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: + C ₂H₆^̇+→Nu^̇+ + C ₂H₆ where, Nu = H₂S, PH₃, H ₂O and NH₃. The computations show that there exist two major classes of ET mechanisms. The first is a concerted ET mechanism which proceeds along a maximum bonding trajectory. The second class is a stepwise ET mechanism which involves shuttles of redox pairs, e.g., H⁺/H ^̇ and CH₃⁺/CH₃^̇, and thereby results in a single electron transfer from the nucleophile to the cation radical. Thus, an apparent ET may be a net result of consecutive steps which by themselves are non-ET steps. The genesis of the mechanistic manifold and variable transition state structure from valence bond (VB) configurations is discussed. It is found that all these mechanisms are typified by electronic structures that share the same set of VB configurations with variable proportions, giving rise thereby to variable transition state structure and an ET mechanistic family. The situation is reminiscent of the mechanistic manifold encountered in physical organic chemistry, e.g., the S_N2 and S _N1 mechanisms in the classical nucleophilic substitution process. For all the Nu:/C₂H_6̇+ combinations we also identified outersphere 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 concerted and stepwise varieties. It appears therefore, that outer-sphere mechanisms should be regarded as default options rather than natural mechanisms of ET-reactions of cation radicals. Indeed, all the computational trends exhibited by the ET mechanistic manifold are shown to be the consequences of maximum bonding.