A crossed molecular beam investigation of the reaction Cl+propane → HCl+C₃H₇ using VUV synchrotron radiation as a product probe

We have used the crossed molecular beam technique to study the hydrogen atom abstraction from propane by atomic chlorine over a wide range of collision energies. The experiments were carried out using a recently constructed crossed molecular beam apparatus that utilizes tunable VUV synchrotron radiation for product photoionization. We have measured laboratory TOF spectra and angular distributions for E_coll=8.0, 11.5, and 31.6 kcal/mol. Center-of-mass flux maps were generated from the measured laboratory distributions. The results demonstrate two distinct reaction mechanisms that depend on the impact parameter of the reactive collision. Large impact parameter collisions proceed via a stripping mechanism resulting in forward scattered products with very little momentum change in going from reactant to product. The stripping reactions are most likely dominated by abstraction of secondary hydrogen atoms. Smaller impact parameter collisions lead to direct reactions with an impulsive recoil and are consistent with a preference for a collinear transition state geometry, C-H-Cl. The larger energy along the line of centers in smaller impact parameter collisions most likely makes the effect of a larger barrier to abstraction of primary hydrogen atoms negligible leaving the ratio of primary to secondary hydrogen abstraction to be dictated by simple statistics.