Forbidden Electron Transfer in the Adiabatic Limit of the Marcus-Inverted Region

Here it is shown that in the adiabatic limit of condensed-phase electron transfer, the onset of barrierless transition occurs at a lower driving force than predicted by the non-adiabatic Marcus formulation. Furthermore, in the adiabatic limit of the Marcus-inverted region, isoenergetic electron transfer is strictly forbidden in the absence of nuclear tunneling. This "forbidden" behavior arises from a topological change in the mapping between the adiabatic and diabatic electronic surfaces, emerging precisely at the onset of the Marcus-inverted region.