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排序方式: 共有626条查询结果,搜索用时 15 毫秒
1.
Jnos Szanyi Ja Hun Kwak Sarah Burton Jose A. Rodriguez Charles H.F. Peden 《Journal of Electron Spectroscopy and Related Phenomena》2006,150(2-3):164-170
Adsorbed ionic NOx species formed upon the interaction of NO2 with dehydrated or hydrated Na- and Ba-Y, FAU zeolites were characterized using FT-IR/TPD, solid state NMR, and XANES techniques. NO2 disproportionates on both dehydrated catalyst materials forming NO+ and NO3− species. These ionic species are stabilized by their interactions with the negatively charged zeolite framework and the charge compensating cations (Na+ and Ba2+), respectively. Although the nature of the adsorbed NOx species formed on the two catalysts is similar, their thermal stabilities are strongly dependent on the charge compensating cations. In the presence of water in the channels of these zeolite materials new paths open for reactions between NO+ and H2O, and NO2 and H2O, resulting in significant changes in the adsorbed ionic species observed. These combined spectroscopic investigations afforded the understanding of the interactions between water and NO2 on these zeolite catalysts. 相似文献
2.
Two results on site percolation on thed-dimensional lattice,d1 arbitrary, are presented. In the first theorem, we show that for stationary underlying probability measures, each infinite cluster has a well-defined density with probability one. The second theorem states that if in addition, the probability measure satisfies the finite energy condition of Newman and Schulman, then there can be at most one infinite cluster with probability one. The simple arguments extend to a broad class of finite-dimensional models, including bond percolation and regular lattices. 相似文献
3.
Treatment of halo--methylphosphonium salts with potassium fluoride and halogen (I2, Br2, ICl, IBr) gives modest yields of halo--methanes. This method of preparation augments the classical Hunsdiecker approach to these materials. 相似文献
4.
Bonvin Y Callens E Larrosa I Henderson DA Oldham J Burton AJ Barrett AG 《Organic letters》2005,7(21):4549-4552
[reaction: see text] Oxidation of alkyl and cycloalkyl arenes with tert-butyl hydroperoxide catalyzed by bismuth and picolinic acid in pyridine and acetic acid gave the corresponding benzylic ketones (48-99%). Alternatively, oxidation of methyl arenes gave the corresponding substituted benzoic acids (50-95%). Preliminary mechanistic studies were consistent with a radical mechanism rather than a bismuth(III)-bismuth(V) cycle. 相似文献
5.
Summary The use ofp-methylaminophenol sulphate (metol) at 100 for the reduction of phosphomolybdic acid in the determination of phosphate in sea-water has a number of advantages over stannous chloride. The molybdenum blue colour once formed, is stable for several hours, and the calibration curve is not dependent upon the batches of reagent used; Beer's law is obeyed up to concentrations of at least 1 mg PO4-P/l. The salt error is very small and the method has approximately 30% greater sensitivity in sea-water than has the stannous chloride method. Arsenic does not interfere at concentrations five times greater than its sea-water concentration. Iron and copper in moderate amounts do not interfere.Evaporation to dryness with a mixture of concentrated nitric and perchloric acids, followed by colorimetric determination of inorganic phosphate, is employed for the determination of total phosphorus in sea-water and marine muds. The method shows a coefficient of variation of ca. 2% with sea-water containing 60g P/l.
Zusammenfassung Die Anwendung von p-Methylaminophenolsulfat (Metol) bei 100 für die Reduktion von PhosphomolybdÄnsÄure bei der Bestimmung der Phosphate in Meerwasser hat gegenüber Zinn(II)chlorid eine Reihe von Vorteilen. Das einmal gebildete MolybdÄnblau ist für einige Stunden stabil und die Eichkurve nicht von der angewendeten Menge Reagens abhÄngig; das Beersche Gesetz ist für Konzentrationen bis zu 1 mg Phosphat-Phosphor/l gültig. Der Salzfehler ist sehr klein und die Methode für Meerwasser etwa um 30% empfindlicher als die Zinn(II)chloridmethode. Arsen stört bei Konzentrationen, die fünfmal größer sind als in Meerwasser, nicht. Eisen und Kupfer in mÄßigen Mengen stören gleichfalls nicht.Für die Bestimmung des Gesamtphosphors in Meerwasser und Seeschlamm wird mit einer Mischung von konz. SalpetersÄure und PerchlorsÄure zur Trockne verdampft und das anorganische Phosphat kolorimetrisch gemessen. Die Methode zeigt eine Schwankungsbreite von etwa 2% für Meerwasser mit 60g P/l.
Résumé L'emploi du sulfate de p-méthylaminophenol (métol) à 100 pour la réduction de l'acide phosphomolybdique lors du dosage des phosphates contenus dans l'eau de mer présente de multiples avantages sur celui du chlorure stanneux. Après sa formation, le bleu de molybdène reste stable pendant plusieurs heures et la courbe d'étalonnage ne varie pas avec le lot de réactif employé; la loi deBeer est applicable jusqu'à des concentrations au moins égales à 1 mg de phosphore phosphorique par litre. L'erreur de sel est très faible et la sensibilité de la méthode appliquée à l'eau de mer est environ 30% plus élevée que la méthode au chlorure stanneux. L'arsenic ne gÊne pas mÊme pour des concentrations cinq fois supérieures à celle qu'il atteint dans l'eau de mer. Des teneurs modérées de fer et de cuivre n'apportent pas de perturbation. On évapore à siecité en présence d'un mélange d'acides nitrique et perchlorique concentrés puis on effectue une détermination colorimétrique des phosphates minéraux pour doser le phosphore total de l'eau de mer et des boues marines. La méthode est sujette à des variations d'environ 2% pour leau de mer contenant 60 g de phosphore par litre.相似文献
6.
Micellar electrokinetic capillary chromatography (MECC) involves the application of a high voltage (10 to 40 kV) across a capillary column (25 to 75 microns i.d.) that is filled with a solution containing micelles. The mobile phase in this work consists of sodium dodecyl sulfate in an aqueous phosphate/borate buffer system. Pyridoxine (vitamin B6) and five of its metabolites are separated, with efficiencies as high as 60,000 theoretical plates/meter. Pyridoxic acid, a metabolite of B6, is separated and quantitated in human urine using laser-excited fluorescence detection. Limits of detection are less than a picogram injected. 相似文献
7.
A kinetic study of the fluoride ion catalyzed isomerization of a series of 2-aryl1-butenes shows the reactions to be pseudo first order in olefin at constant fluoride ion concentration. The resultant Hammett plot is non-linear witb a concave downward break near =0. A two step mechanism involving formation of a carbanionic intermediate is proposed. A change in the rate limiting step causes the break in the Hammett plot. Carbanion trapping experiments are also reported. 相似文献
8.
Hau Sun Sam Chan Amber L. Thompson Kirsten E. Christensen Jonathan W. Burton 《Chemical science》2020,11(42):11592
Laurefurenynes C–F are four natural products isolated from Laurencia species whose structures were originally determined on the basis of extensive nuclear magnetic resonance experiments. On the basis of a proposed biogenesis, involving a tricyclic oxonium ion as a key intermediate, we have reassigned the structures of these four natural products and synthesized the four reassigned structures using a biomimetic approach demonstrating that they are the actual structures of the natural products. In addition, we have developed a synthesis of the enantiomers of the natural products laurencin and deacetyllaurencin from the enantiomer of (E)-laurefucin using an unusual retrobiomimetic strategy. All of these syntheses have been enabled by the use of tricyclic oxonium ions as pivotal synthetic intermediates.The synthesis and structural reassignment of laurefurenynes C–F has been achieved, with the new structures fitting with a proposed biosynthesis. Also reported is the synthesis of ent-laurencin and ent-deacetyllaurencin via a retrobiomimetic approach. 相似文献
9.
Burton A Elomari S Chen CY Medrud RC Chan IY Bull LM Kibby C Harris TV Zones SI Vittoratos ES 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(23):5737-5748
The syntheses, structure solutions, and physicochemical and catalytic characterizations of the novel zeolites SSZ-53 and SSZ-59 are described. SSZ-53 and SSZ-59 were synthesized under hydrothermal conditions with the [1-(4-fluorophenyl)cyclopentylmethyl]trimethyl ammonium cation and 1-[1-(4-chlorophenyl)cyclopentylmethyl]-1-methyl azocanium cation, respectively, as structure-directing agents. The framework topology of SSZ-53 was solved with the FOCUS method, and the structure of SSZ-59 was determined by model building. Rietveld refinement of synchrotron X-ray powder diffraction data confirms each proposed model. SSZ-53 and SSZ-59 each possess a one-dimensional channel system delimited by 14-membered rings. Results from transmission electron microscopy, electron diffraction, catalytic experiments (spaciousness index and constraint index tests), and argon and hydrocarbon adsorption experiments are consistent with the proposed structures. 相似文献
10.
Peter G. Burton 《Chemical physics》1974,6(3):419-430
The key components of a completely theoretical parametrization of the essential-structural-elements molecular orbital (ESE MO) formalism using Slater-type AO basis in the LCAO SCF procedure are discussed. Special attention is paid to the problem of separability into core and valence parts of the total molecular wavefunction, including the case where valence functions strongly overlap neighbouring core orbitals. The use of Huzinaga and Cantu effective hamiltonian is proposed. The parametrization is tested in relation to the SO2 molecules. The role of sulphur 3d functions in bonding as predicted by the present ESE MO calculations and ab initio calculations are compared. The present parametrization appears to adequately handle both the core/valence separation, and the diffuse higher valence sulphur 3d functions in this system. 相似文献