Collision-induced activation of the β-hydride elimination reaction of isobutyl iodide dissociatively chemisorbed on Al(111)

The collision-induced activation of the endothermic surface reaction of isobutyl iodide chemisorbed on an Al(111) surface is demonstrated using inert-gas, hyperthermal atomic beams. The collision-induced reaction (CIR) is highly selective towards promoting the beta-hydride elimination pathway of the chemisorbed isobutyl fragments. The cross section for the collision-induced reaction was measured over a wide range of energies (14-92 kcal/mol) at normal incidence for Ar, Kr, and Xe atom beams. The CLR cross section exhibits scaling as a function of the normal kinetic energy of the incident atoms. The threshold energy for the beta-hydride elimination reaction calculated from the experimental results using a classical energy transfer model is similar to 1.1 eV (similar to 25 kcal/mol). This value is in excellent agreement with that obtained from an analysis of the thermally activated kinetics of the reaction. The measured cross section shows a complex dependence on both the incident energy of the colliding atom and the thermal energy provided by the surface where the two energy modes are interchangeable. The dynamics are explained on the basis of an impulsive, bimolecular collision event where the beta-hydride elimination proceeds via a possible tunneling mechanism. The threshold energy calculated in this manner is an upper limit given that it is derived from an analysis which ignores excitations of the internal modes of the chemisorbed alkyl groups. (C) 1998 American Institute of Physics. [S0021-9606(98)02420-9].