Dynamics of the F2 reaction with propene: the effect of methyl substitution.

The reaction of F2 + C3H6 has been investigated with the crossed molecular beam technique. The only observed primary product channel is F + C3H6F while the HF + C3H5F channel cannot be found. The reaction cross section was measured as a function of collision energy and the reaction threshold was determined to be 2.4 +/- 0.3 kcal/mol. Compared to the reaction threshold of the F2 + C2H4 reaction, the methyl substitution effectively reduces the reaction threshold by about 3 kcal/mol. The product time-of-flight spectra and angular distributions were measured and analyzed. The angular distribution displays strongly backward, indicating that the reaction is much faster than rotation. All experimental results support a rebound reaction mechanism, which agrees with the structure of the calculated transition state. The transition state geometry also suggests an early barrier; such dynamics is consistent with the observed small kinetic energy release of the products. Except for the different values of the reaction thresholds, the dynamics of the F2 + C2H4 and F2 + C3H6 reactions are remarkably similar.