Hydration and ion pair formation in common aqueous La(iii) salt solutions – a Raman scattering and DFT study

Abstract

Raman spectra of aqueous lanthanum perchlorate, triflate (trifluorosulfonate), chloride and nitrate solutions were measured over a broad concentration (0.121–3.050 mol L⁻¹) range at room temperature (23 °C). A very weak mode at 343 cm⁻¹ with a full width at half height at 49 cm⁻¹ in the isotropic spectrum suggests that the nona-aqua La(III) ion is thermodynamically stable in dilute perchlorate solutions (∼0.2 mol L⁻¹) while in concentrated perchlorate solutions outer-sphere ion pairs and contact ion pairs are formed. The La³⁺ nona-hydrate was also detected in a 1.2 mol L⁻¹ La(CF₃SO₃)₃(aq). In lanthanum chloride solutions chloro-complex formation was detected over the measured concentration range from 0.5–3.050 mol L⁻¹. The chloro-complexes in LaCl₃(aq) are fairly weak and disappear with dilution. At a concentration <0.1 mol L⁻¹ almost all complexes disappeared. In LaCl₃ solutions, with additional HCl, a series of chloro-complexes of the type [La(OH₂)_9−nCl_n]⁺³⁻ⁿ (n = 1–3) were formed. The La(NO₃)₃(aq) spectra were compared with a spectrum of a 0.409 mol L⁻¹ NaNO₃(aq) and it was concluded that in La(NO₃)₃(aq) over the concentration range from 0.121–1.844 mol L⁻¹, nitrato-complexes, [La(OH₂)_9−n(NO₃)_n]⁺³⁻ⁿ (n = 1, 2) were formed. These nitrato-complexes are quite weak and disappear with dilution <0.01 mol L⁻¹. DFT geometry optimizations and frequency calculations are reported for a lanthanum-nona-hydrate with a polarizable dielectric continuum in order to take the solvent into account. The bond distances and angles for the cluster geometry of [La(OH₂)₉]³⁺ with the polarizable dielectric continuum are in good agreement with data from recent structural experimental measurements and high quality simulations. The DFT frequency of the La–O stretching mode at 328.2 cm⁻¹, is only slightly smaller than the experimental one.