Electron-Transfer Reactions of Radical Anions: Do They Follow Outer- or Inner-Sphere Mechanisms?

From the valence bond curve crossing model, it is predicted that an electron-transfer (ET) reaction between an organic radical anion and an even-electron organic species should proceed by an inner-sphere ET mechanism. The resonance interaction in the transition state, fi(inner-sphere), is estimated for two model reactions, (a) the self-exchange reaction between a radical anion and its parent compound and (b) the dissociative ET reduction of alky] halides by radical anions. For reactions of type (a) fi(inner-sphere) is estimated to be in the range of 2.3-7 kcal mol_-1(from transfer integrals in organic conductors), 5.7 kcal mol_-1(from an analysis by the Marcus theory of heterogeneous and homogeneous ET rate constants), 2.5 kcal mol_-1(from a comparison between experimental and calculated reorganization energies) and 4.1-5.5 kcal mol_-1(from a consideration of the distance dependence of intramolecular ET rate constants). For reactions of type (b), fi(inner-sphere) comes out at 8.5 kcal mol_-1(from an analysis by the Marcus theory of heterogeneous and homogeneous ET rate constants) or 4.0 kcal mol_-1(from a comparison of experimental and calculated reorganization energies). The distinction between the inner-sphere ET mechanisms (a and b) and their corresponding bond-forming mechanisms is discussed and shown to emerge naturally from considerations of the inherent bonding properties of the configurations which participate in the curve crossing diagram. Accordingly, the transition state for the bond-forming mechanism is shown to be more distorted (relative to reactants), tighter, and with a larger B in comparison to the situation in the inner-sphere ET mechanism.