Two interconverting pentaiodide forms in the cyclomaltohexaose (α-cyclodextrin) polyiodide inclusion complex with sodium ion: dielectric and Raman spectroscopy studies

The polycrystalline inclusion complex of cyclomaltohexaose, (α-CD)2·NaI5·8H2O, has been investigated via dielectric spectroscopy over a frequency range of 0-100 kHz and the temperature range of 125-450 K. Additionally, a Raman spectroscopy study was accomplished in the temperature ranges of (i) 153-298 K and (ii) 303-413 K. The ln σ versus 1/T variation revealed the order-disorder transition of some normal hydrogen bonds to those of a flip-flop type at 200.9 K. From 278.3 up to 357.1 K, the progressive transformation (H2O)tightly bound → (H2O)easily movable takes place resulting in an Arrhenius linear increment of the ac-conductivity with activation energy Ea = 0.32 eV. In the range of 357.1-386.1 K a second linear part with Ea = 0.55 eV is observed, indicating the contribution of sodium ions via the water-net.The rapid decrease of the ac-conductivity at T > 386.1 K is due to the removal of the water molecules from the crystal lattice, whereas the abrupt increase at T > 414.9 K is caused by the sublimation of iodine.The Raman bands at 160 and 169 cm−1 indicate the coexistence of (I2·I−·I2) and (I3-·I2 ↔ I2·I3-) units, respectively.The (I3-·I2 ↔ I2·I3-) units are presented as form (I), and their central I− ion is disordered in occupancy ratio different from 50/50 (e.g., …60/40…70/30…).The(I2·I−·I2) units are displayed by the 2 equiv forms (IIa) and (IIb). In (IIa) the central I− ion is twofold disordered in an occupancy ratio of 50:50, whereas in (IIb) the central I− ion is well-ordered and equidistant from the two I2 molecules. At low temperatures the transformation (I)→(IIa) takes place, whereas at high temperatures the inverse one (IIa)→(I) happens. X-ray powder diffraction and Rietveld analysis revealed a triclinic crystal form with space group P1 and lattice parameters that are in good agreement with the theoretical values.