Aluminium(III) hydration in aqueous solution. A Raman spectroscopic investigation and an ab initio molecular orbital study of aluminium(III) water clusters

Abstract

Raman spectra of aqueous Al(III) chloride, nitrate, and perchlorate solutions were measured over a broad concentration (0.21–3.14 mol L⁻¹) and temperature (25–125°C) range. The weak, polarized band at 525 cm⁻¹ and two depolarized modes at 438 and 332 cm⁻¹ have been assigned to ν₁(a_1g), ν₂(e_g) and ν₅(f_2g) of the hexaaquaaluminium(III) ion, respectively. The IR-active mode at 598 cm⁻¹ has been assigned to ν₃(f_1u). The vibrational analysis of the species [Al(OH₂)₆³⁺] was done on the basis of O_h symmetry (OH₂ as point mass). The polarized mode ν₁(a_1g) AlO₆ has been followed over the full temperature range and band parameters (band maximum, full width at half height and band intensity) have been examined. The position of the ν₁(a_1g) AlO₆ mode shifts only about 3 cm⁻¹ to lower frequencies and broadens about 20 cm⁻¹ for a 100°C temperature increase. The Raman spectroscopic data suggest that the hexaaquaaluminium(III) ion is thermodynamically stable in chloride, nitrate and perchlorate solutions over the temperature and concentration range measured. No inner-sphere complexes in these solutions could be detected spectroscopically. Aluminium sulfate solutions show a different picture and thermodynamically stable aluminium sulfato complexes could be detected. The sulfato complexes are entropically driven, so that their formation is favoured at higher temperatures. Ab initio geometry optimizations and frequency calculations of [Al(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 Al(III) species corresponds to symmetry T_h. The unscaled vibrational frequencies of the [Al(OH₂)₆³⁺] were reported. The unscaled vibrational frequencies of the AlO₆ unit are lower than the experimental frequencies (ca. 15%), but scaling the frequencies reproduces the measured frequencies. The theoretical binding enthalpy for [Al(OH₂)₆³⁺] was calculated and accounts for ca. 64% of the experimental single ion hydration enthalpy for Al(III). Ab initio geometry optimizations and frequency calculations are also reported for the [Al(OH₂)₁₈³⁺] (Al[6 + 12]) cluster with 6 water molecules in the first sphere and 12 water molecules in the second sphere. The global minimum corresponds to T symmetry. Calculated frequencies of the aluminium [6 + 12] cluster correspond with the observed frequencies in solution. The ν₁ AlO₆ (unscaled, HF/6-31G*) mode occurs at 542 cm⁻¹, in fair agreement with the experimental value. The theoretical binding enthalpy for [Al(OH₂)₁₈³⁺] was calculated and is a slightly underestimate of the experimental single ion hydration enthalpy for Al(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 Al(III) ion.