The arrangement of first- and second-shell water molecules in trivalent aluminum complexes: Results from density functional theory and structural crystallography

The structural and energetic features of a variety of gas-phase aluminum ion hydrates containing up to 18 water molecules have been studied computationally using density functional theory. Comparisons are made with experimental data from neutron diffraction studies of aluminum-containing crystal structures listed in the Cambridge Structural Database. Computational studies indicate that the hexahydrated structure Al[H2O](6)(3+) (with symmetry T-h), in which all six water molecules are located in the innermost coordination shell, is lower in energy than that of AI[H2O](5)(3+). [H2O], where only five water molecules are in the inner shell and one water molecule is in the second shell. The analogous complex with four water molecules in the inner shell and two in the outer shell undergoes spontaneous proton transfer during the optimization to give {Al[H2O](2)[OH](2)}(+).[H3O+](2), which is lower in energy than Al[H2O](6)(3+); this finding of H3O+ is consistent with the acidity of concentrated Al3+ solutions. Since, however, Al[H2O](6)(3+) is detected in solutions of Al3+, additional water molecules are presumed to stabilize the hexa-aquo Al3+ cation. Three models of a trivalent aluminum ion complex surrounded by a total of 18 water molecules arranged in a first shell containing 6 water molecules and a second shell of 12 water molecules are discussed. We find that a model with S-6 symmetry for which the Al[H2O](6)(3+) unit remains essentially octahedral and participates in an integrated hydrogen bonded network with the 12 outer-shell water molecules is lowest in energy. Interactions between the 12 second-shell water molecules and the trivalent aluminum ion in Al[H2O](6)(3+) do not appear to be sufficiently strong to orient the dipole moments of these second-shell water molecules toward the Al3+ ion.