Computational Electrochemistry: The Aqueous Ru3Ru2Reduction Potential

We present results of density functional calculations for the standard reduction potential of the Ru3Ru2couple in aqueous solution. The metal cations are modeled as Ru(H2O)nqsurrounded by continuum solvent (q2, 3; n6, 18). The continuum model includes bulk electrostatic polarization as well as atomic surface tensions accounting for the deviation of the second or third hydration shell from the bulk. After consideration of 37 density functionals with 5 different basis sets, it has been found that hybrid and hybrid meta functionals provide the most accurate predictions for the Ru(H2O)nqgeometries and for the corresponding reduction potential in comparison with available experimental data. The gas-phase ionization potentials of Ru(H2O)n2calculated by density functional theory are also compared to results of ab initio computations using second-order Møller−Plesset perturbation theory. The difference in solvation free energies of Ru3and Ru2varies from −10.56 to −10.99 eV for n6 and from −6.83 to −7.45 eV for n18 depending on the density functional and basis set quality. The aqueous standard reduction potential is overestimated when only the first solvation shell is treated explicitly and is underestimated when the first and second solvation shells are treated explicitly.