Reactive cross sections for isotopic variants of the H′+H′′Br abstraction reaction: A classical trajectory study

Differing dynamics for isotopically related reactions (H′+H→H′H\+Br, with H′ = H, D and H\ = H, D in the present work) must be explicable in terms of different patterns of motion under the influence of the same forces; i.e., kinematic effects. We have identified two prominent kinematic effects in the family of reactions under study. Both effects rendered D+HBr [symbolized (D, H)] and H+DBr (H, D) the extreme mass combinations, but the effects were opposite in sign. The alignment effect increased the reactive cross section for (D, H) compared with (H, D), since the former mass combination had longer in which to align (slower approach) and aligned more rapidly (faster molecular rotation). The rebound effect, by contrast, reduced the reactive cross section for (D, H) relative to (H, D), since in the former case the light central atom tended to rebound to re-form reagents. Either the alignment or the rebound effect could be made to dominate, depending on the choice of potential-energy surface; an increased bending force constant favored the former effect, and an earlier barrier the latter.