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-nCl_n]^+3-n (n = 1, 2) were detected.