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881.
The kinetics of the reaction between γ-Al2O3 and COCl2 have been studied by isothermal TG measurements in the temperature range 585–1085 K. The influence of the partial pressure of COCl2 on the reaction rate was investigated at 648, 673 and 698 K in the 104–105 Pa range, using N2 as a diluting gas. The reaction seems to proceed in the chemical control region below 675 K. Above this temperature, however, diffusional processes and decomposition of COCl2 are considered to affect the reaction rate. In the chemical control region an apparent order of reaction of 0.75 in respect of COCl2 and an apparent activation energy of 128–135 kJ mole?1 were found for the chlorination process. 相似文献
882.
Maarten B J Roeffaers Bert F Sels Davey Loos Christopher Kohl Klaus Müllen Pierre A Jacobs Johan Hofkens Dirk E De Vos 《Chemphyschem》2005,6(11):2295-2299
Fluorescence spectroscopy is employed to follow the ion exchange of an emissive dye--a carboxylated perylene imide--on a layered double hydroxide. The exchange of the carboxylate ions starts at the edges of the layered double hydroxide crystals and is followed by diffusion to the basal plane. Such space-resolved observations provide a solid basis for modelling and studying the mechanisms of exchange. 相似文献
883.
Water is capable of reducing the rate of deactivation of gold-titania catalysts for propene epoxidation. 相似文献
884.
Lu J Jayaprakash KN Schlueter U Fraser-Reid B 《Journal of the American Chemical Society》2004,126(24):7540-7547
A congener of the glycosylphosphatidylinositol (GPI) membrane anchor present on the cell surface of the malaria pathogen Plasmodium falciparum has been synthesized. This GPI is an example of a small number of such membrane anchors that carry a fatty acyl group at O-2 of the inositol. Although the acyl group plays crucial roles in GPI biosynthesis, it rarely persits in mature molecules. Other notable examples are the mammalian GPIs CD52 and AchE. The presence of bulky functionalities at three contiguous positions of the inositol moiety creates a very crowded environment that poses difficulties for carrying out selective chemical manipulations. Thus installations of the axial long-chain acyl group and neighboring phosphoglyceryl complex were fraught with obstacles. The key solution to these obstacles in the successful synthesis of the malarial candidate and prototype structures involved stereoelectronically controlled opening of a cyclic ortho ester. The reaction proceeds in very good yields, the desired axial diastereomer being formed predominantly, even more so in the case of long-chain acyl derivatives. The myoinositol precursor was prepared from methyl alpha-d-glucopyranoside by the biomimetic procedure of Bender and Budhu. For the glycan array, advantage was taken of the fact that (a). n-pentenyl ortho ester donors are rapidly and chemospecifically activated upon treatment with ytterbium triflate and N-iodosuccinimide and (b). coupling to an acceptor affords alpha-coupled product exclusively. A strategy for obtaining the GPI's alpha-glucosaminide component from the corresponding alpha-mannoside employed Deshong's novel azide displacement procedure. Thus all units of the glycan array were obtained from a beta-d-manno-n-pentenyl ortho ester, this being readily prepared from d-mannose in three easy, high-yielding steps. The "crowded environment" at positions 1 and 2, noted above, could conceivably be relieved by migration of the acyl group to the neighboring cis-O-3-hydroxyl in the natural product. However, study of our synthetic intermediates and prototypes indicate that the O-2 acyl group is quite stable, and that such migration does not occur readily. 相似文献
885.
van de Water LG Bergwerff JA Leliveld BR Weckhuysen BM de Jong KP 《The journal of physical chemistry. B》2005,109(30):14513-14522
Spatially resolved Raman and UV-vis-NIR microspectroscopy have been used as tools to study the preparation process of supported catalyst bodies. Detailed spectroscopic information on the local coordination geometry of two different metallic species along with their macro-distribution over the catalyst body has been obtained, enabling a good understanding of the physicochemical processes occurring during the drying process of impregnated gamma-Al(2)O(3) bodies. The formation and decomposition of the Keggin-type complex H(x)PMo(11)CoO(40)((7-)(x)-), which is considered to be a potential precursor for CoMoS(2)/gamma-Al(2)O(3) HDS catalysts, inside gamma-Al(2)O(3) bodies is shown to be a function of the composition of the impregnation solutions, the aging time, and the drying conditions applied. This knowledge has been successfully applied to prepare samples with a well-defined distribution of the bimetallic complex, that is, either egg-shell, egg-yolk, or homogeneous distributions. The Raman results are presented in a semiquantitative way by subtraction of a reference spectrum of a sample containing a known amount of H(x)PMo(11)CoO(40)((7-)(x)-) from the spectra recorded along the cross-section of the catalyst bodies. 相似文献
886.
Davis WC Venzie JL Willis B Coffee RL Arya DP Marcus RK 《Rapid communications in mass spectrometry : RCM》2003,17(15):1749-1758
Use of a particle beam glow discharge (PB-GD) source for mass spectrometric determinations of deoxy- and ribonucleosides and nucleotides is described. Use of this combination of sample introduction and ion source decouples the vaporization and ionization steps, leading to very simple spectral structure. The mass spectra of these compounds are EI-like in nature, with clearly identified molecular ions and fragmentation patterns that are easily rationalized. The PB-GDMS combination can be operated in a flow injection mode wherein the analyte is injected directly into the solvent flow, or can also be coupled to a high-performance liquid chromatography (HPLC) system allowing LC/MS analysis of mixtures. Mass spectra obtained for nucleic acid bases, nucleosides, and nucleotides are readily obtained with injections of low-nanomole quantities. Representative PB-GDMS spectra for deoxy- and ribonucleosides, nucleotides, and mixed-base oligonucleotides are presented to demonstrate the capabilities of the GD source. Characteristic fragmentation peaks from the spectra of adenine, cytosine, guanine, and thymine were identified in 22-base sequences of single-stranded DNA. The PB-GD source is capable of producing spectra that may be used to identify the individual bases present in mixed-base DNA and RNA fragments. 相似文献
887.
Hanson EL Schwartz J Nickel B Koch N Danisman MF 《Journal of the American Chemical Society》2003,125(51):16074-16080
A new method is described to prepare strongly bonded, compact monolayer films of alkyl- or arylphosphonates on the native oxide surface of Si (SiO(2)/Si). This method is illustrated for octadecyl- and alpha-quarterthiophene-2-phosphonates. For both cases, AFM shows comprehensive coverage of the SiO(2)/Si surface. The thickness of the continuous film of 4TP/SiO(2)/Si was measured both by AFM and by X-ray reflectivity to be ca. 18 A. Direct gravimetric analysis shows surface coverage by alpha-quarterthiophene-2-phosphonate to be about 0.66 nmol/cm(2), which corresponds to molecular packing in the film close to that of crystalline alpha-quarterthiophene. Coverage by octadecylphosphonate was ca. 0.90 nmol/cm(2), corresponding to a cross-sectional area of about 18.5 A(2)/molecule, consistent with close-packed alkyl chains. 相似文献
888.
L. Costa G. Camino A. Chiotis G. Clouet J. Brossas M. Bert A. Guyot 《Polymer Degradation and Stability》1984,6(3):177-188
On heating at volatilisation temperatures, polystyrene (PS) and telechelic chloromethoxyphosphonated polystyrene (TPPS) behave differently in the very early stage of the process. The initial rate of volatilisation of PS is zero and increases with conversion whereas TPPS polymers volatilise at an initial high rate which decreases with conversion. However, in the main stage of volatilisation (from 10–20 to 70% weight loss) both polymers liberate the same volatile products through the same basic mechanism of depolymerisation. Nevertheless, the overall rate of volatilisation of TPPS polymers in this stage is much lower than that of PS. This is attributed to the creation in degrading TPPS polymers of less reactive chain carriers of the depolymerisation process which are stabilised by the phosphoryl group. 相似文献
889.
DirkJ.A. DeRidder Kees Goubitz Henk Schenk Bert Krijnen JanW. Verhoeven 《Helvetica chimica acta》2003,86(3):799-811
The crystal structures of five N‐arylpiperidin‐4‐one derivatives 2P2, 3P2, 5P2, 1P3 , and 2P3 are presented (Fig. 2 and Tables 1–5) and discussed together with the derivatives 1P2 and 4P2 published previously. In all but one structure, 1P2 , the aryl group is in an equatorial position. The piperidine ring adopts a normal chair conformation. In 1P2 , the piperidine ring central C? C bonds are significantly elongated, which is consistent with the idea that through‐bond interaction is more pronounced in the axial conformation. Through‐bond interaction also influences the pyramidalization at the piperidine C(4)‐atom in such a way that a strong interaction is directing the ethylene C‐atom C(9) into the axial direction. 相似文献
890.
I. Bertóti I. S. Pap T. Székely I. Z. Babievskaya 《Journal of Thermal Analysis and Calorimetry》1987,32(1):281-292
The reactions of hematite and wustite with Cl2, COCl2, CO+Cl2 and gaseous CCl4 were studied by thermogravimetry, X-ray diffraction and computer-assisted thermodynamic calculations. During the chlorination of hematite, only gaseous products were formed. The apparent activation energies were calculated from the isothermal TG curves. The chlorination of wustite involves the formation and transformation of a series of solid chloride products. FeCl2(s), FeCl3(s) and FeOCl(s) were found as intermediates during chlorination with Cl2 or Cl2+CO, while only FeCl2(s) was detected in reactions with COCl2 oe CCl4. The chlorinations of the iron oxides with various agents were compared, and the differences in reactivity sequences were established.
Zusammenfassung Die Reaktionen von HÄmatit und Wustit mit Cl2, COCl2, CO+Cl2 and gasförmigen CCl4 wurden thermogravimetrisch und röntgendiffraktometrisch untersucht. Thermodynamische Berechnungen wurden mit Computern ausgeführt. Bei der Chlorierung von HÄmatit entstehen nur gasförmige Reaktionsprodukte. Die scheinbaren Aktivierungsenergien wurden aus isothermen TG-Kurven berechnet. Bei der Chlorierung von Wustit wird eine Reihe von festen Chloriden gebildet. FeCl2(s), FeCl3(s) und FeOCl(s) treten bei der Chlorierung mit Cl2 oder Cl2 + CO als intermediÄre Produkte auf, wÄhrend bei Reaktionen mit COCl2 oder CCl4 nur FeCl2(s) nachgewiesen wurde. Die verschiedenen Chlorierungsmittel wurden hinsichtlich ihrer Wirksamkeit miteinander verglichen und in einer entsprechenden Reihenfolge eingeordnet.
Cl2, COCl2, CO+Cl2 CCl2 4 , , . . . . Cl2 Cl2 + CO , FeCl2, FeCl3 FeOCl, COCl2 CCl4 — FeCl2. .相似文献