Resolving the Marcus-Rehm-Weller Paradox in Electron Transfer

Marcus theory famously predicts that electron-transfer rates decrease once the thermodynamic driving force exceeds the reorganization energy. Yet many systems instead exhibit Rehm-Weller kinetics, in which the rate saturates rather than decreases. Here we show that these apparently contradictory phenomenologies emerge as opposite physical limits of the same two-level quantum Hamiltonian. In the normal region, the model recovers both Marcus and Rehm-Weller behavior. In the inverted region, however, it predicts Marcus's decreasing rate in the nonadiabatic limit but Rehm-Weller saturation in the adiabatic limit. Using physically realistic reorganization energies and electronic coupling values, we show that Rehm-Weller's data can be quantitatively reproduced within a microscopic quantum model without invoking diffusion limitations or phenomenological corrections.