Reactive scattering from double-minimum potentials

Abstract

We present a crossed-beam study of the elimination reactions of Li⁺ with t-butyl alcohol (TBA), isopropyl chloride (IPC), n-propyl chloride (NPC), and isopropyl bromide (IPB) over the collision energy range 0.6–2.5 eV. Full differential cross-sections at collision energies up to 1.2 eV show increasing asymmetry in the angular distributions as a function of initial collision energy and suggest that the reactions take place on a timescale of 0.2–0.5 ps. The observation of Li⁺ being non-reactively ejected from the initial encounter complex formed by approaching reactants is consistent with a double-minimum reaction coordinate and a comparison of the non-reactive flux with the flux for elimination products allows an estimate of the heights of intermediate isomerization barriers separating the wells on the potential surface. The kinetic energy distributions for Li⁺(HX) and Li⁺(olefin) products and non-reactively scattered Li⁺ are in reasonable agreement with phase-space theory calculations which include all product vibrational modes. The energy dependence of the Li⁺(HX)/Li⁺(olefin) branching ratio can be accounted for with statistical calculations which include product dissociation. The discrepancy between the statistical recoil energy distributions and apparent complex lifetimes several orders of magnitude shorter than the predictions of RRKM theory can be understood in terms of the specific dynamics of passage from the crest of the isomerization barrier to the second well of the potential surface and the apparent failure of the system to re-establish microcanonical equilibrium in that well.