ECE vs DISP Mechanisms in Anodic Electrolysis of Benzyl Alcohols: Computational Prediction of Microscopic Rate Constants

The heterogeneous nature of electrochemical reactions entails unique kinetic control of product yield/selectivity as compared with corresponding homogeneous oxidation/reduction reactions. In direct electrolysis, subsequent elementary steps following the initiating electron transfer may also occur heterogeneously at the electrode surface or homogeneously within the bulk electrolyte, often via a disproportionation step for secondary electron transfer; kinetic control of this branching may have important consequences for product selectivity/yield, due to differences in lifetimes of reactive radical intermediates. In this work, we use computer simulations to predict microscopic rate constants governing the heterogeneous "ECE" electrochemical oxidation of para-methoxybenzyl alcohol to its corresponding aldehyde at a working carbon anode within an aqueous electrolyte. Molecular dynamics simulations are conducted to model the full electrochemical cell at atomistic resolution under conditions approximating controlled potential electrolysis, from which rate constants are predicted via a combination of direct dynamics and free energy sampling methods. Density functional theory-based quantum mechanics/molecular mechanics (DFT-QM/MM) simulations are performed to predict free energy barriers for deprotonation of the cation radical intermediate within the electrical double layer environment. We demonstrate how strong solvophobic forces lead to residence times of ten(s) of nanoseconds for the electrogenerated cation radical intermediates to reside within the anodic double layer, and the relative deprotonation rate is a key factor dictating the heterogeneous vs homogeneous reaction branching. We predict a compelling double-layer modulation for the cation radical deprotonation rate with NaOAc aqueous electrolyte, arising from a combination of preformed "encounter pairs" via ionic interactions and reduction in activation barrier via stereoelectronic effects. Our computational study of this prototypical electrolysis reaction illustrates the substantial role of reaction conditions (solvent, electrolyte, and overpotential) on the microscopic rate constants that kinetically control the reaction pathway/outcome.