Coefficient seebeck et méanismes de conduction dans les polymères (PPP) electroactifs dopés par implantation ionique

A study of the Seebeck coefficient has shown that doping of polyparaphenylene by ion implantation makes it possible to obtain an electronically doped semiconductor at low energy: n-type with alkali metal ions and p-type with halide ions. At the highest energies (E > 100 keV) the p-type conductivity is due to the creation of defects by irradiation. Generally the semiconductor obtained is degenerate with a Seebeck coefficient close to that obtained by chemical doping. Study of the mechanisms of conduction suggests plots of log σ = (T^?l/n); the greater n is, the better is the agreement between the experimental curve and theory. Representation of the conductivity is proposed according to Mott's theories, which are applicable to amorphous semiconductors and involve several conduction processes in the temperature space. For the variable-range-hopping (VRH) mechanism at low temperature, two parameters, α^?1 (representing the spatial separation of hopping sites) and N(E_F) (the density of states at the Fermi Level) are obtained.     

The scaled one-electron Hamiltonian model for open-shelllcao-mo-scf calculations: further corrections to thesoeh energy

The wavefunction generated by the scaled one-electron Hamiltonian (soeh) model has been modified further by including all single excitations from the space of thesoeh function. A perturbation-variation ansatz has been invoked for obtaining the corresponding energy correction (ΔE ₂). [E(soeh)+ΔE ₂] is shown to reproduceE(Roothaan) very closely. It has been demonstrated that by making ΔE ₂ stationary with respect to change in μ results. The correctedsoeh [E(soeh)+ΔE ₂] is shown to be quite useful for the calculation of geometrical parameters of open-shell systems even though it lacks the ‘upper-boundedness’ exhibited byE(soeh).     

Physical and photoelectrochemical characterization of CuCrO2 single crystal

CuCrO₂ single crystal, elaborated by the flux method, is a narrow-band-gap semiconductor crystallizing in the delafossite structure with an indirect optical transition at 2.12 eV. The relatively longer Cu–Cu is consistent with the semi-conducting behavior. The conductivity in the (001) plans is thermally activated and occurs predominantly by small polaron hopping through mixed-valence states Cu^+/2+ in conformity with a classical dielectric behavior. The activation energy (0.05 eV) gave an effective mass of 9 m _o, indicating that the levels in the vicinity of the Fermi level E _f are strongly localized. The oxide shows an excellent chemical stability over the whole pH range; the semi-logarithmic plot gave an exchange current density of 0.7 mA cm⁻² and a corrosion potential of 0.18 V_/SCE in KOH (0.5 M) electrolyte. The electrochemical study is confined in (001) plans, and reversible oxygen intercalation is evidenced from the cyclic voltammetry. The Mott–Schottky plot (C⁻²-V) is characteristic of p type conduction and exhibits a linear plot from which a flat band potential of +0.21 V_/SCE and a holes density N _A of 5.06 × 10¹⁴ cm⁻³ were obtained. The photocurrent is due to Cu⁺: d → d transition and the valence band is positioned at 5.34 eV below vacuum.     

Variable-range hopping conductivity in Lu-doped Bi2Te3

The Seebeck coefficient enhancement due to an increase of density-of-states effective mass of electron has been found in n-type Bi_1.9Lu_0.1Te₃. This enhancement is assumed to be related to forming the narrow and non-parabolic impurity (Lu) band with local maximum of electronic density of states lying near the Fermi level. Minimum in the specific electrical resistivity, ρ, originated from change of conductivity mechanism was observed at temperature T_m ≈ 11 K. Above T_m, the ρ change is due to decrease of electron mobility via acoustic phonon scattering. Below T_m, the variable-range hopping conductivity takes place. The electron hops between the localized states of the impurity energy band occur via tunneling process. Using the temperature and magnetic field dependences of ρ, the localization radius of electron was estimated as ≈6 nm. Two parts in the magnetic field dependence of the electrical resistivity were found at temperature of 2 K. At weak magnetic fields, the ρ change is in agreement with the variable-range hopping conductivity mechanism. At high magnetic fields, the positive and almost linear transverse and longitudinal magnetoresistances were observed at low temperatures. Both variable-range hopping conductivity and positive linear magnetoresistance are characteristics of disordered and inhomogeneous semiconductors.     

Synthesis of diblock copolymers with cellulose derivatives. 3. Cellulose derivatives carrying a single pyrene group at the reducing-end and fluorescent studies of their self-assembly systems in aqueous NaOH solutions

Novel cellobiose and cellulose (DP _n =ca. 30) derivatives, N-(1-pyrenebutyloyl)-4-O-(β-d-glucopyranosyl)-β-d-glucopyranosylamine (6), N-(15-(1-pyrenebutyloylamino)-pentadecanoyl)-4-O-(β-d-glucopyranosyl)-β-d-glucopyranosylamine (7), N-(1-pyrenebutyloyl)-β-cellulosylamine (13), N-(15-(1-pyrenebutyloylamino)-pentadecanoyl)-β-cellulosylamine (14) carrying a pyrene group as a single fluorescent probe at the reducing end, were prepared in order to investigate their self-assembly systems in solutions. The relative intensity of the excimer emission at ca. 480 nm due to dimerized pyrenes (intensity I _E) to the monomer emission at ca. 380 nm due to isolated pyrene (intensity I _M), i.e., I _E/I _M, was monitored in various solutions. In water/dimethyl sulfoxide (DMSO) mixed solvent (0–98%, v/v), the ratio I _E/I _M remained low (0.04) for compound 6 over the range of water concentrations, indicating that pyrenes at C-1 position of compound 6 were diffused. On the other hand, the ratio I _E/I _M increased (0.04–4.96) for compound 7 with the increase in water concentration, indicating that pyrenes at C-1 position were associated. In aqueous NaOH solutions (4.4–17.5%, w/w), compound 14 showed a large increase in the ratio I _E/I _M (0.84–8.14) with the increase in NaOH concentration, compared to compound 13 (0.06–0.41). It was found that the association of hydrophobic groups at the reducing-end of cellulose could be controlled by the hydrophilic–hydrophobic balance of compounds and the solvent polarity.     

New polyhydroxysteroids and steroid glycosides from the far east starfishCeramaster patagonicus

Two new polyhydroxysteroids and five new glycosides were isolated from the starfishCeramaster patagonicus and their structures were elucidated: 5α-cholestane-3β,6α,15β,16β,26-pentol, (22E)-5α-cholest-22-ene-3β,6α,8,15α,24-pentol, (22E)-28-O-[O-(2-O-methyl-β-d-xylopyranosyl)-(1→2)-β-d-galactofuranosyl]-24-hydroxymethyl-5α-cholest-22-ene-3β,4β, 6α,8,15β,16β,28-heptol (ceramasteroside C₁), (22E)-28-O-[O-(2,4-di-O-methyl-β-d-xylopyranosyl)-(1→2)-β-d-galactofuranosyl]-24-hydroxymethyl-5α-cholest-22-ene-3β, 6α,8,15β,16β,28-hexol (ceramasteroside C₂), (22E)-28-O-[O-methyl-β-d-xylopyranosyl)-(1→2)-β-d-galactofuranosyl]-24-hydroxymethyl-5α-cholest-22-ene-3β,6α,8,15β,16β 28-hexol (eramasteroside C₃), (22E)-28-O-[O-(2-O-methyl-β-d-xylopyranosyl)-(1→2)-β-d-galactofuranosyl]-24-methyl-5α-cholest-22-ene-3β,4β,6α,8, 15β, 26-hexol (ceramasteroside C₄), and (22E)-28-O-[O-(2-O-methyl-β-d-xylopyranosyl)-(1→2)-β-d-xylopyranosyl]-5α-cholest-22-ene-3β,6α,8,15β,24-pentol (ceramasteroside C₅)). Three known polyhydroxysteroids (24-methylene-5α-cholestane-3β,6α,8,15β,16β,26-hexol, 5α-cholestane-3β,6α,8,15β,16β,26-hexol, and 5α-cholestane-3β,6β,15α,16β,26-pentol) were also isolated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 190–195, January, 1997.     

Electrical properties and stoichiometry in La2NiO4

The electrical properties of La₂NiO₄ have been studied with respect to the stoichiometry of the material. The conductivity of reduced compositions has been measured between 20 and 1000 K and compared to that of the air-prepared ones. Nonstoichiometry is present in both cases and produces disorder leading to Anderson localization and to the definition of a mobility edge in the σ_x²−y² itinerant band. Air-prepared compounds contain in addition a large number of Ni³⁺ states which may overlap the itinerant σ_x²−y² band. For reduced materials containing small amounts of Ni³⁺, the electrical properties can be described below 200 K by a hopping conductivity at the Fermi level within a sharply peaked density of states. The results are well described within the frame of the Mott theory of variable range hopping. Above 200 K highly reduced materials exhibit direct excitation of holes from Ni³⁺ states to the mobility edge in the itinerant band. Under conditions appropriate to air-prepared materials, the Fermi level is shifted toward the itinerant band and a major contribution to the conductivity arises from hopping at the Fermi level. At high temperature a progressive excitation of carriers from the localized states is anticipated with an eventual exhaustion region. This last assumption is corroborated with a shift of the conductivity maximum to higher temperature for increasingly reduced materials.     

Optical and electrical properties and conduction mechanism of [(CH3)2NH2]2CoCl4

Monoclinic crystals of [(CH₃)₂NH₂]₂CoCl₄ are of the space group P2₁/n. Unit cell dimensions are as follows: a = 8.5393 Å, b = 11.3905 Å, c = 13.4069 Å and β = 91.02°. This compound undergoes several phase transitions. The optical properties were measured by means of UV–visible absorption spectroscopy in the range 200–800 nm. Analysis of the data revealed the existence of direct allowed optical transition mechanisms with bandgap energy equal to 3.84 eV. The electrical conductivity of [(CH₃)₂NH₂]₂CoCl₄ was studied in the frequency range 10⁻¹–10⁶ Hz and in the temperature range 411–449 K by means of impedance spectroscopy. Impedance and modulus analyses indicated temperature-independent distribution of relaxation times and non-Debye behavior in this material. A super-linear power law was observed for the AC conductivity, which was analyzed based on the jump relaxation model σ_ac = σ_dc + A₁ω^s1 + A₂ω^s2. Accordingly, the first AC term A₁ω^s1 corresponds to translational hopping motion and the second term A₂ω^s2 to well-localized hopping and/or reorientational motion. Conduction takes place via correlated barrier hopping in phases I and II.     

New conducting polymers, 3. Doping,stability, electrical,and optical characteristics of poly-(P-phenylphosphoethynediyl)

Studies on direct-current electrical conductivity and optical properties of a new solution of processable conducting polymer are reported. Electrical conductivity of thin films of the polymer on glass plate at room temperature was 6×10⁻⁶ S/cm. Study of conductivity with variation of temperature does not provide any definite thermal activation energy, which is in accordance with the amorphous nature of polymer. Optical absorption data adopting the Bardeen equation showed that maximum ‘optical gap’ (E _g ) is 3.30 eV. Doping with Br₂-vapor was found to be only partially effective in decreasingE _g by 0.43 eV. The polymer was found to be quite stable under normal atmospheric conditions. Environmental stability of both undoped and doped polymer has been discussed. Part 2: [5]     

Acoustical studies on fructose with enzyme amylase in aqueous media at 298.15 K

Ultrasonic velocity (U), density (ρ), and viscosity (η) measurements have been carried out in three ternary mixtures of fructose with amylase in aqueous medium at 298.15 K. The experimental data have been used to calculate some derived parameters such as acoustical impedance (Z), relative association (R _a ), Rao’s constant (R _R), Wada’s constant (W), relaxation time (τ), relaxation amplitude (α/f ²), relaxation strength (r), and some excess thermodynamical properties like excess adiabatic compressibility (β ^E ), excess free length (L _f ^E ) excess free volume (V _f ^E ), excess internal pressure (π _i ^E ), and excess acoustical impedance (Z ^E ). The above parameters have been evaluated and discussed in light of molecular interactions in the mixture.     

Determination of Binding Constants of Polyethylene Glycol Vancomycin Derivatives to Peptide Ligands Using Affinity Capillary Electrophoresis

Vancomycin (Van) from Streptomyces orientalis has been derivatized with polyethylene glycol [PEG; PEG-550 (1), 750 (2), 1,100 (3), 2,000 (4), 5,000 (5), and 8,000 (6) g mol⁻¹] at the N-terminus of the glycopeptide backbone and their binding to d-Ala-d-Ala terminus peptides assessed using affinity capillary electrophoresis (ACE). Utilizing ACE, a plug of Van-PEG and non-interacting standards are injected and electrophoresed. Analysis of the change in the relative migration time ratio of the Van-PEG species, relative to the non-interacting standards, as a function of the concentration of peptide, yields a value for the binding constant (K _b). Values of K _b for N-acetyl-d-Ala-d-Ala, 7 to the Van-PEG derivatives are weaker than those for N _α,N _ε-diacetyl-Lys-d-Ala-d-Ala, 8 (for example, values of K _b for 7-1 and 8-1 are 1.8 and 47.7 × 10³ M⁻¹, respectively). These results demonstrate that derivatization of Van with PEG has little effect on the affinity of d-Ala-d-Ala peptide ligands to it. The findings further prove the versatility of ACE and its ability to estimate binding parameters of ligands to antibiotics.     

Kinetics of the oxidative degradation of d-xylose in the presence and absence of cationic and anionic surfactants

The oxidative degradation of d-xylose by cerium(IV) has been found to be slow in acidic aqueous solution with the evidence of autocatalysis. The reaction is accelerated in the cetyltrimethylammonium bromide (CTAB) micellar medium but sodium dodecyl sulfate (an anionic surfactant) has no effect. The pseudo first-order rate constants have been determined at different [reductant], [oxidant], [H₂SO₄], temperature, and [CTAB]. The reaction rate increased with increasing [d-xylose] and decreased with increase in [H₂SO₄]. The CTAB-micelle-catalyzed kinetic results can be interpreted by the pseudophase model. The kinetic parameters such as association constant (K _s), micellar medium rate constant (k _m), and activation parameters (E _a, ΔH ^# and ΔS ^#) are evaluated and the reaction mechanism is proposed. The reaction rate is inhibited by electrolytes and the results provide an evidence for the exclusion of the reactive species from the reaction site.     

Ab-initio calculation of structural, electronic, and optical characterizations of the intermetallic trialuminides ScAl3 compound

We present first-principles study of the electronic and the optical properties for the intermetallic trialuminides ScAl₃ compound using the full-potential linear augmented plane wave method within density-functional theory. We have employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for calculating the electronic band structure and optical properties. The electronic specific heat coefficient (γ), which is a function of density of states, can be calculated from the density of states at Fermi energy N(E_F). The N(E_F) of the phase L1₂ is found to be lower than that of D0₂₂ structure which confirms the stability of L1₂ structure. We found that the dispersion of the band structure of D0₂₂ is denser than L1₂ phase. The linear optical properties were calculated. The evaluations are based on calculations of the energy band structure.     

Toward the synthesis and biological evaluation of hirsutide

Abstract  

The present investigation deals with the synthesis of a N-methylated cyclotetrapeptide, hirsutide (2), by coupling of the dipeptide units Boc-l-phenylalanyl-l-N-methylphenylalanine-OH and l-valyl-l-N-methylphenylalanine-OMe followed by cyclization of the linear tetrapeptide fragment. The chemical structure was established on the basis of analytical as well as spectroscopic data. The newly synthesized cyclic peptide was subjected to pharmacological screening and found to be highly potent against the gram-negative bacteria Pseudomonas aeruginosa and Klebsiella pneumoniae at 6 μg cm⁻³. In addition, potent antihelmintic activity against the earthworms Megascoplex konkanensis and Pontoscotex corethruses at 1 and 2 mg cm⁻³, and potent cytotoxic activity against Dalton’s lymphoma ascites and Ehrlich’s ascites carcinoma cell lines with IC₅₀ values of 14 and 22 μM were also observed. Studies revealed that the pentafluorophenyl ester method employing a catalytic amount of N-methylmorpholine proved to be better for cyclization of the linear tetrapeptide unit.     

Stereoselective Michael Addition of Carbonyl Compounds to ( E )-β-Nitrostyrene Catalysed by N -Toluensulfonyl-l -proline Amide in Ionic Liquids

Summary. N-Toluensulfonyl-l-prolin amide was tested as catalyst in the enantioselective Michael addition of carbonyl compounds to (E)-β-nitrostyrene in nine ionic liquids under different reaction conditions. The reaction rates and enantioselectivities were strongly dependent on the ionic liquids. Change of enantioselectivity was observed too and it is attributed to both the cation and the anion of ionic liquid. The best yields (up to 98%) and enantioselectivity (70% ee) of product were obtained in a basic ionic liquid [bmim]BF₄ at room temperature.     

Conductivity and modulus formulation in lithium modified bismuth zinc borate glasses

The conductivity and modulus formulation in lithium modified bismuth zinc borate glasses with compositions xLi₂O–(50-x) Bi₂O₃–10ZnO–40B₂O₃ has been studied in the frequency range 0.1 Hz–1.5 × 10⁵ Hz in the temperature range 573 K–693 K. The temperature and frequency dependent conductivity is found to obey Jonscher's universal power law for all the studied compositions, the dc conductivity (σ_dc), crossover frequency (ω_H), and frequency exponent (s) have been estimated from the fitting of the experimental data of ac conductivity with Jonscher's universal power law. Enthalpy to dissociate the cation from its original site next to a charge compensating centre (H_f) and enthalpy of migration (H_m) have been estimated. It has been observed that number of charge carriers and ac conductivity in the lithium modified bismuth zinc borate glasses increases with increase in Li₂O content. Further, the conduction mechanism in the glass sample with x = 0 may be due to overlapping large polaron tunneling, whereas, conduction mechanism in other studied glass samples more or less follows diffusion controlled relaxation model. The ac conductivity is scaled using σ_dc and ω_H as the scaling parameter and is found that these are suitable scaling parameter for conductivity scaling. Non-Debye type relaxation is found prevalent in the studied glass system. Scaling of ac conductivity as well as electric modulus confirms the presence of different type of conduction mechanism in the glass samples with x = 0 and 5 from other studied samples. The activation energy of relaxation (E_R) and dc conductivity (E_dc) are almost equal, suggesting that polarons/ions have to overcome same barrier while relaxing and conducting.     

Enhanced local density of states at the Fermi level of the surface platinum in carbon-supported platinum particles by Nafion ionomer

¹⁹⁵Pt nuclear magnetic resonance (NMR) was employed to study the influence of Nafion ionomers (NIs), a common ingredient used as a proton conducting mediator and binder in a membrane-electrode-assembly (MEA) for proton exchange membrane fuel cells, on the local density of states at the Fermi surface (E_F-LDOS) of Pt/C catalysts. NIs were observed to increase the surface E_F-LDOS of Pt particles, thereby altering of the catalytic properties of metal particles. This correlates well with the reduction of fuel oxidation rates. Thus, ionomers decreasing the surface E_F-LDOS of Pt particles would be desirable. In addition, a new concept to interpret the surface E_F-LDOS was introduced and validated.     

Stereoselective Michael Addition of Carbonyl Compounds to ( E )-β-Nitrostyrene Catalysed by N -Toluensulfonyl-l -proline Amide in Ionic Liquids

N-Toluensulfonyl-l-prolin amide was tested as catalyst in the enantioselective Michael addition of carbonyl compounds to (E)-β-nitrostyrene in nine ionic liquids under different reaction conditions. The reaction rates and enantioselectivities were strongly dependent on the ionic liquids. Change of enantioselectivity was observed too and it is attributed to both the cation and the anion of ionic liquid. The best yields (up to 98%) and enantioselectivity (70% ee) of product were obtained in a basic ionic liquid [bmim]BF₄ at room temperature.