Aqueous Solution Chemistry of Scandium(III) Studied by Raman Spectroscopy and ab initio Molecular Orbital Calculations

Aqueous solutions of Sc(ClO₄)₃,ScCl₃, and Sc₂(SO₄)₃ were studied by Ramanspectroscopy over a wide concentration range. In aqueous perchlorate solutionSc(III) occurs as an hexaaqua cation. The weak, polarized Raman band assignedto the ₁(a _1g) ScO₆ mode of the hexaaqua-Sc (III) ion has been studied as afunction of concentration and temperature. The ₁(a _1g) ScO₆ mode at 442 cm^–1of the hexaaqua—Sc(III) shifts only 3 cm^–1 to lower frequency and broadensabout 20 cm^–1 for a 60°C temperature increase. The Raman spectroscopic datasuggest that the hexaaqua-Sc (III) ion is stable in perchlorate solution within theconcentration and temperature range measured. Besides the polarized componentat 442 cm^–1, two weak depolarized modes at 295 and 410 cm^–1 were measuredin the Raman effect. These two modes of the ScO₆ unit were assigned to ₃(f _2g)and ₂(e ), respectively. The infrared active mode ₃(f _1u) was measured at 460cm^–1. The frequency data confirm the centrosymmetry of the Sc(III) aquacomplex, contrary to earlier Raman results. The powder spectrum of crystallineSc(ClO₄) ₃ · 6H₂O shows the above described Raman modes as well. Thesefindings are in contrast to Sc₂(SO₄)₃ solutions, where sulfate replaces water inthe first hydration sphere and forms thermodynamically strong sulfato complexes.In ScCl₃ solutions thermodynamically weak chloro complexes could be detected.Ab initio molecular orbital calculations were performed at the HF and MP2 levelsof theory using different basis sets up to 6–31 + G(d). Gas-phase structures,binding energies, and enthalpies are reported for the Sc³⁺(OH₂)₆ and Sc³⁺(OH₂)₇cluster. The Sc—O bond length for the Sc³⁺(OH₂)₆ cluster reproduces theexperimentally determined bond length of 2.18 Å (recent EXAFS data) almost exactly.The theoretical binding energy for the hexaaqua Sc(III) ion was calculated andaccounts for ca. 54–59% of the experimental hydration enthalpy of Sc(III). Thethermodynamic stability of the Sc³⁺(OH₂)₆(OH₂) cluster was compared to thatof the Sc³⁺(OH₂)₇ cluster, demonstrating that hexacoordination is inherently morestable than heptacoordination in the scandium (III) system. The calculated ₁ScO₆frequency of the Sc⁺(OH²)⁶ cluster is ca. 12% lower than the experimentalfrequency. Adding an explicit second hydration sphere to give Sc³⁺ (OH²)¹⁸,denoted Sc6 + 12], is shown to correct for the discrepancy. The frequencycalculation and the thermodynamic parameters for the Sc6 + 12] cluster aregiven and the importance of the second hydration sphere is stressed. Calculatedfrequencies of the ScO₆ subunit in the Sc6 + 12] cluster agree very well withthe experimental values (for example, the calculated ₁ScO₆ frequency was foundto be 447 cm^–1, in excellent agreement with the above-reported experimentalvalue). The binding enthalpy for the Sc6 + 12]cluster predicts the single ionhydration enthalpy to about 89%.