Ab Initio Study of Inner Solvent Shell Reorganization in the Fe2+–Fe3+ Aqueous Electron Exchange Reaction

Ab initio RHF molecular orbital calculations are carried out for several hydrate clusters of Fe2+ and Fe3+, using a double‐zeta valence atomic orbital basis set and an effective core potential for inner shell electrons, and comparison is made with all‐electron results at the monohydrate level. The calculations are shown to give a good quantitiative account of the solvent shell radii and breathing vibration frequencies for the hexahydrates. The large decrease (∼0.15 Å) in going from Fe2+(H2O)6 to Fe3+(H2O)6 is related to the interaction of the ag (4s) and ϵg (3d) Fe valence orbitals with the ligand 2p‐orbitals, with little π antibonding character found for the tg orbitals. The expansion of the 3d orbitals in the Fe2+ ion, relative to the Fe3+, is also noted and discussed in connection with the change in solvent shell size for the two complexes. The charge transfer from the ligands involves mainly a depletion of the hydrogen atom electrons, implying increased acidity for the first‐shell water molecules.