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41.
42.
Three fractions of SiC platelets were characterized using polarizing microscopy, X-ray diffraction, high resolution Auger electron spectroscopy (AES), electron spectroscopy for chemical analysis (ESCA) and scanning electron microscopy with energy dispersive analysis (SEM/EDX). The chemical analysis of the three fractions is given. The determination of the SiC polytypes (4H, 6H, 15R, 3C) was done and has been correlated with the chemical analysis. The results of the X-ray diffraction investigations confirm the polarizing microscopy observations. Using microanalytical techniques important conclusions about the impurity elements (e.g. B, Al, Fe) and their local distribution could be drawn. Most of them were found to be present on the outermost surfaces of the platelets. Some conclusions concerning the processing of the platelets are suggested.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday 相似文献
43.
A special type of substrate for surface-enhanced Raman scattering (s.e.r.s.) is evaluated. The substrates consist of silver particles deposited on stochastically arranged SiO2 posts produced by plasma etching of a quartz surface using a silver island film as an etch mask. The optimization of various experimental parameters such as silver layer thickness, silver evaporation angle, and excitation energy are discussed in detail. Comparative studies with p-nitrobenzoic acid as the model compound indicate that this present substrate is at least one order of magnitude more effective than other common s.e.r.s. substrates, such as the silver island film and the crossed-grating surface, which were previously found to induce the strongest s.e.r.s. signals. The preparation of these silver-particle-post substrates avoids the elaborate lithographic procedures required for crossed-grating structures. The quantitation of species in a three-component mixture illustrates the selectivity of the s.e.r.s. technique. 相似文献
44.
The generation of thiirenes in the thermal and photochemical nitrogen elimination of 1,2,3-thiadiazoles is discussed in relation to oxirene formation. Isotope labelling experiments were performed with 4-phenyl-[4-13C]-1,2,3-thiadiazole ( 7 ) as the substrate. 相似文献
45.
Ingo Hartenbach Falk Lissner Tanja Nikelski Steffen F. Meier Helge Müller‐Bunz Thomas Schleid 《无机化学与普通化学杂志》2005,631(12):2377-2382
About Lanthanide Oxotantalates with the Formula MTaO4 (M = La – Nd, Sm – Lu) Besides being a by‐product of solid state syntheses in tantalum ampoules the lanthanide(III) oxotantalates of the formula MTaO4 can be easily prepared by sintering lanthanide sesquioxide M2O3 and tantalum(V) oxide Ta2O5 with sodium chloride as flux. Under these conditions two structure types emerge depending upon the M3+ cationic radius. For M = La – Pr the MTaO4‐type tantalates crystallize in the space group P21/c with lattice constants of a = 762(±1), b = 553(±4), c = 777(±4) pm, β = 101(±1)° and four formula units per unit cell. With M = Nd, Sm – Lu, the monoclinic cell dimensions (space group P2/c) shrink to the lattice constants like a = 516(±9), b = 551(±9), c = 534(±9) pm, β = 96.5(±0.3)° and there are only two formula units present. Both structures show a coordination sphere of eight oxygen atoms for the lanthanide trications shaped as distorted square antiprism for the structure with the larger lanthanides (in the following referred to as A‐type) and as trigonal dodecahedron for the structure with the smaller ones (called as B‐type in the following). The coordination environment about the Ta5+ cations can be described as a slightly distorted octahedron (CN = 6) for the A‐type structure of MTaO4 and a heavily distorted one (CN = 6) for the B‐type. The difference between the two types results from the interconnection of these [TaO6]7? octahedra. Whereas they are connected via four vertices to form corrugated layers according to parallel the bc‐plane in the A‐type, the octahedra of the B‐type MTaO4 structure share edges to built up zig‐zag chains along the c axis. 相似文献
46.
Williamson PT Verhoeven A Ernst M Meier BH 《Journal of the American Chemical Society》2003,125(9):2718-2722
Rotational-resonance magic-angle spinning NMR experiments are frequently used to measure dipolar couplings and to determine internuclear distances. So far most measurements were performed on samples containing isolated spin pairs. Thus, extensive structure elucidation, for example in biomolecules, requires the preparation of a whole set of doubly labeled samples. Here, we describe the analysis of the rotational-resonance polarization-exchange curves obtained from a single, uniformly labeled sample. It is shown experimentally that, at a magnetic field of 14.09 T, the rotational-resonance conditions in uniformly (13)C-labeled threonine are sufficiently narrow to permit the measurement of five distances between the four carbon spins with an accuracy of better than 10%. The polarization-exchange curves are analyzed using a modified two-spin model consisting of the two active spins. The modified model includes an additional offset in the final polarization, which comes from the coupling to the additional, passive, spins. The validity of this approach is experimentally verified for uniformly (13)C-labeled threonine. The broader applicability of such a model is demonstrated by numerical simulations which quantify the errors as a function of the most relevant parameters in the spin system. 相似文献
47.
Summary Phenoxy acid herbicides have been determined by use of high-pressure liquid chromatography (HPLC). The separation efficiency of several stationary phases like octadecyl silica and nitril silica has been estimated using different mobile phases. Regarding the necessary experimental conditions for separating phenoxy acids, the enrichment phases for on-line operation have been tested by use of column switching. The method of sample enrichment is described and the enrichment factors have been calculated.
Bestimmung von Phenoxycarbonsäure-Herbiciden durch HPLC und On-line-AnreicherungI. Möglichkeiten der chromatographischen Trennung durch HPLC unter besonderer Berücksichtigung der On-line-Anreicherung der Herbicid-Verbindungen相似文献
48.
Vass S Grimm H Bányai I Meier G Gilányi T 《The journal of physical chemistry. B》2005,109(24):11870-11874
Slowly diffusing water molecules were found by quasi-elastic neutron scattering (QENS) in a sodium dodecyl sulfate (SDS) micellar solution, and both their diffusion coefficient (4.33 x 10(-6) cm2 x s(-1)) and mole fraction (0.057) were determined. After successfully checking the mean slowing down of solvent molecules by the gradient compensated stimulated spin-echo (GCSTE) pulse sequence NMR method, a similar effect was observed with this technique in the solvent phase of dodecyl trimethylammonium bromide (DTAB) and differing chain length (X = 12, 20, 30, and 40) ethoxylated nonyl phenol (9NX) micellar systems. Following the literature, the experimental results are qualitatively explained by assuming that, apart from ionic hydration, H-bonds may form between the solvent molecules and the O or N atoms present in the hydrophilic (head)groups of the micelle-forming monomers. 相似文献
49.
Ernst M Detken A Böckmann A Meier BH 《Journal of the American Chemical Society》2003,125(51):15807-15810
Proteins are not always available in amounts desirable for solid-state magic-angle spinning (MAS) nuclear-magnetic resonance (NMR) spectroscopy. To maximize the signal-to-noise ratio achievable with small samples, the filling factor must be optimized by using small-diameter MAS rotors. These rotors have the added benefit of allowing higher radio frequency field amplitudes during polarization transfer steps and during decoupling periods as well as allowing higher spinning frequencies. We demonstrate the advantages of relatively fast MAS (30 kHz using a 2.5 mm rotor) compared to MAS at 12 kHz for the 10.4 kDa model protein Crh with 93 residues and show that the signal-to-noise ratio in two-dimensional correlation spectra can be significantly improved by taking advantage of optimized pulse sequences available with rapid MAS. 相似文献
50.
Victoria V. Lipson Irina V. Ignatenko Sergey M. Desenko Svetlana V. Shishkina Oleg V. Shishkin Sergey A. Komykhov Natalya V. Logvinenko Valery D. Orlov Herbert Meier 《Journal of heterocyclic chemistry》2003,40(6):1081-1086
The reaction of 5,7‐diphenyl‐4,7‐dihydro‐1,2,4‐triazolo[1,5‐a]pyrimidine ( 1 ) with α,β‐unsaturated carbonyl compounds 2a‐f led to the formation of the alkylated heterocycles 3a‐f (Figure 1). However, the reaction of 5‐methyl‐7‐phenyl‐4,7‐dihydro‐1,2,4‐triazolo[1,5‐a]pyrimidine ( 5 ) with 2a‐c yielded under the same conditions the triazolo[5,1‐b]quinazolines 6a‐c (Figure 3). In this case, the alkylation is followed by a cyclocondensation. The structure elucidation of the products is based on ir, ms, 1H and 13C nmr measurements and on an X‐ray diffraction study. 相似文献