Impedance measurements using ionic contacts on purified and doped silver bromide crystals

Impedance measurements are made on purified, vacancy-doped and interstitial-doped silver bromide crystals of thicknesses from 0.061 cm to 0.260 cm using constant ionic strength solution contacts at temperatures from 20°C to 35°C. Results characterize the macroscopic transport properties from 316 kHz to 0.01 Hz and at d.c. In all cases the high frequency limit resistive component of the impedance, R_∞, is equal to the d.c. resistance R₀. This result indicates rapid ion exchange at the surfaces and neither Warburg diffusion nor surface kinetic-limitation of transport. As expected, R_∞ is a function of charge carrier concentration, which depends on extrinsic dopant concentration over the temperature range measured. For pure crystals the resistance, R_∞, is linearly dependent on crystal thickness, δ, while thickness correlation could not be tested for the doped crystals as dopant level was not uniform. The geometric capacitance for all crystals, C_g, is linear with δ^?1. The dimensionless dielectric constant, κ, calculated from C_g and crystal dimentsions, is 13.9±0.7. The electric relaxation time, τ_el, defined as the product R_∞C_g, at 25°C is found to be 50±3 μs for pure crystals, 1 to 4 μs for the Cd²⁺-doped crystals and 12 to 66 μs for the S^2?-doped crystals. Temperature dependance of R_∞ allows determination of crystal transport activation energies. For pure and Cd²⁺-doped crystals a value of 0.33±0.02 eV is found. For the S²⁺-doped crystals values from 0.48 to 0.28 eV are found. The parameter α, in the non-ideal Cole-Cole representation of impedance plane arcs, is 0.96±0.01 for purified crystals, 0.93±0.03 for Cd²⁺-doped, and 0.92±0.02 for S^2?-doped crystals.     

Evaluation of the Static Permittivity of Aqueous Electrolytes

The static (relative) permittivity of aqueous electrolyte solutions at 25 °C, ε(ω = 0) where ω is the frequency of the external field, has been obtained from the literature. The limiting dielectric decrements δ = ? lim(c → 0)dε/dc, where c is the electrolyte concentration (≤ 1 mol·dm^?3), are tabulated. The most appropriate additive individual ionic δ _i values are derived. The effect of ion pairing in the solutions is briefly discussed.     

Simultaneous kinetic and microdielectric studies of some epoxy–amine systems

Three reactive epoxy–amine systems based on diglycidyl ether of bisphenol A (DGEBA) with 4,4′-diaminodiphenylsulfone (DDS), 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA), and 4,4′-methylenebis [2,6-diethylaniline] (MDEA), were studied during isothermal curings at 140 and 160°C. The simultaneous kinetic and dielectric studies allow to express conductivity, σ, in terms of conversion, x, and of glass transition temperature, T_g. The conductivity, σ₀, of the initial monomer mixture and, σ_∞ of the fully cured network are measured. It is found that:

• The glass transition temperature, T_g, versus conversion, x, curves follows the equation of Di Benedetto modified by Pascault and Williams
• There exists a linear relation between log σ/log σ₀ and T_g.

So, it is possible to predict both kinetic and dielectric behaviors of these epoxy-amine systems by the knowledge of T_g0, ΔC_p0, and σ₀, respectively, glass transition temperature, heat capacity, and conductivity of initial monomer mixture, T_g∞ and ΔC_p∞, and σ_∞, respectively, glass transition temperature and heat capacity and conductivity of fully cured network. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2911–2921, 1998     

Investigations by electrochemical quartz crystal impedance system-electrodeposition of silver and polyaniline

An electrochemical quartz crystal impedance system (EQCIS) which allows rapid and simultaneous measurements of admittance spectra of piezoelectric quartz crystal resonance during electrochemical processes was developed by combining an HP 4395A Network/Spectrum/Impedance analyzer with an EG & G M283 potentiostat. Non-linear least square regression analyses of simultaneously acquired conductance and susceptance data were discussed in detail, giving that R_m, C_s, 1/C_m (or L_m) and of as estimation parameters is the best choice among various fitting routines. Equivalent electrical circuit parameters of quartz crystal resonance during electrodeposition of silver and polyaniline and electrochemical processes of the deposits were obtained and discussed according to changes in electrode mass, electrode surface roughness and film conductivity etc. The significant changes of motional resistance R_m and static capacitance C, observed in the silver case was believed to result mainly from changes in electrode surface roughness and the linear relationship between them was well explained by the following equation, C_s = C_q+ C_e = ε_qA_q/ h_q + ε_ek²R_m/[h_e(ωρ_Lη_L]1/2.     

Synthesis, structure, and activity evaluation of two silver(I) complexes as Helicobacter pylori urease inhibitors

Two silver(I) compounds, [Ag(R,R-hxn)](C₇H₄BrO₂) · 2H₂O (I) (Chxn = 1,2-diaminocyclohexane) and [Ag(C₅H₆N₂)₂]₂(C₈H₄O₄) · 10H₂O (II), were synthesized and complex I was structurally characterized by X-ray crystallography. Compound I contains a catena-(trans-1,2-diaminocyclohexane) silver polycation ([Ag(Chxn)]_∞) in a roughly linear fashion, while II possesses a linear-type silver monocation. Compounds I and II were evaluated for their inhibitory activities against Helicobacter pylori urease in vitro. Both were found to have strong inhibitory activities against H. pylori urease comparable to that of acetohydroxamic acid.     

Ionic diffusion mastering using crystal-chemistry parameters: τ-Cu1/2Ag1/2V2O5 structure determination and comparison with refined δ-AgxV2O5 and ε-CuxV2O5 ones

τ-Ag_1/2Cu_1/2V₂O₅ compound crystallises in the monoclinic system space group C2/m with cell parameters a=11.757(4) Å, b=3.6942(5) Å, c=9.463(2) Å, and β=114.62(2)°. The structure is build up with V₄O₁₀ D4 double layer. The silver and copper ions are located in two different oxygenated tunnels. Examination of electronic density maps shows that while the silver ions are located in defined crystallographic sites, the copper ones are fully delocalised over the whole tunnel. Comparison with δ-Ag_xV₂O₅ and ε-Cu_xV₂O₅ refined structure allows to define crystal chemistry parameters governing the ionic delocalisation and give clues to predict from structural consideration the expected electrical behaviour with the aim to make possible a structural design to enhance guest species reactivity.     

Silver‐Catalyzed Formal Insertion of Arynes into RfI Bonds

An unprecedented silver‐catalyzed formal insertion of arynes into R_f?I (R_f=CF₃, C₂F₅) bonds has been developed. This protocol provides easy access to various ortho‐perfluoroalkyl iodoarenes under mild conditions. In this insertion reaction, an ionic atom‐transfer reaction of R_fI occurs, and a silver‐mediated metathesis process is involved in the efficient transfer of the electropositive iodine atom.     

Crystal structure and phase transition in the mixed compound KCd0.84Hg0.16Cl3·H2O

The title compound crystallized in the space group Pnma with the following unit cell dimensions: a=8.5100(2), b=4.0580(4), c=18.1510(9) Å, Z=4. Its crystal structure was determined and refined down to R=0.0288 and Rw=0.0732. The atomic arrangement of this structure may be considered as a succession of double layers built up from Hg/CdCl₆ octahedra along the b axis connected by hydrogen and ionic bonds. DSC measurement reveals a transition at T=429 K. Spectroscopic and dielectric measurements were performed to discuss the mechanism of the phase transition.     

Synthesis, crystal structure and thermoelectric properties of a new carbide Zr2[Al3.56Si0.44]C5

A new quaternary layered carbide, Zr₂[Al_3.56Si_0.44]C₅, has been synthesized and characterized by X-ray powder diffraction, transmission electron microscopy and thermopower and electrical conductivity measurements. The crystal structure was successfully determined using direct methods, and further refined by the Rietveld method. The crystal is trigonal (space group R3m, Z=3) with lattice dimensions of a=0.331059(5), c=4.09450(5) nm and V=0.38864(1) nm³. The final reliability indices calculated from the Rietveld refinement were R_wp=6.24%, R_p=4.21% and R_B=0.82%. The crystal structure is composed of electroconductive NaCl-type ZrC slabs separated by Al₄C₃-type [Al_3.56Si_0.44]C₃ layers. This material had thermoelectric properties superior to those of the ternary layered carbides Zr₂Al₃C₄ and Zr₃Al₃C₅, with the power factor reaching 7.6×10⁻⁵W m⁻¹ K⁻².     

The surface tension,density and refractive index of amino acid ionic liquids: [C3mim][Gly] and [C4mim][Gly]

The amino acid ionic liquids (AAILs) [C₃mim][Gly] (1-propyl-3-methylimidazolium glycine) and [C₄mim][Gly] (1-butyl-3-methylimidazolium glycine) have been prepared by the neutralization method and characterized by ¹H NMR spectroscopy and differential scanning calorimetry (DSC). The values of their density, surface tension and refractive index were measured at different temperatures. Since the AAILs can form strong hydrogen bonds with water, small amounts of water are difficult to remove from the AAILs by common methods. In order to eliminate the effect of the impurity as water, the standard addition method (SAM) was applied to these measurements. The values of thermal expansion coefficients, α, surface excess energy, E_a, surface excess entropy, S_a, and molar refraction, R_m, for the AAILs were determined.     

Ring-expanded chiral rhombamine macrocycles for efficient NMR enantiodiscrimination of carboxylic acid derivatives

Novel 46-membered chiral rhombamine macrocycles (R,R,R,R)-8a and 8b were synthesized by [2+2] cyclocondensation reactions of (R,R)-1,2-diaminocyclohexane with the corresponding dialdehydes and subsequent reduction with NaBH₄. The X-ray crystal structure of 1:4 dioxane complex with (R,R,R,R)-8a indicated a rhombus conformation of the chiral macrocycle. Compounds (R,R,R,R)-8a and 8b were tested as chiral shift reagents for a wide range of α-substituted carboxylic acids and amino acid derivatives. Enantiodiscrimination of ¹H NMR signals was observed with ΔΔδ values of up to 0.214 ppm.     

A new organically templated gallium(III)-doped chromium(III) fluorophosphite, (C2H10N2)[Ga0.98Cr0.02(HPO3)F3] hydrothermal synthesis, crystal structure and spectroscopic properties

A new organically templated fluoro-phosphite gallium(III)-doped chromium(III) with formula (C₂H₁₀N₂)[Ga_0.98Cr_0.02(HPO₃)F₃] has been synthesized by using mild hydrothermal conditions under autogeneous pressure. The crystal structure has been solved from X-ray single-crystal data. The compound crystallizes in the P2₁2₁2₁ orthorhombic space group, with the unit-cell parameters a=12.9417(7) Å, b=9.4027(6) Å, c=6.3502(4) Å and Z=4. The final R factors were R1=0.022 (all data) and wR2=0.050. The crystal structure consists of [Ga_0.98Cr_0.02(HPO₃)F₃]²⁻ anionic chains extended along the c-axis, with the ethylenediammonium cations placed in the cavities of the structure delimited by three different chains. The IR and Raman spectra show the characteristic bands of the phosphite oxoanion. The diffuse reflectance spectroscopy allowed us to calculate the Dq and Racah parameters of the Cr(III) cations in octahedral environment. The values are Dq=1375 cm⁻¹, B=780 cm⁻¹ and C=3420 cm⁻¹. The polycrystalline ESR spectra performed at X and Q-bands show the signals belonging to the diluted Cr(III) cation in this phase. From the fit of the X-band ESR spectrum at 4.2 K, the calculated values of the axial (D) and rhombic (E) distortion parameters are 0.075 and 0.042 cm⁻¹, respectively, the components of the g-tensor being g_x=1.98, g_y=1.99 and g_z=1.90.     

Preparation and Double-Helix Icosahedra Structure of δ-Co2Zn15

The first known example of a regular face-sharing icosahedra helix is presented in the novel crystal structure of δ-Co₂Zn₁₅, solved and refined from single-crystal X-ray and neutron powder diffraction data. The compound δ-Co₂Zn₁₅ is also the first example of an intermetallic compound crystallizing in the acentric hexagonal space group P6₂. The parameters from the single crystal refinement are a=11.292(2) Å, c=7.750(1) Å, Z=4, and R_w=0.025.     

External calibration in gas chromatography-combustion-isotope ratio mass spectrometry measurements of endogenous androgenic anabolic steroids in sports doping control

An alternative calibration procedure for the Gas Chromatography–Combustion–Isotope Ratio Mass Spectrometry (GC–C–IRMS) measurements of the World Antidoping Agency (WADA) Accredited Laboratories is presented. To alleviate the need for externally calibrated CO₂ gas for GC–C–IRMS analysis of urinary steroid metabolites, calibration using an external standard mixture solution of steroids with certified isotopic composition was investigated. The reference steroids of the calibration mixture and routine samples underwent identical instrumental processes. The calibration standards bracketed the entire range of the relevant δ¹³C values for the endogenous and exogenous steroids as well as their chromatographic retention times. The certified δ¹³C values of the reference calibrators were plotted in relation to measured m/z¹³CO₂/¹²CO₂ (i.e. R(45/44)) mass spectrometric signals of each calibrator. δ¹³C values of the sample steroids were calculated from the least squares fit through the calibration curve. The effect of the external calibration on δ¹³C values, using the same calibration standards and set of urine samples but different brands of GC–C–IRMS instruments, was assessed by an interlaboratory study in the WADA Accredited Laboratories of Sydney, Australia and Athens, Greece. Relative correspondence between the laboratories for determination of androsterone, etiocholanolone, 5β-androstane-3α,17β-diacetate, and pregnanediacetate means were SD(δ¹³C) = 0.12‰, 0.58‰, −0.34‰, and −0.40‰, respectively. These data demonstrate that accurate intralaboratory external calibration with certified steroids provided by United States Antidoping Agency (USADA) and without external CO₂ calibration is feasible and directly applicable to the WADA Accredited Laboratories for the harmonization of the GC–C–IRMS measurements.     

Cobalt ion-doped polyaniline,poly(N-methylaniline), and poly(N-ethylaniline): electrosynthesis and characterisation using electrochemical methods in acidic solutions

Cobalt ion (Co²⁺)-doped polyaniline (PANI-Co), poly(N-methylaniline) (PNMA-Co), and poly(N-ethylaniline) (PNEA-Co) films were synthesised electrochemically on a pencil graphite electrode (PGE) and their electrochemical properties were investigated for supercapacitor applications. The polymer film-coated electrodes (PGE/PANI-Co, PGE/PNMA-Co, and PGE/PNEA-Co) thus obtained were characterised by scanning electron microscopy (SEM) and different electrochemical methods. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were employed in 0.1 M H₂SO₄ solution to calculate the specific capacitance (C _S) values of the electrodes. The maximum C _S of 192.94 F g^?1, 139.83 F g^?1, and 47.12 F g^?1 were achieved for PGE/PANI-Co, PGE/PNMA-Co, and PGE/PNEA-Co at 1 mV s^?1, respectively. On the other hand, the charge/discharge stability of the electrodes was analysed using the repeating chronopotentiometry (RCP) method. The RCP measurements indicate that the electrodes could be used as an electrode active material for low voltage supercapacitor applications.     

Arene-iridium(I) complexes. Crystal structure of [Ir(Me3TFB)(C6Me6)]ClO4

The crystal structure of [Ir(Me₃TFB)(C₆Me₆)]ClO₄ has been determined by X-ray diffraction. Lattice constants are a 16.9092(7), b 17.7120(7), c 16.8635(6) Å and β 97.984(3)° in a C2/c monoclinic lattice. The agreement factors are R, 0.038 and R_w 0.048 for the 3321 reflexions with I > 3σ(I). The coordination characteristics are similar to those of the corresponding rhodium analogues.     

Some considerations concerning the temperature dependence of the bulk crystallization rate constants of polymeric materials

The experimentally estimated ‘normalized’ rate constants of the bulk crystallization of poly(L-lactic acid) and natural rubber at different temperatures have been used in order to test different equations describing the temperature dependence of the normalized crystallization rate constants. Ten of these expressions are variants of the well-known Turnbull-Fisher relationship k = k₀ exp[-U*/R(T - T_∞)] exp[K_g/T(ΔT)f] which differ essentially by the assumed number of fixed parameters [one or two Williams, Landel, and Ferry (WLF) constants i.e., U* = C₁ and/or C₂]. Two additionally used equations are expressions derived on the basis of the above mentioned general relation, which introduce T_g instead of T_∞ = T_g - C₂ as the low temperature limit of crystallization and two energies of activation, for nucleation and crystal growth, respectively. It is shown that the temperature dependence of the normalized bulk crystallization rate constants is more accurately described if three parameters (beside k₀, U*, and K_g or the respective activation energies) are assumed to be adjustable and when instead of the widely accepted expression for the low temperature limit, T_∞ = T_g - C₂(K) with C₂ = 51.6 or 30°C, respectively, T_g (i.e., C₂ = 0°C) is considered. A qualitative explanation for this finding is suggested in terms of the increasing mobility of the crystallizing segments at temperatures above T_g, taking into account that generally the glass transition is considered to be a ‘freezing in’ process. © 1997 John Wiley & Sons, Inc.     

A high-performance fluorosensor for pH measurements between 6 and 9

This study presents a high-performance ratiometric pH optode based on the fluorophore 6,8-dihydroxypyrene-1,3-disulfonic acid (DHPDS). The two pH-sensitive terminal hydroxy groups of DHPDS facilitated dual excitation/dual emission (F₁: λ_1,ex = 420 nm, λ_1,em = 462 nm; F₂: λ_2,ex = 470 nm, λ_2,em = 498 nm) properties for ratiometric (R_F1,F2 = F₁/F₂) normalization of sensor signal. The sensor demonstrated an exponentially decreasing ratiometric response with increasing pH, with a linear correlation (R² = 0.9936) between ¹⁰log(R_F1,F2) and pH within the pH interval 6-9. Precision determined as the IUPAC pooled standard deviation for the pH values 6.00, 7.01 and 9.01, was 0.0057 pH units for the fluorosensor and 0.0054 for a commercially available pH electrode used for comparison. Between the end-points of calibration at pH 7.01, the precision of the sensor was 0.0037 pH units. Effects from changes in ionic strength (I_tot, 10-700 mM) were more pronounced for the electrode, with a linear (R² = 0.9976) increase in response (δE/δpH) with increasing I_tot. The DHPDS-based fluorosensor, however, retained sensitivity (δ¹⁰log(R_F1,F2)/δpH = 0.8024 ± 0.0145), though with an overall increase in ratiometric signal with increasing I_tot. The preserved sensitivity despite changes in ionic strength was possibly a consequence from the dual photo-acidic properties of DHPDS. Analytical characteristics of immobilized DHPDS therefore not only facilitated high-performance measurements over a wide pH range, but also opened for straightforward simultaneous measurements of pH and ionic strength.     

Theoretical study of the electronic transition moment for the d 3IIga 3IIu band system of the C2 molecule

Large scale ab initio SCF and CI calculations are employed to study the potential curves for the d ³II_g, a ³II_u and X¹Σ⁺_g states of the C₂ radical. The electronic transition moment R_e′e″ for the Swan bands (d ³II_ga ³II_u) is calculated in various AO and MO basis sets as a function of the internuclear CC distance. The form of the Σ|R_e′e″|² curve is in very good agreement with that obtained recently from measurements of Danylewych and Nicholls and Tatarczyk et al.; the calculated value for Σ|R_e′e″² at 2.44 bohr is found to be 5.2 au² compared to the most recent experimental values of |R_e(r_oo)|² = 3.57 au² of Tatarczyk et al.     

The influence of alkylene spacers on conformational and thermal properties of poly (aryl methacrylates)

The effect of introducing alkylene spacers on the thermal and conformational properties of poly(aryl methacrylates) has been studied. Poly(2-phenylethyl-1-methacrylate) (PPEMA) and poly(4-phenylbutyl-1-methacrylate) (PPBMA) samples of narrow poly-dispersities and covering a very broad range of molecular weights were produced by free radical solution polymerization, followed by fractionation. These materials were studied by a combination of differential scanning calorimetry, light scattering, and viscometry measurements to allow evaluation of their glass transition temperatures (T_g) and characteristic ratios (C_∞). These results are compared with literature data available for poly(phenyl methacrylate) (PPMA), which has no alkylene spacer, and poly(benzylmethacrylate) (PBMA), which has a methylene spacer. A progressive decrease in T_g is observed as the length of the alkylene spacer is increased, reflecting the enhancement of side group (local) flexibility. C_∞ decreases substantially on going from PPMA to PBMA but then increases on going to PPEMA and PPBMA. This behavior is attributed to the fact that while side group flexibility increases for the materials with longer alkylene spacers, and this would be expected to decrease C_∞, the side group size is progressively increasing. Larger side groups generally increase C_∞ of polymethacrylates. Thus the observed C_∞ behavior for these polymers reflects both the size and flexibility of the substituents. © 1994 John Wiley & Sons, Inc.