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排序方式: 共有164条查询结果,搜索用时 15 毫秒
1.
Saig A Finkelstein Y Danon A Koresh JE 《The journal of physical chemistry. B》2005,109(22):11180-11185
He and Ne in contact with molecular sieves in the form of crystalline A zeolites and amorphous carbon molecular sieves fibers (CMSF) were studied by adsorption measurements. Classification of the effective enclosure of zeolitic apertures and of graphitic constrictions, as determined by recent temperature-programmed desorption mass spectrometry (TPD-MS) studies of adsorption of He and Ne onto these materials, was utilized in making a prudent choice of samples and experimental conditions. In view of the former TPD information, the behaviors of adsorption and volumetric measurements reported herein are straightforwardly interpreted. The combined TPD, adsorption isotherms, and dead volume data deepen the understanding of the physicochemical nature of adsorbed gas, where gas adsorption in the vicinity of pore constrictions and/or apertures as well as on the inner surface areas of pores and/or cages could be resolved. Previous conclusions that the huge activation energies measured for Ne/CMSF at high temperatures are unlikely to characterize chemical desorption but reflect those required for overcoming the barrier of effectively constricted apertures were confirmed by the volumetric data presented here. At 77 K, considerable He adsorption was observed in the porous solids and found to be responsible for abnormal deduced values of dead volumes. The occurrence of significant adsorption of He onto A zeolites and CMSF at 77 K warrants the realization that in cases concerning porous materials, volumetrically deduced quantities should not be taken for granted, but should be carefully considered and uniquely interpreted in relation to the specific experimental conditions under which they are taken. 相似文献
2.
Simon A. Butler Gillian W. Harris David S. Moss Beatrice A. Gorinsky Margaret J. Adams Sheila Gover 《Journal of chemical crystallography》1994,24(1):1-3
We report the estimation of random errors in the refinement of the rigid body displacements of the -helices of the enzyme 6-phosphogluconate dehydrogenase. Least-squares refinement of the TLS parameters of the helices has been carried out using X-ray reflection data of 2.1 Å resolution, resulting in anR-factor of 19.5%. Standard deviations were estimated from the normal matrix. The results show that the translational mean-square displacements of nearly all the helices are significant at this resolution. However the libration parameters are only significant when the helices have at least four turns. Screw-rotation tensor values cannot be determined at this resolution. 相似文献
3.
Ze'ev V. I. Zaretskii Albert Avraham Ben-Bassat David Lavie 《Journal of heterocyclic chemistry》1975,12(5):837-840
The mass spectra of α-, β- and γ-pyridine aldoximes and the respective O-methyl ethers were studied. The mass spectral behaviour of α-pyridine aldoxime is characterized by the elimination of NO, while the molecular ion of the γ-isomer expels H2 CN. In the case of the β-isomer the formation of the m/e 67 ion (C4H5N) in a concerted process is the main feature. In the α-pyridine aldoxime methyl ether, in sharp contrast to the hydroxy analog, the M-30 peak was found to be due to the elimination of CH2O, the expulsion of NO being absent. The mechanism of the fragmentation reactions is discussed, the conclusion drawn being based on the high resolution measurements as well as on the spectra of the respective deuterioanalogs and on the metastable transitions. 相似文献
4.
Multichannel homogeneous immunoassay for detection of 2,4,6-trinitrotoluene (TNT) using a microfabricated capillary array electrophoresis chip 总被引:6,自引:0,他引:6
A high-throughput homogeneous immunoassay for the sensitive detection of 2,4,6-trinitrotoluene (TNT) has been developed using radial capillary array electrophoresis microdevices. Samples consisting of equilibrium mixtures of anti-TNT antibody (Ab), fluorescein-labeled TNT, and various concentrations of unlabeled TNT were electrokinetically injected into 48 channels of a radial capillary array electrophoresis microchannel plate. The rapid electrophoretic separation allows us to analyze the equilibrium ratio formed by the competition between the labeled and the unlabeled TNT for Ab binding. The simultaneous parallel TNT separations facilitate determination of a calibration curve for the TNT assay, which has high sensitivity (LOD, 1 ng/mL) and a wide dynamic range (1-300 ng/mL). 相似文献
5.
Danon A Saig A Finkelstein Y Koresh JE 《The journal of physical chemistry. B》2005,109(44):21197-21201
Temperature-programmed desorption mass spectrometry (TPD-MS) measurements on [(18)O]water-enriched copper sulfate pentahydrate (CuSO(4).5H(2)(18)O) reveal an unambiguous occurrence of efficient oxygen isotope exchange between the water of crystallization and the sulfate in its CuSO(4) solid phase. To the best of our knowledge, the occurrence of such an exchange was never observed in a solid phase. The exchange process was observed during the stepwise dehydration (50-300 degrees C) of the compound. Specifically, the exchange promptly occurs somewhere between 160 and 250 degrees C; however, the exact temperature could not be resolved conclusively. It is shown that only the fifth, sulfate-associated, anionic H(2)O molecule participates in the exchange process and that the exchange seems to occur in a preferable fashion with, at the most, one oxygen atom in SO(4). Such an exchange, occurring below 250 degrees C, questions the common conviction of unfeasible oxygen exchange under geothermic conditions. This new oxygen exchange phenomenon is not exclusive to copper sulfate but is unambiguously observed also in other sulfate- and nitrate-containing minerals. 相似文献
6.
Israel Zilbermann Avraham Meshulam Haim Cohen Dan Meyerstein 《Supramolecular chemistry》2013,25(2):325-332
Abstract The mechanisms and kinetics of oxidation of ascorbate, AH?, by Ni(III)Li aq and by LiNi(III) (HPO4)2 ? complexes (L1 = meso-(5,12)-7,7,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; L2 = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) in neutral aqueous solutions have been investigated. The oxidation of ascorbate by the LiNi(III) (HPO4)2 ? and Ni(III)L1 aq proceeds via two consecutive reactions well separated in time. The products of the first reaction are the A.? radical anion and the corresponding Ni(II) complex. The oxidations by the LiNi(III)(HPO4)2 ? complexes proceed via the outer sphere mechanism, whereas the detailed mechanism of reaction of Ni(III)L1 aq cannot be determined. The rate of reaction decreases with the increase in the concentration of phosphate, thus indicating that LiNi(III)(HPO4)(H2O)+ and LiNi(III)OH2+ are stronger oxidizing agents than LiNi(III)(HPO4)? 2. The oxidation of ascorbate by Ni(III)L2 aq proceeds via three consecutive reactions which are well separated in time. Thus the results clearly point out that this process occurs via the inner sphere mechanism. The first transient observed is tentatively identified as L2(H2O)Ni(II)(A.?)2+, i.e., an unexpected complex of the ascorbate anion radical. Also in this process the last transient observed is the A.? anion radical. The stabilization of the ascorbyl radical in a transient complex might be of biological significance. 相似文献
7.
We address the problem of scheduling in programs involving the production of multiple units of the same product. Our study was motivated by a construction program for fast naval patrol boats. Other applications of this problem include procurement of multiple copies of aircraft, spacecraft, and weapon systems. In this problem we must decide how many units of the product to assign to each of a number of available crews (individuals, teams, subcontractors, etc.). These types of problems are characterized by two potentially conflicting considerations: 1) the need to complete each unit by its contractual due date, and 2) learning effects. Because of the first consideration, there is a tendency to use multiple crews for simultaneous production, so that meeting due dates is assured. However, the second consideration encourages assigning many units to a single crew so that learning effects are maximized. We study this scheduling problem with two different penalty cost structures and develop models for both versions. The models trade-off the penalty associated with late deliveries and the savings due to learning (and possibly incentive payments for early completion). We discuss different heuristic algorithms — simulated annealing, a genetic algorithm, and a pair-wise swap heuristic — as well as an exhaustive search to determine a baseline for comparisons. Our computational results show that the pair-wise swap algorithm is the most efficient solution procedure for these models. 相似文献
8.
Suzuki Y Sarachik MP Chudnovsky EM McHugh S Gonzalez-Rubio R Avraham N Myasoedov Y Zeldov E Shtrikman H Chakov NE Christou G 《Physical review letters》2005,95(14):147201
Local time-resolved measurements of fast reversal of the magnetization of single crystals of Mn12-acetate indicate that the magnetization avalanche spreads as a narrow interface that propagates through the crystal at a constant velocity that is roughly 2 orders of magnitude smaller than the speed of sound. We argue that this phenomenon is closely analogous to the propagation of a flame front (deflagration) through a flammable chemical substance. 相似文献
9.
We show that on conformal manifolds of even dimension there is no conformally invariant natural differential operator between density bundles with leading part a power of the Laplacian for n/2$">. This shows that a large class of invariant operators on conformally flat manifolds do not generalise to arbitrarily curved manifolds and that the theorem of Graham, Jenne, Mason and Sparling, asserting the existence of curved version of for , is sharp.
10.
Raz Gvishi Avraham Englander Gadi Peleg 《Journal of Sol-Gel Science and Technology》2008,48(1-2):18-23
We present a feasibility study to apply terahertz (THz) spectroscopy and THz imaging as non-destructive diagnostic tools for sol–gel analysis, manufacturing and quality control. By performing THz spectroscopy on liquid and solid samples we were able to follow several key parameters during the sol–gel formation process and of the final product. Sol–gel transformations were monitored by THz absorption, whereas density changes have been observed through changes in refractive indices. Time domain spectroscopy (TDS), both in transmission and reflection geometries, was used to monitor the properties of fast sol–gel resins. THz imaging of gold coated, thin-film sol–gel enables us to determine inhomogeneities and defects in their internal structure. We demonstrated that THz spectroscopy can be implemented as an online analytical tool for multi-parameter evaluation of the sol–gel process during fabrication, and of the final product. 相似文献