Reduction Energy of 1 M Aqueous Ruthenium(III) Hexaammine in the Gas Phase: A Route toward Establishing an Absolute Electrochemical Scale

Abstract

The internal energy deposited into gas-phase Ru(NH3)6(H2O)n3+ when reduced by thermal electrons is investigated as a function of cluster size. For n ≥ 40, reduction results exclusively in the loss of water molecules from the reduced precursor ion; loss of water is accompanied by the loss of a single ammonia molecule for smaller clusters. The average number of ligands lost from the reduced precursor decreases with cluster size for n ≤ 31, presumably because of increased binding energy of the ligands to the smaller, doubly charged clusters. For Ru(NH3)6(H2O)553+, which corresponds to a concentration or activity of about 1 M, reduction results in a mean loss of 18.2 water molecules, from which an average and maximum energy deposition of 7.9 and ∼8.2 to 8.7 eV, respectively, is determined. To the extent that the dissociation is statistical, the internal energy deposited corresponds to the reduction energy of the hydrated precursor ion by a thermal electron in the gas phase. This measured value is combined with the electron affinity of water and the difference in solvation energies of the precursor and reduced cluster ions to provide an absolute value for the reduction energy for 1 M Ru(NH3)63+ by a solvated electron in bulk water of about −3.8 eV at 0 K. This route toward establishing absolute half-cell reduction potentials has the advantages that effects of counterions, solvent, and chemical form can be easily controlled and quantified, and redox reactions not readily observed in solution can be measured.