John C. Papaioannou

Crystal Structures of the Inclusion Complexes of β-Cyclodextrin with Aliphatic Monoacids Tridecanoic Acid and (Z)-Tetradec-7-enoic Acid. Formation of [3]Pseudorotaxanes

The structures of the inclusion complexes of beta cyclodextrin with the aliphatic mono-acids tridecanoic acid (1) and (Z)-tetradec-7-enoic acid (2) have been determined at room temperature. Both compounds crystallise in P1, a = 15.654(6) Å, b = 15.650(6) Å, c = 15.937(6) Å, α = 101.58(1)°, β = 101.59(1)°, γ = 103.58(1)°, Z = 1, for 1 and a = 15.6259(9) Å, b = 15.623(1) Å, c = 15.935(1) Å, α = 101.547(2)°, β = 101.555(2)°, γ = 103.642(2)°, Z = 1, for 2. One molecule of the monoacids threads through two cyclodextrin macrocycles arranged in dimers thus forming [3]pseudorotaxanes. The host dimers are aligned along a channel in order to create a hydrophobic environment for the terminal methyl group of the guest and isolate it from the aqueous environment that surrounds the cyclodextrin dimeric units. The guests exhibit disorder over two orientations resulting in hydrogen bonding between the carboxyl groups of adjacent guest molecules along the channel and formation of carboxylic dimers. This crystal packing differs from that of β-CD complexes of homologous dicarboxylic acids.

Dielectric relaxation of α-cyclodextrin-polyiodide complexes (α-cyclodextrin)2 · BaI2 · I2 · 8H2O and (α-cyclodextrin)2 · KI3 · I2 · 8H2O

The frequency and temperature dependence of the real (ε′) and imaginary (ε″) parts of the dielectric constant of the polycrystalline complexes (α-CD)2 · Bal2 · I2 · 8H2O and (α-CD)2 · KI3 · I2 · 8H2O (α-CD = α-cyclodetrin) have been investigated over the frequency and temperature ranges 0–100 kHz and 120–300 K, respectively. The temperature dependences of ε′, ε″ and the phase shift φ show two steps, two peaks and two minima, respectively, revealing the existence of two kinds of water molecule, the tightly bound and the easily movable water molecules, in both complexes. The first peak of (T) or the first minimum of φ(T) presents the transformation of flip-flop hydrogen bonds to the normal state. The second ε″ (T) peak or φ(T) minimum corresponds to the easily movable water molecules or to a partial transformation of tightly bound to easily movable water molecules. For T > 270K both samples show semiconductive behaviour with energy gaps of 1.84eV for the (α-CD)2 · BaI2 · I2 · 8H2O complex and 1.36eV for the (α-CD)2 · KI3 · I2 · 8H2O complex. The conductivity at room temperature decreases in the order: (α-CD)2 · BaI2 · I2 · 8H2O > (α-CD)2 · LiI3 · I2 · 8H2O > (α-CD)2 · KI3 · I2 · 8H2O > (α-CD)2 · Cd0.5 · I5 · 26H2O. The relaxation time varies in a Λ-like curve (from 120 to 250 K) and rises rapidly for temperatures greater than 250 K, indicating the process of ionic movements. The activation energies around the transition temperature 0.98–1.09 k B T trans for (α-CD)2 · BaI2 · I2 · 8H2O and 1.06-1.55 k B T trans for (α-CD)2 · KI3 · I2 · 8H2O reveal the greater stability of the α-K complex against that of the α-Ba complex.

Organisation of long aliphatic monocarboxylic acids in β-cyclodextrin channels: crystal structures of the inclusion complexes of tridecanoic acid and (Z)-tetradec-7-enoic acid in β-cyclodextrin

In the crystalline state, infinite channels of β-cyclodextrin dimers host infinite arrays of self associated linear aliphatic monocarboxylic acids, thus enclosing the hydrophilic carboxy ends inside the hydrophobic channels.

Dielectric relaxation of α-cyclodextrin—polyiodide complexes (α-cyclodextrin)2 · LiI3 · I2 · 8H2O and (α-cyclodextrin)2 · Cd0.5 · I5 · 26H2O

The frequency and temperature dependence of real (ε′) and imaginary (ε″) parts of the dielectric constant of polycrystalline complexes (α-CD)2 · LiI3 · I2 · 8H2O and (α-CD)2 · Cd0.5 · I5 · 26H2O (α-CD = α-cyclodextrin) has been investigated over the frequency and temperature ranges of 0–100 kHz and 12–300 K. The dielectric behaviour is described well by Debye type relaxation (α-dispersion). Both systems exhibit an additional Ω dispersion at low frequencies which is attributed to ionic conductance and is much greater in the case of Li due to the greater mobility of cations Li+. The temperature dependence of ε′ reveals the existence of two kinds of water molecule in the case of the (α-CD)2 · Cd0.5 · I5 · 26H2O complex; these can be classified as tiqhtly bound and easily movable water molecules that cause two steps in ε′ versus T plots. In the case of the (α-CD)2 · LiI3 · I2 · 8H2O complex the water molecules are tightly bound and as a result only one step is observed in these graphs. These finding are also confirmed from the ε″max versus T plots, which exhibit the same number of steps with ε′, and from calorimetric measurements. The order-disorder transition or the transformation of normal hydrogen bonds to flip-flop type has been observed as a peak in ε″ versus T plots that is more intense and narrow in the case of Li and less high but more broad in Cd. The relaxation time vanes in a α-like curve (from 120 K to 240 K) and rises rapidly for temperatures greater than 240 K, indicating the existence of a new process involving the breaking of hydrogen bonds (normal or flip-flop type). The calculated values of activation energy (0.35–0.62 kBTtrans) reveal the greater stability of the Li compared with the Cd complex. The starting value of 8.2–8.4 μs for τ is the same as observed in β-CD complexes with guest 4-t-butylbenzyl alcohol (TERB). However, the activation energies of these are greater (1.1–1.7kBTtrans), indicating greater stability for β-CD complexes.

Impedance spectroscopy study of nickel electrodeposits

Abstract Nickel electrodeposits prepared in NiSO4 and NiCl2 electrolytes with thicknesses of 43–49.4 μm were examined by the impedance spectroscopy (IS) method for a–c frequencies 1 Hz–100 kHz. The Nyquist diagrams were single, almost perfect semicircles, suggesting the applicability of the in parallel combination of the Ra–Ca elements characterised by a single relaxation time, 8.27×10−6 s, combined in series with another Rb element, Rb≪Ra. This relaxation time predicted that the incorporated hydrogen atoms are involved in the process of conduction by a hopping/diffusion mechanism, which is differentiated in the cases of NiSO4–Ni and NiCl2–Ni deposits. The diffusion coefficient of hydrogen, 1.25×10−11 cm2 s−1, was determined. The microstructure of Ni electrodeposits was also studied and characterised by electron microscopy. Finally, a general model for the microstructure of Ni electrodeposits, consistent with the results of impedance spectroscopy, was formulated.

Dielectric relaxation of β-cyclodextrin–polyiodide complexes (β-cyclodextrin)2·LiI7·8H2O and (β-cyclodextrin)2·KI7·8H2O

The frequency and temperature dependence of real (ε′) and imaginary (ε′′) parts of the dielectric constant of polycrystalline complexes (β-CD)2·LiI7·8H2O and (β-CD)2·KI7·8H2O (β-CD = β-cyclodextrin) have been investigated over the frequency and temperature ranges of 0–100 kHz and 120–300 K. The temperature dependence of ε′, ε′′ and phase shift ϕ showing two steps, two peaks and two minima respectively, reveals the existence of two kinds of water molecule, the tightly bound and the easily movable water molecules, in both complexes. The first peak of ε′′(T) or the first minimum of ϕ(T) present the transformation of flip-flop hydrogen bonds to the normal state. The second ε′′(T) peak or ϕ(T) minimum correspond to the easily movable water molecules or to a partial transformation of tightly bound to easily movable water molecules. Both samples for T>275 K show semiconductive behaviour with energy gaps 0.72 eV for the (β-CD)2·LiI7·8H2O and 0.58 eV for the (β-CD)2·KI7·8H2O complex. The conductivity at temperatures T>220 K is greater for the Li complex and at T < 220 K both complexes have similar conductivity values. The relaxation time varies in a Λ-like curve (from 180 K to 260 K) and rises rapidly for temperatures greater than 260 K, indicating the process of ionic movements. The activation energies around the transition temperature 0.40–0.50 k B T trans for the (β-CD)2·LiI7·8H2O and 0.78–1.00 k B T trans for the (β-CD)2·KI7·8H2O reveal the greater stability of the β-K complex against that of the β-Li complex.

Electronic properties of sodium-C222-sodide

The electrical conductivity of both polycrystalline and single-crystal Na+C222 · Na− was measured as a function of temperature from 220 to 280 K and the dielectric constant of polycrystalline samples was studied as a function of frequency from 1 kHz to 10 MHz at several temperatures. The results show both electronic and ionic conductivity for single crystals but the ionic contribution is suppressed in polycrystalline samples. The activation energies for ionic and electronic transport are 1.6 ± 0.2 eV and 1.2 ± 0.1 eV, respectively. After being subjected to a dc potential of >2.0 V for a period of time, single crystals had an open circuit voltage that decayed only slowly with time, showing that the system behaves as an electrochemical cell. When the potential was <2.0 V, the effect was absent. The results are in agreement with ionic conduction by Na+ superimposed on a background of electronic conduction.

An insight into the disorder properties of the α-cyclodextrin polyiodide inclusion complex with Sr2+ ion: dielectric, DSC and FT-Raman spectroscopy studies

At T < 250 K, the polyiodide inclusion complex (α-cyclodextrin)2·Sr0.5·I5·17H2O displays two separate relaxation processes due to both the frozen-in proton motions in an otherwise ordered H-bonding network and the order–disorder transition of some normal H-bonds to flip-flop ones. At T>250 K, the AC-conductivity is dominated by the combinational contributions of the disordered water network, the mobile Sr2+ ions, the polyiodide charge-transfer interactions and the dehydration process. The evolution of the Raman spectroscopic data with temperature reveals the coexistence of four discrete pentaiodide forms. In form (I) (I− 3·I2 ↔ I2·I− 3), the occupancy ratio (x/y) of the central I− ion differs from 50/50. In form (IIa) (I2·I− ·I2) x/y = 50/50, whereas in its equivalent form (IIb) (I2·I− ·I2) * as well as in form (III) (I− 3·I2), x/y = 100/0 (indicative of full occupancy). Through slow cooling and heating, the inverse transformations (I) → (IIa) and (IIa) → (I) occur, respectively.

Dielectric behaviour of α-cyclodextrin, heptakis (2,3,6-tri-Omethyl)-β-cyclodextrin, randomly methylated β-cyclodextrin and low frequency Raman spectra of α- and β-cyclodextrins

The frequency and temperature dependence of the real (ε′) and imaginary (ε″)parts of the dielectric constant of α-cyclodrextrin (form 1; α-CD. 5.9H2O) and α-cyclodextrin (form III; α-CD.7.6H2O) and of the corresponding dried forms (α-CD.1.1H2O, α-CD.2.9H2O, respectively) has been investigated over the frequency range 0–100 k Hz and temperature range 130–350 K. In addition the dielectric behaviour has been investigated of heptakis-(2,3,6-tri-O-methyl)-β-cyclodextrin (β-CD.TRIME. 0.3H2O) and randomly methylated β-cyclodextrin (β-CD.RAME.0.8H2O). The dielectric behaviour is described well by Debyetype relaxation (α-disperson). All α-CD systems exhibit an additional ω-dispersion at low frequencies, which is attributed to proton transport. The fact that the ε′ values of α-CD form III are larger than those of α-CD form I is attributed to the different numbers and different strengths of the intramolecular hydrogen bonds. Form III has a stable conformation which is shown by the constant values of ε′ in the temperature range 125–250 K. By contrast, in form I the ε′ values increased linearly with temperature, indicating that the system passed through succesive conformations. The temperature dependence of ε″ and phase shift ∞ in all the specimens of α-CD (forms I, III) and fully methylated β-CD do not reveal any orderdisorder transition, because the developed hydrogen bonds ae not of the flip-flop type according to their crystal structures. The partially methylated β-CD reveals the characteristics of the order-disorder transition, which was observed before in the systems dried β-CD.2H2O and non-dried β-CD.9.8H2O. There is a direct relation between the hydroxyl groups of β-CD and the order-disorder transition. The order-disorder transition could also be shown in dried β-CD.2.4H2O and non-dried β-CD.9.8H2O samples but not in dried α-CD.1.1H2O and non-dried a-CD.5.9H2O samples, by low frequency Raman spectroscopy (< 180 cm−1). The step-like temperature dependence of the band at 33.7cm−1 reveals a transition at about 223K for both β-CD samples. In the case of α-CD samples the almost linear temperature dependence of the band at 49.1 cm−1 does not reveal any transition.

Four‐probe single‐crystal holder for conductivity measurements

The development and testing of a four‐probe small single‐crystal holder for conductivity studies down to liquid‐helium temperature is described. A single crystal of the reactive sodide, Na+C222⋅Na–, was successfully mounted, but its resistance was too high to permit determination of the conductivity. The method should be useful for single‐crystal conductivity measurements of reactive and/or thermally unstable crystals.