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.