Cuaq+/Cuaq2+ Redox Reaction Exhibits Strong Nonlinear Solvent Response Due to Change in Coordination Number

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Abstract
We have carried out extensive Born−Oppenheimer molecular dynamics simulations to characterize the structure and energetics of the Cu+/Cu2+ redox pair. Our simulations support recent experimental evidence suggesting that Cu2+ adopts a 5-fold coordination in aqueous solution and not the traditionally assumed octahedral coordination. Cu+ forms a linear dihydrate structure with a third water molecule occasionally binding in the equatorial plane. We show that the change in ligand coordination number from 2 for aqueous Cu+ to 5 for aqueous Cu2+ leads to marked deviations from the linear response assumption underlying Marcus theory of oxidation. The diabatic free energy curves deviate from parabolic behavior and the reorganization free energies for Cu+ and Cu2+ are asymmetric and differ by 1.0 eV. The calculated reorganization free energies can semiquantitatively explain the exceptionally small electron self-exchange rate between Cu+ and Cu2+ in aqueous solution.