The Hammond postulate: A quantitative interpretation

Abstract

The Hammond postulate is considered in terms of the model of intersecting parabolas. It is shown that, in radical detachment reactions of the type Xf· + HXi → XfH + Xi· with a symmetrical reaction center Xi…H…Xf, the H atom in the transition state is equidistant from the Xi and Xf atoms if the enthalpy of the reaction ΔH = 0. The Xi…H distance increases and the Xf…H distance decreases linearly as ΔH grows. The dependence remains linear over the range ΔHmin ≤ ΔH ≤ Hmax. The same result was obtained in quantum-chemical calculations for reactions of the type Rf· + RiH. In reactions of the type X· + HY → XH + Y· with an asymmetric reaction center X…H…Y, the X…H and Y…H interatomic distances in the transition state at ΔH = 0 depend on the force constants and lengths of the X-H and Y-H bonds. The Y…H distance increases and the X…H distance decreases linearly as ΔH grows. A similar picture is observed in the model of intersecting Morse curves, where the dependence of interatomic distances on ΔH in the transition state is nonlinear. Equations describing interatomic distances in the transition state as functions of the enthalpy of the reaction are presented.