title: Two interconverting pentaiodide forms in the cyclomaltohexaose (alpha-cyclodextrin) polyiodide inclusion complex with sodium ion: dielectric and Raman spectroscopy studies.
abstract: The polycrystalline inclusion complex of cyclomaltohexaose, (alpha-CD)(2) x NaI(5) x 8H(2)O, 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 sigma 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 357.1K, the progressive transformation (H(2)O)(tightly bound)-->(H(2)O)(easily movable) takes place resulting in an Arrhenius linear increment of the ac-conductivity with activation energy E(a)=0.32 eV. In the range of 357.1-386.1K a second linear part with E(a)=0.55 eV is observed, indicating the contribution of sodium ions via the water-net. The rapid decrease of the ac-conductivity at T>386.1K 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 (I(2) x I(-) x I(2)) and (I3(-) x I(2)<-->I(2) x I3(-)) units, respectively. The (I3(-) x I(2)<-->I(2) x 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(I(2) x I(-) x I(2)) 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 I(2) 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.
authors.affiliation: Laboratory of Physical Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, PO BOX 64004, 157 10 Zografou, Athens, Greece.
authors.forename: Vasileios G
authors.initials: VG
authors.lastname: Charalampopoulos
authors.forename: John C
authors.initials: JC
authors.lastname: Papaioannou
mesh_terms: D004563:Electrochemistry; D006860:Hydrogen Bonding; D007454:Iodides / Q000737:chemistry*; D044367:Phase Transition; D012964:Sodium / Q000737:chemistry; D013059:Spectrum Analysis, Raman; D013696:Temperature; D014867:Water / Q000737:chemistry; D014961:X-Ray Diffraction; D047391:alpha-Cyclodextrins / Q000737:chemistry*
chemical_list: D007454:Iodides; D047391:alpha-Cyclodextrins; D014867:Water; C056733:pentaiodide; D012964:Sodium; C032613:alpha-cyclodextrin