Raman Spectroscopic Measurements of Scandium(III) Hydration in Aqueous Perchlorate Solution and ab Initio Molecular Orbital Studies of Scandium(III) Water Clusters: Does Sc(III) Occur as a Hexaaqua Complex?

Abstract

We have shown that Sc(III) in aqueous perchlorate solution occurs as a hexaaqua cation. The weak, polarized Raman band assigned to the ν1(a1g)(ScO6) mode of the hexaaqua Sc(III) ion has been studied as a function of concentration. Besides the isotropic component at 442 cm-1, two weak depolarized modes at 295 and 410 cm-1 were measured in the Raman effect. These two modes of the ScO6 unit were assigned to ν5(f2g) and ν2(eg), respectively. The infrared active mode,ν3(f1u), was found at 460 cm-1. The Raman spectroscopic data suggest that the hexaaqua Sc(III) ion is stable in perchlorate solution within the concentration range measured. Furthermore, the frequency data confirm the centrosymmetry of the Sc(III) aqua complex, contrary to earlier Raman results. The ScO6 unit possesses Oh symmetry (water seen as point mass). These findings are in contrast to ScCl3 solutions, where chloride replaces water in the first hydration sphere and forms one or more chloro complexes. Gas-phase structures, binding energies, and enthalpies are reported for small [Sc(OH2)n]3+ clusters, with one to nine water molecules in the first sphere. Ab initio molecular orbital calculations were performed at the HF and MP2 levels of theory using different basis sets up to 6-31+G*. The water molecules in these clusters coordinate the Sc3+ in highly symmetric arrangements that tend to enhance electrostatic charge−dipole interactions while minimizing ligand−ligand repulsion. The Sc−O bond length for the [Sc(OH2)6]3+ cluster reproduces the experimentally determined bond length (EXAFS) of 2.18 Å. The theoretical binding energy for the [Sc(OH2)6]3+ ion was calculated and accounts for ca. 54−59% of the experimental single ion hydration enthalpy of Sc(III). The stability of the hexaaquascandium(III) cluster could be demonstrated by comparing the stability of the [Sc(OH2)6(OH2)]3+ cluster with the [Sc(OH2)7]3+ cluster, resulting in the stability of the former. The frequencies of ν1(ScO6) of the [Sc(OH2)6]3+ cluster are ca. 11% lower than the experimental frequency. The reason for this discrepancy was discussed and found to lie in the lack of the second hydration sphere which was subsequently modeled as the [Sc(OH2)18]3+ cluster, denoted Sc[6+12]. Frequencies of the ScO6 unit for the Sc[6+12] cluster were calculated and agree with the experimental values. The binding enthalpy resembles that of the single ion hydration enthalpy.