Hydration and Ion Pair Formation in Aqueous Lu3+- Solution

Abstract

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−nCln]+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.

On the Hydration of the Rare Earth Ions in Aqueous Solution

“…The peak positions of the totally symmetric stretching mode, ν 1 Ln-(OH) 2 , for the rare earth ions in the middle of the series show a slight concentration effect which has been observed also for the light and heavy rare earth ions even though perchlorate is known as a weak complex forming anion [28][29][30]35]. The concentration dependence of the 1 Ln-(OH 2 ) mode of the RE ions in the middle of the series may be illustrated by the spectra of Gd(ClO 4 ) 3 (aq) and Dy(ClO 4 ) 3 (aq).…”

“…The concentration dependence of the 1 Ln-(OH 2 ) mode of the RE ions in the middle of the series may be illustrated by the spectra of Gd(ClO 4 ) 3 (aq) and Dy(ClO 4 ) 3 (aq). This phenomenon was described for a variety of lanthanide solutions in our previous contributions [28][29][30]35]. These earlier Raman spectroscopic contributions characterizing rare earth ions in aqueous perchlorate solutions showed that the solute concentration must be sufficiently small (~ 0.3 mol·L -1 , which translates to a solute to water ration at ca.…”

“…2.8 cm -1 Δ for the HRE between neighboring ions. It should be pointed out that this frequency difference between nona-and octa-hydrates is much larger than the uncertainty in determining 1 with an accuracy of ± 1 cm -1 [30,31,35] and, therefore, reflects the coordination change well.…”

“…1 ps for establishment of an altered geometry [45]. The vibrational periods of the 1 Ln-OH 2 vibrations range from 0.097 ps for La 3+ to 0.082 ps for Lu 3+ [30,31,35] so that the modes vibrate several hundred thousand times before the hydrated Ln 3+ species experiences a water exchange. (The establishment of slightly altered geometry according to Rode et al [45,46] is of the order of a ps and allows only a few vibrational periods to occur.…”

“…In addition to the ν 1 Ln-(OH 2 ) mode, the totally symmetric stretching mode, other modes, broad, and of much weaker intensity could be observed as well for this ions (see especially[28][29][30]35]). …”