Infrared and Raman spectroscopic and theoretical studies of hexaaqua metal ions in aqueous solution

Far-infrared spectra of the hexaaqua complexes, [M(OH2)(6)](n+), of divalent (M-II = Mg, Zn, Cd and Hg) and trivalent (M-III = Al, Ga. In and TI) metal ions in aqueous solution have been recorded. Together with existing or remeasured Raman spectra this has enabled a complete assignment for all Six MO6 fundamentals for the first time. Force constant calculations have been performed for the hexaaquametal ions in T-h symmetry. The normal coordinate treatment has been extended to the hydrated trivalent cations (M-III = Ti, V, Cr, Fe, Co, Rh and Ir) using experimental frequencies in the literature, obtained for solid or glassy state samples. A satisfactory linear correlation was found between the averaged metal-oxygen stretching frequencies and the parameter Z(1/2)/R-0(2), which characterises the ion-dipole moment interaction, where Z is the charge of the metal ion and R-0 the M-O bond length. The averaged stretching frequencies seem to be more directly related to the electrostatic contributions to the bond strength than the widely used nu(1)(A(g)) symmetric stretching mode. The obtained M-O stretching force constants were also fairly well correlated to the Z(1/2)/R-0(2) parameter. (C) 2003 Elsevier B.V. All rights reserved.