Gallium(III) hydration in aqueous solution of perchlorate, nitrate and sulfate. Raman and 71-Ga NMR spectroscopic studies and ab initio molecular orbital calculations of gallium (III) water clusters

摘要

Raman spectra of aqueous Ca(III)-perchlorate, -nitrate and -sulfate solutions were measured. In Ga(III)- perchlorate solutions, a strongly polarized mode of medium intensity at 526 cm~(-1) and two depolarized modes at 430 cm~(-1) and 328 cm~(-1) have been assigned to v_1(a_(1g)), v_2(e_g) and v_5(f_(2g)) of the hexaaquagallium (III) ion, [Ga(OH_2)_6~(3+)] (Oh symmetry), respectively. The infrared active mode at 510 cm~(-1) has been assigned to v_3(f_(1u)). The polarized mode v_1(a_(1g)) GaO_6 has been followed over a temperature range from 25 to 75 deg C and band parameters (band maximum, full width of half height and band intensity) have been examined. The position of the v_1(a_(1g))GaO_6 mode shifts by only about 2 cm~(-1) to lower frequencies and broadens by about 10 cm~(-1) for a 50 deg C temperature increase. The Raman spectroscopic data suggest that the hexaaquagallium (III) ion is thermodynamically stable in perchlorate solutions (no inner-sphere complex formation) over the temperature and concentration range measured. In a concentrated Ga(NO_3)_3 solution, most of the gallium (III) exist in the form of an outer-sphere ion pair, [Ga(OH_2)_6~(3+) NO_3~-], except for a few percent of the gallium (III) which occurs as an inner-sphere complex, [Ga(OH_2)_5ONO_2]~(2+). The nitrato complex is thermodynamically weak and disappears completely upon dilution. Gallium sulfate solutions show a different picture and a thermodynamically stable gallium (III) sulfato complex could be detected using Raman spectroscopy and 71-Ga NMR. The formation of the sulfato complex is favoured with increasing temperature and is thus entropically driven. At higher temperatures a basic gallium (III) sulfate of the alunite type is precipitated and was characterised by wet chemical analysis and X-ray diffraction (XRD). Ab initio geometry optimizations and frequency calculations of [Ga(OH_2)_n~(3+)] clusters with n = 1 to 6 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 aqua Ga(III) species was reported. The unscaled vibrational frequencies of the [Ga(OH_2)_6~(3+)] cluster were reported and do not correspond well experimental values because of the missing second hydration sphere. The theoretical binding entalpy for [Ga(OH_2)_6~(3+)] was calculated and accounts for ca.62% of the experimental single ion hydration enthalpy for Ga(III). Ab initio geometry optimizations and frequency calculations are also reported for a [Ga(OH_2)_(18)~(3+)](Ga[6+12]) cluster with 6 water molecules in the first sphere and 12 water molecules in the second sphere (T symmetry). The calcualted v_1 GaO_6 mode of the gallium [6+12] cluster occurs at 524 cm~(-1) (HF/6-31G~*), in good agreement with the experimental value at 526 cm~(-1). The other frequencies of the gallium [6+12] cluster also correspond well with the observed frequencies in solution. The theoretical binding enthalpy for [Ga(OH_2)_(18)~(3+)] was calculated and is slighly underestimated compared to the experimental single ion hydration enthalpy for Ga(III).