Beyond the Dielectric Continuum: The Effect of the Electrolyte on the Rate of Electron Transfer Reactions from a Quantum Rate Dynamics Perspective

High Resolution Image Download MS PowerPoint Slide The solvent environment shapes electron transfer (ET) reactions, which researchers predominantly model using the semi-classical Marcus theory. This theory relies on reorganization energy (λ 0 ) and describes the solvent as a dielectric continuum (implicit solvation). However, this approach often overlooks how electrolyte cation/anion pairs and the electronic structure of reactants directly influence ET. To address these gaps, we present quantum-rate (QR) theory─a more general framework that explicitly incorporates classical and quantum energy states, as well as the discrete effects of electrolyte composition and reactant structure. QR theory extends Marcus theory by offering a unified quantum-dynamic perspective that treats Marcus ET theory as a special case. Specifically, QR theory reinterprets λ 0, directly linking it to the balance between internal quantum energy ( E ∝ e 2 / C q ) and external electrostatic screening energy ( E e ∝ e 2 / C e ) determined by electrolyte properties─thus connecting λ 0 to measurable circuit parameters ( C q and C e ). To validate the applicability of QR theory, we experimentally analyze diffusionless ET rate dynamics in redox-tagged monolayers. The study distinguishes two solvation regimes: (i) we probe “Explicit Solvation Effects” by varying the water/acetonitrile ratio with a fixed electrolyte to reveal how solvent composition changes the rate via electronic coupling (κ) and (ii) explore “Implicit/Continuum Solvation Effects” by varying both solvent and cation/anion pairs, finding agreement with Marcus-like continuum analysis in specific regimes. This work advances quantum electrochemistry by clarifying the definition and measurement of λ 0 and specifying when to use implicit or explicit solvent models in quantum-dynamic ET descriptions.