Parallel and sequential isomerism of activated complexes in organic reactions

Abstract

A general unimolecular process is considered for which the analysis of potential hypersurfaces has proved the existence of complicating factors of two types not differentiated by experiment. One of them consists of the fact that between the reactant and the product there are not one but several pathways, each having its own activated complex, i.e. parallel isomerism. The other consists of the presence of a third local energy minimum also participating in the rate process considered, namely as an intermediate, which implies the existence of not one but two activated complexes at the reaction pathway given (sequential isomerism). These factors indicate a more complicated structure of the activation parameters obtained in experiment under the presumption of conventional simple, concerted behaviour and mechanism of the process studied. This makes itself felt in the values and in the course of these activation parameters. Special attention is paid to the activation heat capacity term. In the case of sequential isomerism a pronounced time dependence has been found for this term. The results are important for the interpretation and prediction, as well as confrontation, of theoretical and experimental data on the kinetics of organic reactions.