Raman spectroscopic characterization of light rare earth ions: La3+, Ce3+, Pr3+, Nd3+ and Sm3+ - hydration and ion pair formation.

Raman spectra of aqueous La3+, Ce3+, Pr3+, Nd3+ and Sm3+ - perchlorate solutions were measured and weak strongly polarized Raman bands were detected at 343 cm-1, 344 cm-1, 347 cm-1, 352 cm-1 and 363 cm-1, respectively. The full width at half height for these bands is quite broad (∼50 cm-1) in the isotropic spectrum and the band width increases with increasing solute concentration. The polarized Raman bands were assigned to the breathing modes of the nona-aqua ions of the mentioned rare earth ions. Published structural results confirmed that these ions exist as nona-hydrates in aqueous solutions [Ln(H2O)9]3+. The Ln-O bond distances of these rare earth ions correlate well with the band positions of the nona-aqua ions [Ln(OH2)9]+3 (Ln = La3+, Ce3+, Pr3+, Nd3+ and Sm3+) and the force constants were calculated for these breathing modes. The strength of the force constants increase with decreasing the Ln-O bond distances (La-O > Ce-O > Pr-O > Nd-O > Sm-O). While the fully hydrated ions are stable in dilute perchlorate solutions (∼0.2 mol L-1), in concentrated perchlorate solutions outer-sphere ion pairs and contact ion pairs are formed (C > 1.5 mol L-1). In a hydrate melt at 161 °C of Ce(ClO4)3 plus 6H2O, the contact ion pairs are the dominate species. The Raman bands of the ligated perchlorate and the Ce-O breathing mode of the partially hydrated ion pair at 326 cm-1 were measured and characterized. In cerium chloride solutions chloro-complex formation was detected over the measured concentration range from 0.270-2.167 mol L-1. The chloro-complexes in CeCl3(aq) are weak and diminish rapidly with dilution and disappear at a concentration <0.1 mol L-1. In a CeCl3 solution, with additional HCl, a series of chloro-complex species of the type [Ce(OH2)9-nCln]+3-n (n = 1, 2) were detected.