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

Abstract

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 Fe(2+)((5)D, 3d(6)) or Fe(3+)((6)S, 3d(5)) and six neutral water molecules. Upon hydration, the ground state of Fe(2+)(H2O)6 is a triply ((5)Tg) degenerate one, with the doubly ((5)Eg) degenerate state lying ∼20 kcal/mol higher in energy. Similarly, the Fe(3+)(H2O)6 cluster has a ground state of (6)Ag 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 Fe(2+)(H2O)6-Fe(3+)(H2O)6 gas-phase complex for different Fe-Fe distances. For distances shorter than 6.0 Å, 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.

New insights into the competition between antioxidant activities and pro-oxidant risks of rosmarinic acid

“…The secondary antioxidant activities of rosmarinic acid based on its ferrous and ferric ion chelation in preventing the formation of the reactive hydroxyl radical (HOc) via the Haber-Weiss reaction 1,95,96 were evaluated. The hydrated Fe(II) and Fe(III) ions existed in the octahedral-coordinated structures with six water molecules, as largely proposed in the literature 9,82,97,98 in which the Fe(II) or Fe(III) ion is located at the center and H 2 O in the corner.…”

Characterization of Fe2+ Aqueous Solutions with Liquid Jet X-ray Photoelectron Spectroscopy: Chloride Depletion at the Liquid/Vapor Interface Due to Complexation with Fe2+

“…Fundamental work focused on characterization and understanding of the first solvation sphere of Fe 3+/2+ complexes has relied on the interpretation of the electronic structure through ultraviolet/visible, electron paramagnetic resonance and Mössbauer spectroscopies or by using X-ray absorption spectroscopy to evaluate the extended structure of the ligand environment and the outer solvation spheres. − The accepted primary coordination sphere of Fe 2+/3+ coordination complexes in acidic aqueous solution are an octahedral arrangement of aqua ligands around a central Fe center. − These aqua ligands, as expected, are extremely sensitive to the pH of the solution. As the pH increases, the aqua ligands around the Fe 2+ center will deprotonate to form hydroxido ligand coordination before eventually collapsing to insoluble Fe 2+ -hydroxido nanoparticles. ,, Fe 3+ complexes have more complicated aqueous solution chemistry as Fe 3+ -hydroxyl complexes are stable as monomers in solution but can also form dimers, trimers, and polymers .…”

Polarizable and Non-Polarizable Force Field Representations of Ferric Cation and Validations

“…Taking into account the CT explicitly improved the calculated hydration energy to −940.4 kcal/mol. This suggests that charge transfer contributes to the Fe–water interaction to form a stable aqua complex of ferric ion when solvated in aqueous solution, as in the case of Fe 2+ (aq) …”