Mechanistic conclusions from the curvature of solvent isotope effects

Abstract

The variation of k_x/k₀ with x the atom fraction of D in H₂O/D₂O mixtures is generally curved. Models of the transition state to fit this curve are described by fractionation factor theory. Nine general conclusions are drawn about the relationship between the model of the transition state and degree of curvature. The handling of data is much simplified by using a binomial expansion about x=½. It is shown how the different models of the transition states which give the same curvature and overall solvent isotope effect may be represented by simple diagrams both for a single transition state and for reactions proceeding through two parallel or consecutive transition states. Some data for the solvolysis of t-butyl chloride are used to exemplify the approach.