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 ν1(a1g), ν2(eg) and ν5(f2g) of the hexaaquagallium(III) ion, [Ga(OH2)63+] (Oh symmetry), respectively. The infrared active mode at 555 cm−1 was assigned to ν3(f1u). 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)63+NO3−], but a small amount of the gallium(III) occurs as an inner‐sphere complex, [Ga(OH2)5ONO2]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 [Ga(OH2)63+] 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 hexaaqua Ga(III) species corresponds to symmetry Th. The unscaled vibrational wavenumbers of the [Ga(OH2)63+] are reported. The unscaled vibrational wavenumbers of the GaO6 unit are lower than the experimental values (∼15%). Scaling the wavenumbers reproduces only partially the measured values for the GaO6 unit. The theoretical binding enthalpy for [Ga(OH2)63+] was calculated and accounts for ∼60% of the experimental single ion hydration enthalpy for Ga(III). Ab initio geometry optimizations and wavenumber calculations are also reported for a [Ga(OH2)183+] (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 ν1 GaO6 (unscaled) mode occurs at 524 cm−1, which is in good agreement with the experimental value. The theoretical binding enthalpy for [Ga(OH2)183+] 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.