Aqueous solution chemistry of scandium(III) studied by Raman spectroscopy and ab initio molecular orbital calculations

Aqueous solutions of Sc(ClO4)(3), ScCl3, and Sc-2(SO4)(3) were studied by Raman spectroscopy over a wide concentration range. In aqueous perchlorate solution sc(III) occurs as an hexaaqua cation. The weak, polarized Raman band assigned to the nu (1)(a(1g)) ScO6 mode of the hexaaqua-Sc (III) ion has been studied as a function of concentration and temperature. The nu (1)(a(1g)) ScO6 mode at 442 cm(-1) of the hexaaqua-Sc(III) shifts only 3 cm(-1) to lower frequency and broadens about 20 cm(-1) for a 60 degreesC temperature increase. The Raman spectroscopic data suggest that the hexaaqua-Sc (III) ion is stable in perchlorate solution within the concentration and temperature range measured. Besides the polarized 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 nu (5)(f(2g)) and nu (2)(e(g)) respectively. The infrared active mode v(3)(f(1u)) was measured at 460 cm(-1). The frequency data confirm the centrosymmetry of the Sc(III) aqua complex contrary to earlier Raman results. The powder spectrum of crystalline Sc(ClO4)(3) . 6H(2)O shows the above described Raman modes as well. These findings are in contrast to Sc-2(SO4)(3) solutions, where sulfate replaces water in the first hydration sphere and forms thermodynamically strong sulfate complexes. In ScCl3 solutions thermodynamically weak chloro complexes could be detected. Ab initio molecular orbital calculations were performed at the HF and MP2 levels of 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)(7) cluster. The Sc-O bond length for the SC3+(OH2)(6) cluster reproduces the experimentally determined bond length of 2.18 A (recent EXAFS data) almost exactly. The theoretical binding energy for the hexaaqua Sc(III) ion was calculated and accounts for ca. 54-59% of the experimental hydration enthalpy of Sc(III). The thermodynamic stability of the SC3+(OH2)(6)(OH2) cluster was compared to that of the Sc3+(OH2)(7) cluster, demonstrating that hexacoordination is inherently more stable than heptacoordination in the scandium (III) system. The calculated sigma 1ScO6 frequency of the SC3+(OH2)(6) cluster is ca. 12% lower than the experimental frequency. Adding an explicit second hydration sphere to give SC3+(OH2)(18), denoted Sc[6 + 12], is shown to correct for the discrepancy. The frequency calculation and the thermodynamic parameters for the Sc[6 + 12] cluster are given and the importance of the second hydration sphere is stressed. Calculated frequencies of the ScO6 subunit in the Sc[6 + 12] cluster agree very well with the experimental values (for example, the calculated nu 1ScO6 frequency was found to be 447 cm(-1), in excellent agreement with the above-reported experimental value), The binding enthalpy for the Sc[6 + 12]cluster predicts the single ion hydration enthalpy to about 89%.