Orientation isotope effect in ion–molecule reactions

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Abstract
A simple kinematic model is proposed to account for the kinetic isotope effect (KIE) upon the reactions of simple ions X+ with HD. This model is based upon (1) the fact that the displacement of the center of polarizability from the center of mass in the HD molecule will affect the alignment of the reactants, (2) the fact that, for many ions X+, reaction must occur by a surface crossing mechanism, and (3) the assumption that the ratio XH+/(XH++XD+) equals the fraction of intimate collisions in which the H end of HD is oriented towards the ion at the moment the reactants pass over the centrifugal barrier in the effective radial potential (or cross over to the X–HD+ surface if the crossing occurs before the centrifugal barrier is reached). Use of the ion–induced dipole potential for the reactants permits the derivation of an analytic expression for the KIE. With no adjustable parameters, this model accounts quantitatively for the very different KIE’s observed in the reactions of Ar+ and Kr+ with HD at low collision energies.