The translational energy dependence of the F+C2H4 →H+C2H3F reaction cross section near threshold

We have carried out crossed molecular beam studies of the substitution reaction F+C2 H4 →H+C2 H3 F in the collision energy range 0.8–2.5 kcal/mol using a velocity selected F atom beam. The relative substitution cross section is found to decrease with increasing collision energy indicating that if there is a potential energy barrier to F atom addition to C2 H4 it is much less than 0.8 kcal/mol and that the transition state for addition occurs early along the reaction coordinate. These results agree well with ab initio calculations. Although the product translational energy distributions are similar to those obtained from earlier work at higher collision energies, the C2 H3 F angular distributions suggest that a wider range of reactant approach geometries lead to products at lower energies.