Ground and Excited States of the [Fe(H2O)6]2+ and [Fe(H2O)6]3+ Clusters: Insight into the Electronic Structure of the [Fe(H2O)6]2+-[Fe(H2O)6]3+ Complex

We report the ground and low-lying electronically excited states of the [Fe(H2O)(6)](2+) and [Fe(H2O)(6)](3+) clusters using multiconfiguration electronic structure theory. In particular, we have constructed the potential energy curves (PECs) with respect to the iron-oxygen distance when removing all water ligands at the same time from the cluster minima and established their correlation to the long-range dissociation channels. Due to the fact that both the second and third ionization potentials of iron are larger than the one for water, the ground-state products asymptotically correlate with dissociation channels that are repulsive in nature at large separations, as they contain at least one H2O+ fragment and a singly positively charged metal center (Fe+). The most stable equilibrium structures emanate, via intersections and/or avoided crossings, from the channels consisting of the lowest electronic states of Fe2+(D-5, 3d(6)) or Fe3+(S-6, 3d(5)) and six neutral water molecules. Upon hydration, the ground state of Fe2+(H2O)(6) is a triply (T-5(g)) degenerate one, with the doubly (E-5(g)) degenerate state lying similar to 20 kcal/mol higher in energy. Similarly, the Fe3+(H2O)(6) cluster has a ground state of 6Ag symmetry under Th symmetry, which is well-separated from the first excited state. We also examine a multitude of electronically excited states of many possible spin multiplicities and report the optimized geometries for several selected states. The PECs of those states exhibit a high density of states. Focusing on the ground and the first few excited states of the [Fe(H2O)(6)](2+) and [Fe(H2O)(6)](3+) clusters, we studied their mutual interaction in the gas phase. We obtained the optimal geometries of the Fe2+(H2O)(6)-Fe3+(H2O)(6) gas-phase complex for different Fe-Fe distances. For distances shorter than 6.0 angstrom, the water molecules in the respective first solvation shells located between the two metal centers were found to interact via weak hydrogen bonds. We examined a total of 10 electronic states for this complex, including those corresponding to the electron transfer (ET) from the ferrous to the ferric ion. The ET process is discussed and a possible path via a quasi-symmetric transition state is suggested.