Hydration and ion pair formation in aqueous Y3+–salt solutions

Abstract

Raman spectra of aqueous yttrium perchlorate, triflate (trifluoromethanesulfonate), chloride and nitrate solutions were measured over a broad concentration range (0.198–3.252 mol L⁻¹). The spectra range from low wavenumbers to 4200 cm⁻¹. A very weak mode at 384 cm⁻¹ with a full width at half height at 50 cm⁻¹ in the isotropic spectrum suggests that the Y³⁺ – octa-aqua ion is thermodynamically stable in dilute perchlorate solutions (∼0.5 mol L⁻¹) while in concentrated perchlorate solutions outer-sphere ion pairs and contact ion pairs are formed. The octa-hydrate, [Y(OH₂)₈]³⁺ was also detected in a 1.10 mol L⁻¹ aqueous Y(CF₃SO₃)₃ solution. Furthermore, very weak and broad depolarized modes could be detected which are assigned to [Y(OH₂)₈]³⁺(aq) at 100, 166, 234 and 320 cm⁻¹ confirming that a hexa-hydrate is not compatible with the hydrated species in solution. In yttrium chloride solutions contact ion pair formation was detected over the measured concentration range from 0.479–3.212 mol L⁻¹. The contact ion pairs in YCl₃(aq) are fairly weak and disappear with dilution. At a concentration <0.2 mol L⁻¹ almost all complexes have disappeared. In YCl₃ solutions, with additional HCl, chloro-complexes of the type [Y(OH₂)_8−nCl_n]⁺³⁻ⁿ (n = 1,2) are formed. The Y(NO₃)₃(aq) spectra were compared with a spectrum of a dilute NaNO₃ solution and it was concluded that in Y(NO₃)₃(aq) over the concentration range from 2.035–0.198 mol L⁻¹ nitrato-complexes [Y(OH₂)_8−n(NO₃)l_n]⁺³⁻ⁿ (n = 1,2) are formed. The nitrato-complexes are weak and disappear with dilution <0.1 mol L⁻¹. DFT geometry optimizations and frequency calculations are reported for both the yttrium–water cluster in the gas phase and the cluster within a polarizable continuum model in order to implicitly describe the presence of the bulk solvent. The bond distance and angle for the square antiprismatic cluster geometry of [Y(OH₂)₈]³⁺ with the polarizable dielectric continuum is in good agreement with data from recent structural experimental measurements. The DFT frequency of the Y–O stretching mode of the [Y(OH₂)₈]³⁺ cluster, in a polarizable continuum, is at 372 cm⁻¹ in satisfactory agreement with the experimental value.