Substitution, addition and elimination reactions: a quantum chemical study of intermolecular complexes

Abstract

The importance of the formation of intermolecular complexes (van der Waals species) to the overall reaction mechanism for some fundamental prototypes of gas-phase organic reactions is described. The role of the van de Waals species is at least an order of magnitude larger with ion-molecule than with molecule-molecule reactions. For numerous ion-molecule reactions it is characteristic that the channel of reactants is separated from the channel of products by a rather low energy barrier. Frequently, reactants and products are not separated by an energy barrier at all. In these instances there exist numerous stationary points (minima, saddle points and saddle points of higher order) on the potential energy surface which are lower in energy than the reactants and products. General considerations are illustrated by specific reactions: substitution reactions (e.g. CH 4+H −, CH 4 + F −, CH + 3 + H 2 and CH − 3 + H 2), addition reactions with metastable products (e.g. CH 4 + + CH 4 and CH 3F + + CH 3F), addition reactions with stable products (e.g. C 2:H 4 + HF and C 2H 4 + H 2O), eliminations and isomerizations of unnatural forms of ions and radical ions (e.g.CH 5 − and H 3C −·H 2⇌H 3CH·H −).