Dipole relaxation and proton transport in polycrystalline γ-cyclodextrin hydrate: A dielectric spectroscopy study

The polycrystalline γ-cyclodextrin hydrate (γ-CD•12. 2H2O) has been investigated via dielectric spectroscopy over a frequency range of 0-100 kHz and the temperature ranges of 108.0-298.5 K (cooling) and 109.0-433.0 K (heating). At T < 250.0 K, the electrical properties of the sample accept a great contribution from the flip-flop proton orientational disorder and a much lesser one from the positional fluctuations of the water molecules. Moreover, a strong synergy is observed between the stability of the γ-CD molecules and the dynamic disorder of the infinite flip-flop chains. This type of disorder disappears upon cooling (T trans = 186.7 K) and reappears upon heating (Ttrans = 194.5 K). At T > 250.0 K, the dielectric permittivity ε′ and loss ε″ increase abruptly due to the proton dc-conductance of γ-CD•12.2H2O which has been interpreted in terms of a theoretical model (Pnevmatikos, 1988) being consistent with the generation of ionic defects and their combination with the dipole reorientations in a collective motion of soliton-type. The influence of the simultaneous dehydration process on this charge transport mechanism relies on the very sensitive balance between the diffusive motion of water molecules (exchange between symmetry related positions) and their removal from the crystal lattice. The Arrhenius semiconductive behavior of the ac-conductivity in the ranges of 257.1-313.2 K (Ea = 0.42 eV) and 331.2-385.1 K (Ea = 0.39 eV) implies the dominance of water diffusion which conserves the structural integrity of the endless hydrogen-bonded chains and the proton transfer along them. The limited decrease of the ac-conductivity from 313.2 to 331.2 K along with its rapid decrease above 385.1 K, indicates that the removal of the water molecules rules out their diffusive motion. The Cole-Cole diagrams (ε″ vs. ε′) make clear that during the heating process the grain boundary polarization gradually becomes more significant than the grain interior one. In the range of 348.0-385.1 K, the constrictive grain boundary resistances are totally eliminated allowing the extensive proton transport through the grains of the polycrystalline specimen. © 2011 Published by Elsevier B.V

High density flip-flop hydrogen-bonding networks in the β-cyclodextrin heptaiodide inclusion complexes with Bi3+ and Te4+ ions. Combined dielectric relaxation, Raman scattering and thermal analysis

The polycrystalline inclusion complexes (β-CD)2·TeI7·17H2O and (β-CD)2·BiI7·17H2O have been investigated via dielectric spectroscopy over a frequency range of 0-100 kHz and the temperature range of 140-425 K. Furthermore, a DSC study was carried out in the range of 273-423 K, whereas the Raman spectra (303-393 K) of β-Te were compared to the previously examined ones of β-Bi. In the case of β-Te an important percentage of normal H-bonds is transformed into flip-flop ones (Ttrans = 216.8 K) as it comes out by the corresponding ε′ (T), ε″ (T) and φ(T) variations at T < 250 K (Δε′ = 18.6, ε″max = 4.8, φmin = 69.9°). In β-Bi the greatest percentage of normal H-bonds is transformed into those of the flip-flop type (Ttrans = 223.6 K, Δε′ = 49.6, ε″max = 16, φmin = 58.6ο) producing a disordered H-bonding network of a much higher density than that of β-Te. At T > 250 K, the ac-conductivity (lnσ vs. 1 / T) of these systems follows an Arrhenius behaviour with activation energies 0.54 and 0.46 eV for β-Te and 0.38, 0.68 and 0.58 eV for β-Bi. This exponential increment reflects the combined contributions of the water network, the oscillating cations and the dehydration process. The abrupt increase of the ac-conductivity at T > 398.5 K is caused by the sublimation of iodine. The temperature-dependent Raman spectra of β-Te exhibit the band shift of 178 → 172 cm- 1 which is identical to that of β-Bi, implying a similar elongation of their I2 units. The high density flip-flop hydrogen-bonding network in the latter complex seems to play a key role in limiting the Lewis base character of I-3. © 2008 Elsevier B.V. All rights reserved

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

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 0-100kHz 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 0.52eV when all the water molecules exist in the sample and 0.99eV when the easily movable water molecules have been removed. In the case of β-Cd the corresponding activation energies are 0.57eV and 0.33eV. The Raman peaks at 179cm-1, 170cm-1 and 165-166cm-1 are due to the charge transfer interactions in the polyiodide chains. © 2005 Taylor & Francis Group Ltd

Dielectric and Raman spectroscopy of the heptaiodide complex (β-Cyclodextrin)2·CsI7·13H2O

The frequency and temperature dependence of the real and imaginary parts of the dielectric constant (ε′,ε″), the phase shift (φ) and the ac-conductivity (σ) of the polycrystalline (β-Cyclodextrin) 2·CsI7·13H2O (β-Cs) have been investigated over the frequency and temperature ranges of 0-100 kHz and 140-425 K, respectively, in combination with Raman spectroscopy and DSC. The ε′(T), ε″(T) and φ(T) variations at frequency 300 Hz in the range 140 K<T<300 K show two sigmoids, two bell-shaped curves and two minima, respectively, revealing the existence of two kinds of water molecules, tightly bound and easily movable ones. β-Cs shows the transition of normal hydrogen bonds to those of flip-flop type at 199.9 K. As the temperature increases most of the thirteen water molecules per cyclodextrin dimer remain tightly bound and only a small number of them become easily movable. The DSC trace shows a small endothermic peak with an onset temperature of 80 °C, which is related to the easily movable water molecules. Strong peaks at 115 and 135 °C are caused by the tightly bound water molecules and the sublimation of iodine, respectively. The Cs+ ions contribute to the ac-conductivity via a Grotthuss mechanism with an activation energy 0.64 eV when all the water molecules exist in the crystal lattice and 0.45 eV when the easily movable water molecules start to escape. The Raman peaks at 179, 170 and 166 cm-1 are due to the I2·I-3·I2 polyiodide chains consisting of I-3 units indicating charge transfer interactions and lengthening of I2 units, respectively. The charge of I-7 units remains localized with negligible contribution to the conductivity until the sublimation of iodine starts. © 2005 Elsevier SAS. All rights reserved

A transformation I2·I-·I2 ↔ I3-·I2 in the pentaiodide complex (α-Cyclodextrin)2·Cd0.5·I5·26H2O, detected via dielectric and Raman spectroscopy

The ac-conductivity and the phase shift of the polycrystalline complex (α-CD)2·Cd0.5·I5·26H2O (α-CD = α-Cyclodextrin) have been investigated over the frequency and temperature ranges of 0-100 kHz and 240-425 K. A Raman spectroscopy study is also accomplished in the temperature ranges of (i) 153-293 K and (ii) 303-383 K. From 276.2 up to 335.6 K - where all the water molecules of the crystal lattice exist - the transformation (H2O)tightlybound → (H2O)easilymovable takes place, resulting in the linear increment of the ac-conductivity in the lnσ vs. 1/T plot with activation energy Ea = 0.51 eV. In the range of 335.6-377.0 K a second linear part with Ea = 0.67 eV is observed attributed to the contribution of Cd2+ ions via the water-net. At T > 377.0 K, the abrupt decrease of the ac-conductivity up to 401.6 K is due to the removal of all the water molecules from the lattice. The phase shift presents a topical minimum at 404.3 K directly related to an order-disorder transition of the I- ions in some pentaiodide units and an abrupt decrease at T > 412.9 K due to the sublimation of iodine molecules. The Raman spectra at room temperature present two bands at 160 and 168 cm- 1 indicating the coexistence of two kinds of pentaiodide units I2·I-·I2 and I3-·I2 ↔ I2·I3-, respectively. Because of the inverse transformation (I3-·I2 ↔ I2·I3-) ↔ (I2·I-·I2) the band at 168 cm- 1 disappears as the temperature decreases whereas the band at 160 cm- 1 disappears during the heating process. The X-ray powder diffraction and the Rietveld analysis revealed a tetragonal crystal form with space group P42212 and lattice parameters that are in good agreement with the theoretical values. © 2007 Elsevier B.V. All rights reserved

Metal-heptaiodide interactions in cyclomaltoheptaose (β-cyclodextrin) polyiodide complexes as detected via Raman spectroscopy

The Raman spectra of the cyclomaltoheptaose (β-cyclodextrin, β-CD) polyiodide complexes (β-CD)2·NaI7·12H2O, (β-CD)2·RbI7·18H2O, (β-CD)2·SrI7·17H2O, (β-CD)2·BiI7·17H2O and (β-CD)2·VI7·14H2O (named β-M, M stands for the corresponding metal) are investigated in the temperature range of 30-140 °C. At room temperature all systems show an initial strong band at 178 cm-1 that reveals similar intramolecular distances of the disordered I2 units (∼2.72 Å). During the heating process β-Na and β-Rb display a gradual shift of this band to the final single frequency of 166 cm-1. In the case of β-Sr and β-Bi, the band at 178 cm-1 is shifted to the final single frequencies of 170 and 172 cm-1, respectively. These band shifts imply a disorder-order transition of the I2 units whose I-I distance becomes elongated via a symmetric charge-transfer interaction I2←I3-→I2. The different final frequencies correspond to different bond lengthening of the disordered I2 units during their transformation into well-ordered ones. In the Raman spectra of β-V, the initial band at 178 cm-1 is not shifted to a single band but to a double one of frequencies 173 and 165 cm-1, indicating a disorder-order transition of the I2 molecules via a non-symmetric charge-transfer interaction I2←I3- → I2. The above spectral data show that the ability of I3- to donate electron density to the attached I2 units is determined by the relative position of the different metal ions and their ionic potential q/r. The combination of the present results with those obtained from our previous investigations reveals that cations with an ionic potential that is lower than ∼1.50 (Cs+, Rb+, Na+, K+ and Ba2+) do not affect the Lewis base character of I3-. However, when the ionic potential of the cation is greater than ∼1.50 (Li+, Sr2+, Cd2+, Bi3+ and V3+), the Mn+⋯I3- interactions become significant. In the case of a face-on position of the metal (Sr2+, Bi3+) relative to I3-, the charge-transfer interaction is symmetric. On the contrary, when the metal (Li+, Cd2+, V3+) presents a side-on position relative to I3-, the charge-transfer interaction is non-symmetric. © 2007 Elsevier Ltd. All rights reserved

Dielectric spectroscopy investigation of proton transfer processes in carboxymethyl alpha-cyclodextrin polymer cross-linked by epichlorohydrin

The carboxymethyl-α-cyclodextrin polymer (cross-linked by epichlorohydrin) is investigated by dielectric spectroscopy over a frequency range of 0.1–100 kHz and the temperature ranges of 137.2–297.6 K (cooling) and 137.2–472 K (heating). Upon cooling to 288.1 K, the ac-conductivity invariance is attributed to slight changes in the topology of the H-bonded chains. From 288.1 to 244.0 K, the ac-conductivity decreases abruptly (following the Arrhenius law with Eα = 0.40 eV), whereas below 244.0 K it presents no important variations. During heating from 137.2 to 302.6 K, no thermal hysteresis is observed. From 302.6 to 364.9 K, the ac-conductivity increases (Eα = 0.71 eV), whereas above 383 K it decreases up to 436.7 K since the dehydration process has been completed and the H-bonded chains can no longer be retained. From 436.7 to 472 K, the ac-conductivity increases again (Eα = 0.76 eV) indicating the formation of “new” H-bonded chains. Curve fitting of various relaxation processes is done by Havriliak-Negami equation at selective temperatures. © 201

AC-conductivity and Raman spectra of polyiodide inclusion compounds (β-cyclodextrin)2·KI7·16H2O and (β-cyclodextrin)2·LiI7·14H2O during the dehydration process

The frequency and temperature dependence of ac-conductivity and phase shift of polycrystalline inclusion compounds (β-CD)2·KI7·16H2O and (β-CD)2·LiI7·14H2O (β-CD=β-cyclodextrin) has been investigated over the frequency and temperature ranges of 0-100 kHz and 240-420 K. A Raman spectroscopic study and calorimetric measurements are also accomplished. The Arrhenius exponential behaviour σ = σ0 exp (- EW / 2 KB T) of the ac-conductivity for T>275 K is caused by the contribution of the metal cations K+, Li+. This contribution is facilitated by the water-net via the Grotthuss mechanism. The ac conductivity starts deviating from the exponential behaviour with lower increasing rate, at 347 K for β-K and at 353 K for β-Li reaching a maximum value at 371.1 and 361.8 K, respectively, and then decreases rapidly due to the gradual removal of all the water molecules. The values 371.1 and 361.8 K are characterized as semiconductor to metal transition temperatures. The shift of the initial Raman peak at 179 cm-1 to the final value 165 cm-1 as the temperature increases reveals the lengthening of I2 units via a charge transfer interaction in I-7 units. A second topical maximum value of conductivity appears at 399.7 K for β-K and 403 K for β-Li, attributed to the sublimation of I2. © 2006 Elsevier Ltd. All rights reserved

Significant modification of the I - 3 Lewis base character in the β-cyclodextrin polyiodide inclusion complex with Co 2+ ion: An FT-Raman investigation

The β-cyclodextrin (β-CD) polyiodide inclusion complex (β-CD) 2·Co 0.5·I 7· 21H 2O has been synthesized, characterized and further investigated via FT-Raman spectroscopy in the temperature range of 30-120 °C. The experimental results point to the coexistence of I - 7 units (I 2·I - 3·I 2) that seem not to interact with the Co 2+ ions and I - 7 units that display such interactions. The former units exhibit a disorder-order transition of both their I 2 molecules above 60 °C due to a symmetric charge-transfer interaction with the central I - 3 [I 2 ← I - 3 → I 2], whereas in the latter units only one of the two I 2 molecules becomes well-ordered above 30 °C. The other I 2 molecule remains disordered presenting no charge-transfer phenomena. The Co 2+ ion induces a considerable asymmetry on the geometry of the I - 3 anion and a significant modification of its Lewis base character. Complementary dielectric measurements suggest no important involvement of H⋯I contacts in the observed modification of the I - 3 electron-transfer properties. © 2011 Elsevier B.V