Molecular beam kinetics: Exchange reactions of deuterium atoms with hydrogen halides

A crossed-beam study finds the reactions of D atoms with HCl, HBr, and HI give large yields of DCl, DBr, and DI, corresponding to reaction cross sections of the order 1--10 A2. This demonstrates that the exceptionally small steric factors or transmission coefficients previously inferred from analysis of chain reaction mechanisms are low by several orders of magnitude. The reactively scattered deuterium halides recoil predominantly into the backward hemisphere with respect to the incident D atoms. In each case, the reaction exoergicity is approx.1 kcal/mole and the mean initial collision energy is approx. 9 kcal/mole (with the D beam at 2800 degreeK, the hydrogen halide at 250 degreeK). The most probable final relative translational energy of the products is approximately-greater-than4 kcal/mole for the HCl and HBr reactions and 7 kcal/mole for the HI reaction. The magnitude of the reaction cross sections and the predominant backward recoil and translational energy of the products are consistent with recent classical trajectory calculations based on semiempirical potential surfaces