ELECTRON-TRANSFER MECHANISTIC MANIFOLD AND VARIABLE TRANSITION-STATE CHARACTER - A THEORETICAL INVESTIGATION OF MODEL ELECTRON-TRANSFER PROCESSES BETWEEN NUCLEOPHILES AND ETHANE CATION-RADICAL

This paper addresses the question of an electron transfer mechanistic manifold by ab initio computations of the model systems Nu: + C2H6.+ - -> Nu(.+) --> C2H6 where, Nu = H2S, PH3, H2O and NH3. The computations show that there exist two major classes of ET mechanisms. The first i s a concerted ET mechanism which proceeds along a maximum bonding traj ectory. The second class is a stepwise ET mechanism which involves shu ttles of redox pairs, e.g., H+/H-. and CH3+/CH3., and thereby results in a single electron transfer from the nucleophile to the cation radic al. Thus, an apparent ET may be a net result of consecutive steps whic h by themselves are non-ET steps. The genesis of the mechanistic manif old and variable transition state structure from valence bond (VB) con figurations is discussed. It is found that all these mechanisms are ty pified by electronic structures that share the same set of VB configur ations with variable proportions, giving rise thereby to variable tran sition state structure and an ET mechanistic family. The situation is reminiscent of the mechanistic manifold encountered in physical organi c chemistry, e.g., the S(N)2 and S(N)1 mechanisms in the classical nuc leophilic substitution process For all the Nu:/C2H6.+ combinations we also identified outer-sphere transition state analogues that avoid the bonding between the reactants. All the outer-sphere saddle points are found to be higher in energy than the bonded mechanisms of the concer ted and stepwise varieties. It appears therefore, that outer-sphere me chanisms should be regarded as default options rather than natural mec hanisms of ET-reactions of cation radicals. Indeed, all the computatio nal trends exhibited by the ET mechanistic manifold are shown to be th e consequences of maximum bonding.