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

Abstract

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.