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

Raman spectra of aqueous Ga(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 v1(a1g), v2(eg) and v5(f2g) of the hexaaquagallium(III) ion, [Ga(OH2)63+] (Oh symmetry), respectively. The infrared active mode at 510 cm−1 has been assigned to v3(f1u). The polarized mode v1(a1g) GaO6 has been followed over a temperature range from 25 to 75 °C and band parameters (band maximum, full width of half height and band intensity) have been examined. The position of the v1(a1g) GaO6 mode shifts by only about 2 cm−1 to lower frequencies and broadens by about 10 cm−1 for a 50 °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(NO3)3 solution, most of the gallium (III) exist in the form of an outer-sphere ion pair, [Ga(OH2)63+ NO3−], except for a few percent of the gallium (III) which occurs as an inner-sphere complex, [Ga(OH2)5ONO2]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(OH2)n3+] clusters with n = 1 to 6 were carried out at the Hartree–Fock and second order Møller–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(OH2)63+] cluster were reported and do not correspond well with experimental values because of the missing second hydration sphere. The theoretical binding enthalpy for [Ga(OH2)63+] 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(OH2)183+] (Ga[6+12]) cluster with 6 water molecules in the first sphere and 12 water molecules in the second sphere (T symmetry). The calculated v1 GaO6 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(OH2)183+] was calculated and is slightly underestimated compared to the experimental single ion hydration enthalpy for Ga(III).