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

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 (similar to 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-n)Cl-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.