Intramolecular Electron Transfer: Independent (Ground State) Adiabatic (Chemical) and Nonadiabatic Reaction Pathways in Bis(hydrazine) Radical Cations

Two different mechanistic processes in intramolecular electron transfer chemistry have been studied with the complete active space self-consistent field quantum chemical method for a model bis(hydrazine) radical cation. These correspond to (a) a chemical electron transfer, where a reaction intermediate or a transition structure exist with the charge localized on the linker, and (b) a nonadiabatic electron transfer, where the bridge remains neutral. These processes coexist on the same potential energy surface. They are characterized by very different reaction coordinates and are thus distinct elements of the mechanistic spectrum of intramolecular electron transfer in organic radical cations. The energetically favored chemical electron-transfer process involves conventional reaction paths. In contrast, the nonadiabatic electron-transfer process involves an unconventional reaction path, which connects reactant and products via an un-avoided (i.e., real) crossing seam (i.e, an (n-1)-dimensional intersection, where n is the number of vibrational degrees of freedom of the system) between two different adiabatic potential energy surfaces. Our results, computed for a model compound, differ from Nelsen's experimental results, and thus demonstrate the importance of the hydrazine substituents and the aromatic spacer.