Theory for Outer Sphere Electron Transfer Coupled with Ion Transfer Kinetics on Atomically Stepped Metal
Neha, Rama Kant
Abstract
A semimicroscopic theory is developed for the kinetics of outer-sphere heterogeneous electron transfer (OS-HET) on an atomically stepped metal electrode coupled with electroactive ion relocation and solvent reorganization. Central postulate of our theory is that the transition state for OS-HET is formed through fluctuation induced matching of the quasi-Fermi level of the electrode to the frontier molecular orbital (FMO) energy level of the electroactive molecule. The pivotal component in our formalism is the fractional charge of activation (δ ‡ ) required to match the energy levels gap. The fractional charge of activation mainly depends on the work function and Fermi level of the metal, the participating FMO and their solvent/ligand reorganization induced energy level fluctuations. The fractional charge of activation is proportional to the magnitude of the activation barrier for OS-HET. The free energy of activation is the product of the electrochemical work function and fractional charge of activation. An additional complexity in HET arises due to influence of ion transfer kinetics, which is essential for understanding barrierless kinetics and is accounted for through the entropy barrier between the bulk and the Stern layer, providing a limiting rate constant. Finally, theoretical predictions for the standard rate constant and exchange current density show good agreement with the multiple experimental data for metals with nanocorrugated atomic steps and different electroactive species.