An Extension of the Hammond Postulate. Structural Effects on the Classification of Chemical Reactions

Abstract

Combination of the reaction energy, a thermodynamic index, with structural descriptors, such as the force constants associated with the reactant and products potential wells, allows us to define a new index that quantifies the position of the transition state and classifies reactions in terms of their Hammond or anti-Hammond character. Switching from the classical restricted Hammond behavior to a general behavior, in which the position of the transition state is not only conditioned by energy comparisons, occurs through adding a new term accounting for the structural effects in the potential-energy function. It is shown that these effects strongly influence the position of transition states but leave the barrier height practically unchanged. Analysis of 27 chemical reactions of different types, such as rotational isomerizations, SN2, and intramolecular reordering reactions, provides strong support to the model presented here.