Density-Functional Theory Calculations of Aqueous Redox Potentials of Fourth-Period Transition Metals

Abstract

Aqueous M 3+ /M 2+ redox potentials for nine of the ten fourth-period transition metals, M, have been calculated with the use of DFT methodology in combination with the COSMO continuum model. Entropy contributions to the potentials are taken from experiments. The model introduces no adjustable parameters beyond those present in the underlying theoretical models. Inclusion of two solvation spheres (18 water molecules) is necessary. For the ions studied, the average absolute difference from experimental values is 0.29 V, with four out of nine potentials (those of V, Cr, Fe, Cu) reproduced with better than 0.1 V accuracy.

Computational Redox Potential Predictions: Applications to Inorganic and Organic Aqueous Complexes, and Complexes Adsorbed to Mineral Surfaces

“…Interestingly, the predicted redox potentials for the negatively charged complexes (3−/4−) were significantly underestimated, which could be due to large solvation errors. Even the effect of explicit water molecules did not show any significant improvement for the computed redox potentials of the cationic complexes whereas significant improvement was observed for the anionic complexes, the latter one in agreement with an earlier study [79]. In addition, the QM/MM model approach was employed in which the thermodynamic integration method was used to predict the free energy of redox processes, and then the reduction potentials were predicted with respect to the SHE reference potential for TM complexes in aqueous solution.…”

“…This explicit solvation (hydration) approach (six (first hydration) + twelve (second hydration) = eighteen water molecules) predicted the reduction potentials within a MUE of 0.29 eV of experimental redox potentials, a significant improvement by 1 eV over the previous approach. The local hydrogen bonding effects were important in order to obtain more accurate reduction potentials [79,130]. This confirms that the solvation process is usually an important aspect of the reduction potentials of TM complexes.…”

“…Explicit solvation by adding explicit water molecules around the solute can describe the hydrogen bonding environment more accurately [60,79] than a pure dielectric fluid approach. However, this method is computationally expensive, and therefore less commonly used because the CPCM solvation captures the solvation process comparatively well.…”

“…Uudsemaa et al [79] computationally determined the reduction potentials of transition metal (3+/2+) aqua complexes using the DFT BP86 method, water solvation effects modeled by the implicit COSMO solvation model, and the SHE as a reference potential. The predicted reduction potentials for these TM aqua complexes including the water molecules of the first solvation sphere (six first hydration sphere water molecules) were overestimated by 1.3 eV compared to the corresponding experimental reduction potentials.…”

Hybrid density functional calculations of redox potentials and formation energies of transition metal compounds

“…Owing to their importance, the development of first-principles techniques to study redox reactions has therefore been an area of considerable research interest. [1][2][3][4][5] In redox reactions, electrons are transferred from one species to another. Previous work 3,4 has shown that the standard local-density ͑LDA͒ and generalized gradient approximation ͑GGA͒ to density functional theory ͑DFT͒ lead to considerable errors in calculated redox energies.…”