Abstract
Raman spectra of aqueous Ho3+, Er3+, Tm3+, Yb3+, and Lu3+-perchlorate solutions were measured over a large wavenumber range from 50–4180 cm−1. In the low wavenumber range (terahertz region), strongly polarized Raman bands were detected at 387 cm−1, 389 cm−1, 391 cm−1, 394 cm−1, and 396 cm−1, respectively, which are fairly broad (full widths at half height at ~52 cm−1). These isotropic Raman bands were assigned to the breathing modes, ν1 Ln–O of the heavy rare earth (HRE) octaaqua ions, [Ln(H2O)8]3+. The strong polarization of these bands (depolarization degree ~0) reveals their totally symmetric character. The vibrational isotope effect was measured in Yb(ClO4)3 solutions in H2O and D2O and the shift of the ν1 mode in changing from H2O to D2O further supports the character of the band. The Ln–O bond distances of these HRE ions (Ho3+, Er3+, Tm3+, Yb3+, and Lu3+) follow the order of Ho–O > Er–O > Tm–O > Yb–O > Lu–O which correlates inversely with the band positions of the breathing modes of their corresponding octaaqua ions [Ln(OH2)8]3+. Furthermore, the force constants, kLn–O, were calculated for these symmetric stretching modes. Ytterbium perchlorate solutions were measured over a broad concentration range, from 0.240 mol·L−1 to 2.423 mol·L−1, and it was shown that with increasing solute concentration outer-sphere ion pairs and contact ion pairs were formed. At the dilute solution state (~0.3 mol·L−1), the fully hydrated ions [Yb(H2O)8]3+ exist, while at higher concentrations (CT > 2 mol·L−1), ion pairs are formed. The concentration behavior of Yb(ClO4)3 (aq) shows similar behavior to the one observed for La(ClO4)3(aq), Ce(ClO4)3(aq) and Lu(ClO4)3(aq) solutions. In ytterbium chloride solutions in water and heavy water, representative for the behavior of the other HRE ions, 1:1 chloro-complex formation was detected over the concentration range from 0.422–3.224 mol·L−1. The 1:1 chloro-complex in YbCl3(aq) is very weak, diminishing rapidly with dilution and vanishing at a concentration < 0.4 mol·L−1.
Highlights
The heavy rare earth (HRE) ions of holmium, erbium, thulium, ytterbium, and lutetium exist in the tervalent state [1] and, with their high charge to radius ratio, are strongly hydrated [2,3]
The hydration geometry of heavy rare earth ions in aqueous solution was determined by X-ray (XRD) and neutron diffraction (ND) [4,5,6] as well as extended X-ray absorption fine structure (EXAFS) [7,8,9,10] techniques
4 )43)(aq), 4 )43)3(aq),: First, First, we present results onon below by present and anddiscuss discussthe thespectroscopic spectroscopic results
Summary
The HRE ions of holmium, erbium, thulium, ytterbium, and lutetium exist in the tervalent state [1] and, with their high charge to radius ratio, are strongly hydrated [2,3]. It was shown that light rare earth ions form chloro-complex species, [14,15,16,17]. In order to clarify whether chloro-complex species may form in these HRE chloride solutions, YbCl3 solutions were measured at varying concentrations in water and heavy water. Inner-sphere chloro- complexes were detected in aqueous solutions on a variety of rare earth chloride solutions using Raman spectroscopy [14,15,16,17]. The influence of Cl− on the fully hydrated Ln3+ (aq) was studied exemplarily on YbCl3 solutions in water and heavy water These results are discussed in connection with recently measured aqueous LuCl3 solutions [17]
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