Issue 18, 2002

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

Abstract

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).

Supplementary files

Article information

Article type
Paper
Submitted
13 Mar 2002
Accepted
03 Jul 2002
First published
12 Aug 2002

Phys. Chem. Chem. Phys., 2002,4, 4319-4327

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

W. W. Rudolph and C. C. Pye, Phys. Chem. Chem. Phys., 2002, 4, 4319 DOI: 10.1039/B202567C

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