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排序方式: 共有1468条查询结果,搜索用时 343 毫秒
31.
Mathieu Allix Philip P.C. Frampton Rocío Ruiz-Bustos 《Journal of solid state chemistry》2006,179(3):775-792
Polycrystalline samples of bilayered brownmillerite-like Ca2.5Sr0.5GaCo0.15Mn1.85O8 and Ca2.5Sr0.5Ga1.2Mn1.8O8 have been prepared and characterised by magnetometry and neutron diffraction over a wide temperature range. The structural chemistry and magnetic properties are compared to those of Ca2.5Sr0.5GaMn2O8. Ga enrichment has a significant effect on the former but not on the latter, whereas changes in both occur when paramagnetic Co3+ cations enter the parent phase on the 4-coordinate sites. The coupling between the environment around the 4-coordinate cations and the transition to an antiferromagnetic ordered state that was observed in Ca2.5Sr0.5GaMn2O8 is not apparent in the cation-substituted compositions, although both show long-range antiferromagnetic order at low temperatures. 相似文献
32.
Marie-Hélène Paclet Cécile F. Rousseau Campion Yannick Françoise Morel Anthony W. Coleman 《Journal of inclusion phenomena and macrocyclic chemistry》2006,55(3-4):353-357
The effects of para-Sulphonato-calix[4]arene, para-Sulphonato-calix[6]arene and para-Sulphonato-calix[8]arene on the activation of NADPH oxidase in neutrophils has been studied. All three molecules do not induce NADPH oxidase activation, and hence do not stimulate neutrophils. Measurement of cell viability demonstrates that these three water-soluble calix[n]arene derivatives are not cytotoxic. 相似文献
33.
Sergei V. Levchik Giovanni Camino Maria Paola Luda Luigi Costa Bruno Costes Yves Henry Emile Morel George Muller 《先进技术聚合物》1995,6(2):53-62
Tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) undergoes homopolymerization on heating. Intramolecular reactions which compete with crosslinking favor the formation of cyclic structures with increasing thermal and fire resistance of the resin, whereas physical mechanical properties tend to decrease. The mechanism of thermal decomposition of TGDDM is studied by thermogravimetry, differential scanning calorimetry and thermal volatilization analysis with characterization of volatiles evolved and residue left. Thermal degradation of poly-(TGDDM) starts at 260°C with elimination of water from secondary alcoholic groups which is a typical pathway for epoxy resin degradation. Resulting unsaturations weaken bonds in the β-position and provoke the first chain breaking at allyl–amine and allyl–either bonds. With increasing temperature, saturated alkyl–ether bonds and alkyl carbon–carbon bonds are broken first, followed by the most stable alkyl–aryl bonds at T>365°C. The combustion performance of TGDDM is discussed on the basis of the thermal degradation behavior. 相似文献
34.
Christine Bonal Jean-Pierre Morel Nicole Morel-Desrosiers 《Journal of solution chemistry》1998,27(4):361-372
The apparent equilibrium constants and enthalpies of complexation of Nd3+, Sm3+, Eu3+, and Gd3+ by xylitol in aqueous solutions containing NaNO3 at an ionic strength of 2.0 mol-kg–1 have been determined by microcalorimetry at 25°C. Since nitrate anion weakly complexes the lanthanide cations, these values are analyzed in terms of competition between xylitol and NO
3
-
The method leads to the apparent equilibrium constants and enthalpies of complexation of the lanthanide cations by NO
3
-
at this particular ionic strength. Despite the difficulties encountered in characterizing rather weak associations, the results are, whenever comparison is possible, in good agreement with those obtained by direct microcalorimetry. The advantage of this competition method is that it can be used when the enthalpic effects are too weak and insufficiently concentration dependent for direct microcalorimetric determination. In the present case, it allows us to thermodynamically characterize the formation of SmNO
3
2+
and EuNO
3
2+
, processes we have not been able to study directly. 相似文献
35.
K. P. Kotchi R. Castanet J. C. Mathieu 《Journal of Thermal Analysis and Calorimetry》1987,32(5):1477-1488
The heat effects corresponding to the dissolution of solid PbSe in liquid Pb-Se alloys starting from pure Pb were measured with a direct reaction calorimeter. The partial enthalpy of formation of PbSe in the melt and the integral enthalpy of formation of the Pb-PbSe system in the liquid state referred the liquid Pb and PbSe were deduced. Then, from a very simple change of reference state, and in the knowledge of the enthalpy of PbSe, the integral enthalpy of formation of Pb-Se liquid alloys with reference to pure liquid components could be derived. The results agreed very well with those obtained previously on the dissolution of Se in Pb. The method used can be applied whenever the component to be added to the bath has a high vapor pressure.
Zusammenfassung Die bei der Überführung von festem PbSe in flüssige Pb-Se-Legierungen auftretenden Wärmeeffekte wurden mittels direkter Reaktionskalorimetrie bestimmt. Die auf flüssiges Pb und PbSe bezogene partielle Bildungsenthalpie des Pb-PbSe-Systems im flüssigen Zustand wurde ermittelt. Durch eine geringfügige Veränderung des Referenzzustandes konnte bei Kenntnis der Enthalpie von PbSe die integrale Bildungsenthalpie von flüssigen Pb-Se-Legierungen in Bezug auf die reinen Komponenten abgeleitet werden. Die Ergebnisse stimmen gut mit denen überein, die früher für die Auflösung von Se in Pb erhalten wurden. Die Methode ist anwendbar, wenn dem Bade zuzugebende Komponenten einen hohen Dampfdruck haben.
Pb-Se Pb-Se, . Pb-Se Pb-PbSe Pb Pb-Se. PbSe, Pb-Se . , . , .相似文献
36.
Hu D Croutxé-Barghorn C Feuillade M Carré C 《The journal of physical chemistry. B》2005,109(32):15214-15220
Hybrid sol-gel materials have been prepared by hydrolytic polycondensation of an alkoxysilane. The sol-gel process of methacryloxypropyltrimethoxysilane (MAPTMS) has been followed by fluorescence spectroscopy with 2-naphthol as a probe. The experimental results showed that this photoprobe was dramatically sensitive to the microenvironment polarity. Spectroscopic studies revealed fluctuations of the maximum emission intensity and wavelength as a function of time. These fluctuations were attributed to the amphiphilic behavior of the hydrolyzed precursor. The maximum emission wavelength of the probe corresponding to its protonated form was higher than in pure water. All the results suggest that the presence of water molecules, tightly bonded to the polar head of the silanols, increased locally the sol polarity and induced a red-shifted emission. Fluorescence spectroscopy emphasized the reversibility of monomeric silanol aggregates and the changes in hydroxy group number of the silica network during the sol maturation. The behavior of this system upon shaking confirmed this statement. 相似文献
37.
Turbiez M Frère P Allain M Videlot C Ackermann J Roncali J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(12):3742-3752
Hybrid oligothiophenes based on a various combinations of thiophene and 3,4-ethylenedioxythiophene (EDOT) groups have been synthesized. UV/Vis absorption spectra show that the number and relative positions of the EDOT groups considerably affect the width of the HOMO-LUMO gap and the rigidity of the conjugated system. Analysis of the crystallographic structure of two hybrid quaterthiophenes confirms that insertion of two adjacent EDOT units in the middle of the molecule leads to a self-rigidification of the conjugated systems by intramolecular SO interactions. Cyclic voltammetry data shows that the first oxidation potential of the oligomers decreases with increasing chain length and increasing number of EDOT groups for a given chain length. Electrochemical studies and theoretical calculations show that the positions of the EDOT units in the conjugated chain control the potential difference (DeltaE(p)) between the first and second oxidation steps. Moving the EDOT groups from the outer to the inner positions of the conjugated system increases DeltaE(p). Theoretical calculations confirm that this phenomenon reflects an increase of the intramolecular coulombic repulsion between positive charges in the dication. A thin-film field-effect transistor was fabricated by vacuum sublimation of a pentamer with alternating thiophene-EDOT structure, and the hole mobility was determined. 相似文献
38.
The reaction of [(CO)PPh3)2Re(μ-H)2(μ-NCHPh)Ru(PPh3)2(PhCN)] (2) with HBF4-Me2O generates [(CO)PPh3)2Re(μ- H)2(μ,η1,η2HNCHPh)Ru(PPh3)2(PhCN)][BF4] (3). Monitoring the reaction by NMR spectroscopy shows the intermediate formation of [(CO)(PPh3)2 HRe(μ-H)2(μ-NCHPh)Ru(PPh3)2(PhCN)][BF4] (4). Attempted reduction of the imine ligand by a nucleophile (H− or CN−) failed, regenerating 2. Under dihydrogen at 50 atm, 3 is slowly transformed into [(CO)(PPh3)2HRe(μ-H)3Ru(PPh3)2(PhCN)][BF4] (5) with liberation of benzyl amine. 相似文献
39.
Aziz Ghoufi Pierre Archirel Jean-Pierre Morel Nicole Morel-Desrosiers Anne Boutin Patrice Malfreyt 《Chemphyschem》2007,8(11):1648-1656
We report potential of mean force (PMF) calculations on the interaction between the p-sulfonatocalix[4]arene and a monovalent cation (Cs(+)). It has been recently shown from microcalorimetry and (133)Cs NMR experiments that the association with Cs(+) is governed by favourable cation-pi interactions and is characterized by the insertion of the cation into the cavity of the macrocycle. We show that the PMF calculation based upon a classical model is not able to reproduce both the thermodynamic properties of association and the insertion of the cation. In order to take into account the different contributions of the cation-pi interactions, we develop a new methodology consisting of changing the standard PMF by an additional contribution resulting from quantum calculations. The calculated thermodynamic properties of association are thus in line with the microcalorimetry and (133)Cs NMR experiments and the structure of the complex at the Gibbs free-energy minimum shows the insertion of the cation into the cavity of the calixarene. 相似文献
40.
The use of a so-called trihedral and a T-shaped cross-flow pneumatic nebulizer with dual solution loading for inductively coupled plasma optical emission spectrometry has been studied. By these devices analyte clouds from two solutions can be mixed during the aerosol generation step. For both nebulizers the correction of matrix effects using internal standardization and standard addition calibration in an on-line way was investigated and compared to elemental determinations using a conventional cross-flow nebulizer and calibration with synthetic standard solutions without matrix matching. A significant improvement of accuracy, both for calibration with internal standardization and standard addition, was obtained in the case of four synthetic solutions containing each 40 mmol L− 1 Na, K, Rb and Ba as matrix elements and 300 μg L− 1 Cd, Co, Cr, Cu, Fe, Mn, Ni and Pb as analytes. Calibration by standard addition in the case of dual solution loading has been shown to be very useful in the determination of elements at minor and trace levels in steel and alumina reference materials. The results of analysis for minor concentrations of Cr, Cu and Ni in steel as well as for Ca, Fe, Ga, Li, Mg, Mn, Na, Si and Zn in alumina powder certified reference materials subsequent to sample dissolution were found to be in good agreement with the certificates. Limits of detection were found to be only slightly above those for a conventional cross-flow nebulizer and a precision better than 3% was realized with both novel nebulizers. 相似文献