Quantitative Hammond postulate

Abstract

In this paper four assumptions are made: (1) Pauling's relation (2) Bond order conservation, (3) An approximation for the potential energy along the reaction coordinate and (4) a symmetry principle. The main results which follow are: (a) An expression for the bond order of the transition state, a result which is compared with potential energy surfaces, BEBO and Marcus' theories. (b) A theoretical fit for (nonlinear) Brönsted plots over a mild range of ΔpK. In addition, other theories which relate kinetics to thermodynamics (in particular: BEBO, Marcus' theory and Thornton's model) are analysed. Two empirical relations of Mok and Polanyi and the Evans–Polanyi relation are derived by using the symmetry principle. Marcus' theory may be related to a “weak symmetry principle”; this theory is given a new proof by Thornton's model of the inverted parabola. Thus these theories appear to have much in common; it is possible that they all follow from one general theory.