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

We have carried out extensive Born-Oppenheimer molecular dynamics simulations to characterize the structure and energetics of the Cu(+)/Cu(2+) redox pair. Our simulations support recent experimental evidence suggesting that Cu(2+) 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 Cu(2+) 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 Cu(2+) 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 Cu(2+) in aqueous solution.