Molecular dynamics simulation of the hydration of transition metal ions: the role of non-additive effects in the hydration shells of Fe2+ and Fe3+ ions

The hydration of Fe2+ and Fe3+ ions in aqueous solution was studied by molecular dynamics simulation using ab initio pairwise interactions potential plus three-body correction terms. The simulations were performed at 298.16 K using the CF2 flexible water model. Radial distribution functions and their integration for Fen+–O show that six water molecules reside in the first hydration shell for both Fe2+ and Fe3+ ions, with RFeO being 2.15 and 2.05 Å, respectively. The second hydration shell contains about 13 and 15 water molecules for Fe2+ and Fe3+ ions, respectively, forming hydrogen bonds to the water molecules in the first shell. Water exchange between the second shell and bulk occurs frequently. Librational and vibrational spectra of second shell water molecules are almost identical to those in the bulk, whereas for first shell ligands remarkable differences are observed.