Study of gallium(III) nitrate hydrate and aqueous solutions:: Raman spectroscopy and ab initio molecular orbital calculations of gallium(III) water clusters

Raman,spectra of aqueous Ga(III) perchlorate and nitrate solutions were measured at room temperature. From Ga(ClO4)(3) solutions, a strongly polarized band at 526 cm(-1) and two depolarized modes at 430 and 328 cm(-1) were assigned to v(1)(a(1g)), v(2)(e(g)) and v(5)(f(2g)) of the hexaaquagallium(III) ion, [Ga(OH2)(6)(3+)] (O-h symmetry), respectively. The infrared active mode at 555 cm(-1) was assigned to v(3)(f(1u)). The Raman spectroscopic data suggest that the hexaaquagallium(III) ion is thermodynamically stable in perchlorate solutions (no inner-sphere complex formation). In a supersaturated solution and in concentrated Ga(NO3)(3) solutions most of the gallium(III) exists in the form of an outer-sphere ion pair, [Ga(OH2)(6)3+NO3-], but a small amount of the gallium(III) occurs as an inner-sphere complex, [Ga(OH2)(5)ONO2](2+), which could be confirmed using Raman spectroscopy. Upon dilution, the fraction of the weak inner-sphere nitrato complex diminishes and only the outer-sphere complex is observable. Ab initio geometry optimizations and wavenumber calculations of IGa(OH2)(6)(3+)] were carried out at the Hartree-Fock and second order Moller-Plesset levels of theory, using various basis sets up to 6-31 + G*. The global minimum structure of the hexaaqua Ga(III) species corresponds to symmetry T-h The unscaled vibrational wavenumbers of the [Ga(OH2)(6)(3+)] are reported. The unscaled vibrational wavenumbers of the GaO6 unit are lower than the experimental values (similar to15%). Scaling the wavenumbers reproduces only partially the measured values for the GaO6 unit. The theoretical binding enthalpy for [Ga(OH2)(6)(3+)] was calculated and accounts for similar to60% of the experimental single ion hydration enthalpy for Ga(III). Ab initio geometry optimizations and wavenumber calculations are also reported for a [Ga(OH2)(18)(3+)] (Ga[6 + 12]) cluster with six water molecules in the first sphere and 12 water molecules in the second sphere. The global minimum occurs for T symmetry. Calculated wavenumbers of the gallium [6 + 12] cluster correspond well with the observed wavenumbers in solution. The v(1) GaO6 (unscaled) mode occurs at 524 cm(-1), which is in good agreement with the experimental value. The theoretical binding enthalpy for [Ga(OH2)(18)(3+)] was calculated and is slightly, underestimated compared with the experimental single ion hydration enthalpy of Ga(III). The water molecules of the first sphere form strong H-bonds with water molecules in the second hydration shell because of the strong polarizing effect of the Ga(III) ion. Copyright (C) 2002 John Wiley Sons, Ltd.