Electronic structure and solvation of copper and silver ions: A theoretical picture of a model aqueous redox reaction

Electronic states and solvation of Cu and Ag aqua ions are investigated by comparing the Cu2+ + e(-) --> Cu+ and Ag2+ + e(-) --> Ag+ redox reactions using density functional-based computational methods. The coordination number of aqueous Cu2+ is found to fluctuate between 5 and 6 and reduces to 2 for Cu+, which forms a tightly bound linear dihydrate. Reduction of Ag2+ changes the coordination number from 5 to 4. The energetics of the oxidation reactions is analyzed by comparing vertical ionization potentials, relaxation energies, and vertical electron affinities. The model is validated by a computation of the free energy of the full redox reaction Ag2+ + Cu+ --> Ag+ + Cu2+. Investigation of the one-electron states shows that the redox active frontier orbitals are confined to the energy gap between occupied and empty states of the pure solvent and localized on the metal ion hydration complex. The effect of solvent fluctuations on the electronic states is highlighted in a computation of the UV absorption spectrum of Cu+ and Ag+.