Computing Redox Potentials in Solution:  Density Functional Theory as A Tool for Rational Design of Redox Agents

High-level density functional theory in combination with a continuum solvation model was employed to compute standard redox potentials in solution phase for three different classes of electrochemically active molecules:  small organic molecules, metallocenes, and M(bpy)₃^x (M = Fe, Ru, Os; x = +3, +2, +1, 0, −1). Excellent agreement with experimentally determined redox potentials is found with an average deviation of approximately 150 mV when four different solvents commonly in use for electrochemical measurements were included. To obtain quantitative agreement between theory and experiment, the use of a large basis set is crucial especially when the redox couple includes anionic species. Whereas the addition of diffuse functions improved the results notably, vibrational zero-point-energy corrections and addition of entropy effects are less important. The computational protocol for computing redox potentials in solution, which has been benchmarked, is a powerful and novel tool that will allow a molecular-level understanding of the features dictating the properties of redox-active species.