulated product translational ener gy distribution for the C2H4F double dagger --> C2H3F + H reaction is broad ened by relative translation-vibrational couplings in the exit-channel and is similar to the distribution used to fit crossed molecular beam data. App roximately 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.