Raman Spectroscopic Studies of Scandium(III) Hydration in Aqueous Solution - about the First Hydration Sphere of Sc(III) in Solution

It has been shown conclusively that Sc(III) in aqueous perchlorate solution occurs as an hexaaqua cation. The weak, polarized Raman band assigned to the v₁(a_1g)ScO₆ mode of the hexaaqua-Sc(III) ion (T_h symmetry for the whole complex ion or O_h symmetry for the ScO₆ unit) has been studied as a function of concentration and temperature. The isotropic scattering geometry in R-format was employed in order to measure the true vibrational contribution of the band and account for the Boltzmann temperature factor B and the wavenumber factor v. The v₁(a_1g)ScO₆ mode at 442 cm^-1 of the hexaaqua-Sc(III) shifts only 3 cm^-1 to lower wavenumbers and broadens about 20 cm^-1 for a 60°C temperature increase. The Raman spectroscopic data suggest that the hexaaqua Sc(III) ion is stable in perchlorate solution within the temperature range measured.

Besides the polarized component at 442 cm^-1, two weak depolarized modes at 295 cm^-1 and 410 cm^-1 were measured in the Raman effect. These two modes of the ScO₆ unit were assigned to v₅(f_2g) and v₂(e_g) respectively. The infrared active mode, v₃(f_1u), was measured to be at 460 cm^-1. The wavenumber data confirm the centrosymmetry of the Sc(III)aqua-complex, in opposite to earlier Raman results.

These findings are also in contrast to the results found in Sc₂(SO₄)₃ solutions, where sulfate replaces water in the first hydration sphere and forms quite strong sulfato complexes. In ScCl₃ solutions weak chloro-complex(es) could be detected.