Density-functional molecular-dynamics study of the redox reactions of two anionic, aqueous transition-metal complexes

The thermochemistry of the RuO(4)(2-)+MnO(4)(-)-->RuO(4)(-)+MnO(4)(2-) redox reaction in aqueous solution is studied by separate density-functional-based ab initio molecular-dynamics simulations of the component half reactions RuO(4)(2-)-->RuO(4)(-)+e(-) and MnO(4)(2-)-->MnO(4)(-)+e(-). We compare the results of a recently developed grand-canonical method for the computation of oxidation free energies to the predictions by the energy-gap relations of the Marcus theory that can be assumed to apply to these reactions. The calculated redox potentials are in good agreement. The subtraction of the half-reaction free energies gives an estimate of the free energy of the full reaction. The result obtained from the grand-canonical method is -0.4 eV, while the application of the Marcus theory gives -0.3 eV. These should be compared to the experimental value of 0.0 eV. Size effects, in response to increasing the number of water molecules in the periodic model system from 30 to 48, are found to be small ( approximately 0.1 eV). The link to the Marcus theory also has enabled us to compute reorganization free energies for oxidation. For both the MnO(4)(2-) and RuO(4)(2-) redox reactions we find the same reorganization free energy of 0.8 eV (1.0 eV in the larger system). The results for the free energies and further analysis of solvation and electronic structure confirm that these two tetrahedral oxoanions show very similar behavior in solution in spite of the central transition-metal atoms occupying a different row and column in the periodic table.