Vibrational spectra of Cs2Cu(SO4)2·6H2O and Cs2Cu(SeO4)2·nH2O (n = 4, 6) with a crystal structure determination of the Tutton salt Cs2Cu(SeO4)2·6H2O

• Solubility in the systems Cs2SO4–CuSO4–H2O and Cs2SeO4–CuSeO4–H2O at 25 °C.
• Crystal structure of Cs2Cu(SeO4)2·6H2O.
• Infrared and Raman spectra of Cs2Cu(SO4)2·6H2O and Cs2Cu(SeO4)2·nH2O (n = 4, 6).
• Hydrogen bond strength.

The solubility in the three-component systems Cs2SO4–CuSO4–H2O and Cs2SeO4–CuSeO4–H2O have been studied at 25 °C. The experimental results show that double salts, Cs2Cu(SO4)2·6H2O and Cs2Cu(SeO4)2·4H2O, crystallize from the ternary solutions within large concentration ranges. Crystals of Cs2Cu(SeO4)2·6H2O were synthesized at somewhat lower temperatures (7–8 °C). The thermal dehydration of the title compounds was studied by TG, DTA and DSC methods and the respective dehydration schemes are proposed. The calculated enthalpies of dehydration (ΔHdeh) have values of: 434.2 kJ mol−1 (Cs2Cu(SeO4)2·6H2O), 280.9 kJ mol−1 (Cs2Cu(SeO4)2·4H2O), and 420.2 kJ mol−1 (the phase transition of Cs2Cu(SO4)2·6H2O into Cs2Cu(SO4)2·H2O).The crystal structure of Cs2Cu(SeO4)2⋅6H2O was determined from single crystal X-ray diffraction data. It belongs to the group of Tutton salts, crystallizing isotypic to the respective sulfate in a monoclinic structure which is characterized by isolated Cu(H2O)6 octahedra and SeO4 tetrahedra, interlinked by hydrogen bonds and [9]-coordinated Cs+ cations.Infrared spectra of the cesium copper compounds are presented and discussed with respect to both the normal modes of the tetrahedral ions and the water molecules. The analysis of the infrared spectra of the double compounds reveals that the distortion of the selenate tetrahedra in Cs2Cu(SeO4)2·4H2O is stronger than those in Cs2Cu(SeO4)2·6H2O in agreement with the structural data. Matrix-infrared spectroscopy was applied to confirm this claim − Δν3 for SO42−SO42− ions matrix-isolated in Cs2Cu(SeO4)2·6H2O has a value of 35 cm−1 and that of the same ions included in Cs2Cu(SeO4)2·4H2O – 84 cm−1. This spectroscopic finding is due to the formation of strong covalent bands Cu–OSO3 on one hand, and on the other to the stronger deformation of the host SeO42−SeO42− tetrahedra in Cs2Cu(SeO4)2·4H2O as compared to those in Cs2Cu(SeO4)2·6H2O.The strength of the hydrogen bonds as deduced from the frequencies of νOD of matrix-isolated HDO molecules (spectral range of 2500−2200 cm−1) is discussed. An interesting feature of the spectra is the appearance of two groups of infrared bands corresponding to νOD, which are separated with 71 cm−1 (sulfate compound) and about 120 cm−1 (selenate compounds) due to the existence of two types of water molecules − those coordinated to the copper ions via shorter Cu–OH2 bonds and those coordinated to the copper ions via longer Cu–OH2 bonds. The spectroscopic experiments reveal that the equatorial water molecules in the hexahydrates exhibit a local molecular symmetry close to C2v (at least at ambient temperature), while those forming long Cu–OH2 bonds are strongly asymmetrically hydrogen bonded. The differences in the wavenumbers of the bands corresponding to the wagging modes have values larger than 100 cm−1, thus indicating strong distortions of the coordination sphere of the copper ions due to the Jahn–Teller effect.

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