Quantum Chemical Study of the Redox Potential of the Co(OH2)62+/3+ Couple and the Singlet-Quintet Gibbs Energy Difference of the Co(OH2)63+ Ion.

The geometry and vibrational frequencies of Co(OH2)62+ in the quartet state and Co(OH2)63+ in the singlet and quintet states were computed with quantum mechanics/molecular mechanics (QM/MM), whereby the LC-BOP-LRD functional was used for the QM part involving the Co(OH2)6n+ ( n = 2, 3) ions. The surrounding 124 MM water molecules were treated with the MMFF94 force field. The hydration energy differences between low-spin Co(OH2)63+ and Co(OH2)62+ or Co(OH2)63+ in the quintet state were also calculated using this method. The electronic energy of the Co(OH2)6n+ ( n = 2, 3) ions was calculated with wave function theory, multistate extended general multiconfiguration quasi-degenerate second-order perturbation theory and spin-orbit configuration interaction. The redox potential of the Co(OH2)62+/3+ couple, and the singlet-quintet (adiabatic) Gibbs energy difference of Co(OH2)63+, computed based on these data, agree with the experiment.