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排序方式: 共有135条查询结果,搜索用时 31 毫秒
31.
[reaction: see text] A highly enantioselective Meerwein-Schmidt-Ponndorf-Verley (MSPV) reduction of N-phosphinoyl ketimines by (BINOL)Al(III)/2-propanol is reported. Yields ranging between 79 and 85% with high enantiomeric excesses (93-98%) are observed for a wide range of structurally diverse ketimines. A [2.0.4] bicyclic chelation model is proposed to account for this high selectivity. 相似文献
32.
Stephen J. Graves 《Discrete Mathematics》2010,310(17-18):2435-2439
33.
Extreme osmotic compression of nanoemulsion droplets, achieved by ultracentrifugation, can create solidlike biliquid foams without causing significant droplet coalescence. Using small-angle neutron scattering (SANS), we probe the structure of a uniform silicone oil-in-water nanoemulsion stabilized by sodium dodecyl sulfate over a wide range of volume fractions, phi, up to and beyond the limit associated with maximal random jamming of spheres, phiMRJ = 0.64. Although some features in the structure can be understood at lower phi using simple predictions for hard spheres, the anionic repulsion and deformability of the droplet interfaces creates departures from these predictions at higher phi. For phi near and beyond phiMRJ, the effective structure factor, Seff, as a function of wavenumber, q, exhibits a primary peak that is subunity. We speculate that this striking feature is due to the deformation of the droplets into nonspherical shapes as the system begins to approach the limit of a random array of nanoscopic thin films, Plateau borders, and vertexes characteristic of a polyhedral foam. 相似文献
34.
Simple Zeros of the Riemann Zeta-Function 总被引:1,自引:0,他引:1
Assuming the Riemann Hypothesis, Montgomery showed by meansof his pair correlation method that at least two-thirds of thezeros of Riemann's zeta-function are simple. Later he and Taylorimproved this to 67.25 percent and, more recently, Cheer andGoldston increased the percentage to 67.2753. Here we proveby a new method that if the Riemann and Generalized LindelöofHypotheses hold, then at least 70.3704 percent of the zerosare simple and at least 84.5679 percent are distinct. Our methoduses mean value estimates for various functions defined by Dirichletseries sampled at the zeros of the Riemann zeta-function. 1991Mathematics Subject Classification: 11M26. 相似文献
35.
It is shown that differential double-layer capacitance data can be obtained simultaneously with the recording of the Raman spectra of adsorbates at a roughened silver microelectrode. The use of such data to provide information about the structure of the silver electrode/aqueous KCl (0.1 M)/pyridine (0.1 M) interface is demonstrated. 相似文献
36.
37.
A cleanup procedure based on electrodialysis is described for the preparation of aqueous bread extracts for bromate determination by chemiluminescence. The technique utilizes electrophoresis with 3 chambers separated by semipermeable membranes. The relative merits of reverse osmosis (RO), ultrafiltration, and nanofiltration membranes with various molecular weight cutoffs were evaluated. The best results were obtained with an RO membrane manufactured from thin-film (composite) polysulfone as support for polyamide. A 0.14 M sodium sulfate solution in the center or collection chamber provides optimum conductivity. Aqueous hydroxylamine sulfate (30 mM) was selected for the anode compartment as a reductant for the anode oxidation products. The constant current mode at 150 mA with a potential of ca 100 volts was used. After electrophoretic separation, the bromate concentration in the collection chamber was typically 2 to 3 times greater than the concentration in the bread extract. The chemiluminescent reaction of bromate with sulfite with hydrocortisone as the enhancer was selected for detection of bromate. The emission, with a wavelength maximum at 575 nm, was found to "glow" rather than "flash" after the reagents were mixed; therefore, it was possible to optimize the light collection period. The method was validated with a variety of commercial bread products. White bread, hot dog buns, hamburger rolls, and a multigrain bread from 7 different manufacturers were studied. 相似文献
38.
Abrams C.F. Graves D.B. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1999,27(5):1426-1432
Energy and angular distributions of reflections and sputtered atoms are essential inputs for feature profile evolution simulations. Molecular dynamics simulations are used to compute the three-dimensional energy and angular distributions for reflected and sputtered products when both Ar+ and Cu+ ions bombard a copper surface. We term these “spatiokinetic” distribution functions (SKDF's). We show by example that SKDF's for reflected Ar+ ions focus as the incident angle &thetas;i (normal=0°) is increased from 60-75° and broaden as the incident energy Ei is increased from 55-175 eV. We show that the SKDF's for glancing-angle reflected Cu+ ions focus when Ei is increased from 55-175 eV. We show that the SKDF's for copper atoms sputtered by 175 eV Ar+ are insensitive to &thetas;i;. We report total sputter yields for Ar+ and Cu+ ions at 55 and 175 eV for incident angles between 0° and 85°, and sticking probabilities for Cu+ ions for these energies and angles. Comparison to representative experimental results (Doughty et al., 1997) is given 相似文献
39.
Brown J Buckley D Coulthard A Dixon AK Dixon JM Easton DF Eeles RA Evans DG Gilbert FG Graves M Hayes C Jenkins JP Jones AP Keevil SF Leach MO Liney GP Moss SM Padhani AR Parker GJ Pointon LJ Ponder BA Redpath TW Sloane JP Turnbull LW Walker LG Warren RM 《Magnetic resonance imaging》2000,18(7):765-776
The imaging and analysis protocol of the UK multicentre study of magnetic resonance imaging (MRI) as a method of screening for breast cancer in women at genetic risk is described. The study will compare the sensitivity and specificity of contrast-enhanced MRI with two-view x-ray mammography. Approximately 500 women below the age of 50 at high genetic risk of breast cancer will be recruited per year for three years, with annual MRI and x-ray mammography continuing for up to 5 years. A symptomatic cohort will be measured in the first year to ensure consistent reporting between centres. The MRI examination comprises a high-sensitivity three-dimensional contrast-enhanced assessment, followed by a high-specificity contrast-enhanced study in equivocal cases. Multiparametric analysis will encompass morphological assessment, the kinetics of contrast agent uptake and determination of quantitative pharmacokinetic parameters. Retrospective analysis will identify the most specific indicators of malignancy. Sensitivity and specificity, together with diagnostic performance, diagnostic impact and therapeutic impact will be assessed with reference to pathology, follow-up and changes in diagnostic certainty and therapeutic decisions. Mammography, lesion localisation, pathology and cytology will be performed in accordance with the UK NHS Breast Screening Programme quality assurance standards. Similar standards of quality assurance will be applied for MR measurements and evaluation. 相似文献
40.
William H Graves Dennis Sentilles 《Journal of Mathematical Analysis and Applications》1979,68(1):228-264
Every vector measure μ factors uniquely by μ = gm ° χ where χ is a universal measure and gm is a linear map continuous on simple functions in the strict topology. The completion of χ describes the dual of the space of measures by uniform closure based on χ-measurable sets and also as the completion of the simple functions in the strict topology. 相似文献