Hydration and Ion Pair Formation in Aqueous Lu³⁺- Solution

Aqueous solutions of Lu³⁺- perchlorate, triflate and chloride were measured by Raman spectroscopy. A weak, isotropic mode at 396 cm^-1 (full width at half height (fwhh) at 50 cm^-1) was observed in perchlorate and triflate solutions. This mode was assigned to the totally symmetric stretching mode of [Lu(OH₂)₈]³⁺, ν₁LuO₈. In Lu(ClO₄)₃ solutions in heavy water, the ν₁LuO₈ symmetric stretch of [Lu(OD₂)₈]³⁺ appears at 376.5 cm^-1. The shift confirms the theoretical isotopic effect of this mode. In the anisotropic scattering of aqueous Lu(ClO₄)₃, five bands of very low intensity were observed at 113 cm^-1, 161.6 cm^-1, 231 cm^-1, 261.3 cm^-1 and 344 cm^-1. In LuCl₃ (aq) solutions measured over a concentration range from 0.105⁻3.199 mol·L^-1 a 1:1 chloro-complex was detected. Its equilibrium concentration, however, disappeared rapidly with dilution and vanished at a concentration < 0.5 mol·L^-1. Quantitative Raman spectroscopy allowed the detection of the fractions of [Lu(OH₂)₈]³⁺, the fully hydrated species and the mono-chloro complex, [Lu(OH₂)₇Cl]²⁺. In a ternary LuCl₃/HCl solution, a mixtrure of chloro-complex species of the type [Lu(OH₂)_8-nCl_n]^+3-n (n = 1 and 2) were detected. DFT geometry optimization and frequency calculations are reported for Lu³⁺- water cluster in vacuo and with a polarizable dielectric continuum (PC) model including the bulk solvent implicitly. The bond distance and angle for [Lu(OH₂)₈]³⁺ within the PC are in good agreement with data from structural experiments. The DFT frequencies for the Lu-O modes of [Lu(OH₂)₈]³⁺ and its deuterated analog [Lu(OD₂)₈]³⁺ in a PC are in fair agreement with the experimental ones. The calculated hydration enthalpy of Lu³⁺ (aq) is slightly lower than the experimental value.