Distribution of reaction products (theory). Part 12.—Microscopic branching in H + XY → HX + Y, HY + X (X, Y = halogens)

Abstract

3D trajectory studies are reported for several potential energy surfaces that could serve as models for the reaction H + ICl. This reaction exhibits macroscopic branching to give HCl + I or HI + Cl. The surfaces yielded product energy distributions suggestive of significant bimodality in the HCl product, but not the HI; i.e., there was evidence of microscopic branching for the macroscopic branch involving reaction with the more electronegative of the halogen atoms, X. All of the surfaces were characterised by a barrier to approach of H at the Cl end of ICl, but attraction at the I end, in conformity with evidence from molecular beam studies regarding the stability of complexes HYX in contrast to HXY (electronegativity χX > χY). Extensive calculations were performed on one of these surfaces for room temperature (T0TRANS= 300 K) and elevated translational temperature (T0TRANS= 2685 K). The findings were in qualitative accord with the bimodal vibration–rotation distributions of HX observed in infrared chemiluminescence studies of reactions of the type H + XY. The bimodal distribution could be identified with two dynamically different paths for HCl formation (microscopic branching). The HCl formed with the lower internal energy, E′int, resulted from reaction of H directly at the Cl end of ICl, whereas the HCl formed with higher E′int was produced by migration of H from the I to the Cl, following a lingering interaction of H with I. Migration occurred late in the encounter, by insertion of H into the extended I–Cl bond. The collision energy dependence of these two microscopic branches (“direct” and “migratory”) differed notably. The probability of direct reaction, since it involved barrier-crossing (Ec= 1.6 kcal mol–1), increased steeply with collision energy, whereas the probability of the migratory dynamics fell (Ec= 0 kcal mol–1 from the I end). As a consequence the HCl product vibration-rotation distribution altered markedly in going from E0TRANS= 300 to 2685 K, in qualitative accord with the findings from an infrared chemiluminescence study. By contrast the energy distribution for the other product, HI, showed insignificant bimodality at 300 K, and no dramatic change in product vib–rotational distribution in going to 2685 K; microscopic branching appeared to be negligible for reaction to form HI.