Correlation of dielectric properties, Raman spectra and calorimetric measurements of β-cyclodextrin-polyiodide complexes (β-cyclodextrin)₂·BaI₇·11H₂O and (β-cyclodextrin)₂·CdI₇·15H₂O

Abstract

The frequency and temperature dependence of real and imaginary parts of the dielectric constant (ε′, ε″), the phase shift (ϕ) and the ac-conductivity (σ) of polycrystalline complexes (β-CD)₂·BaI₇·11H₂O and (β-CD)₂·CdI₇·15H₂O (β-CD = β-cyclodextrin) has been investigated over the frequency and temperature ranges 0–100 kHz and 140–420 K in combination with their Raman spectra, DSC traces and XRD patterns. The ε′(T), ε″(T) and ϕ(T) values at frequency 300 Hz in the range T<330 K show two sigmoids, two bell-shaped curves and two minima respectively revealing the existence of two kinds of water molecule, the tightly bound and the easily movable. Both complexes show the transition of normal hydrogen bonds to flip-flop type at 201 K. In the β-Ba complex most of the eleven water molecules remain tightly bound and only a small number of them are easily movable. On the contrary, in the β-Cd case the tightly bound water molecules are transformed gradually to easily movable. Their DSC traces show endothermic peaks with onset temperatures 118°C, 128°C for β-Ba and 106°C, 123°C, 131°C for β-Cd. The peaks 118°C, 106°C, 123°C are related to the easily movable and the tightly bound water molecules, while the peaks at 128°C, 131°C are caused by the sublimation of iodine. The activation energy of Ba²⁺ ions is 0.52 eV when all the water molecules exist in the sample and 0.99 eV when the easily movable water molecules have been removed. In the case of β-Cd the corresponding activation energies are 0.57 eV and 0.33 eV. The Raman peaks at 179 cm⁻¹, 170 cm⁻¹ and 165–166 cm⁻¹ are due to the charge transfer interactions in the polyiodide chains.

Acknowledgements

This work was partly supported by Grant No. 70/4/3347SARG, NKUA. We thank Professor K. Viras for his assistance with the Raman spectra and DSC thermograms.