A reassessment of the isoniversion relationship

Abstract

Calculations have been carried out on model systems with two potentially rate determining step on a single reaction path. Taking the ratio of the diastereoisomers as P, the mazima found on plots of ln P vs. 1/T are shown to derive from the intersection of a line characteristic of one rate-determining step and a second line characteristic of the transitional region in which no single step is rate-determining. On this interpretation, the quantities previously calculated from the difference between the slopes of these lines and termed δΔΔH‡ and δΔΔS‡ have no simple physical meaning. The isoinversion relationship involving these quantities is not therefore a conventional enthalpy–entropy relationship for those sets of reactions that involve little change in the inversion temperautre, the isoinversion relationship is shown to be a necessary consequence of the geometry of the plot and the definition of the terms.