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排序方式: 共有3279条查询结果,搜索用时 15 毫秒
1.
Pan Huang Dawen Xu Robert M. Reich Felix Kaiser Boping Liu Fritz E. Kühn 《Tetrahedron letters》2019,60(24):1574-1577
A first example of an Et2Zn mediated silylation of 1-aklynes is reported. A series of functional groups are tolerated in this reaction. Mechanistic studies support Zn alkynilides as intermediates in the reaction. This reaction protocol provides a practical method for the preparation of alkynylsilanes and expands the application of organometallic zinc in organic synthesis. 相似文献
2.
Marcel Oliver 《Theoretical and Computational Fluid Dynamics》1997,9(3-4):311-324
The so-called lake equations arise as the shallow-water limit of the rigid-lid equations—three-dimensional Euler equations
with a rigid-lid upper boundary condition—in a horizontally periodic basin with bottom topography. We prove an a priori estimate in the Sobolev space H
m
for m≥ 3 which shows that a solution to the rigid-lid equations can be approximated by a solution of the lake equations for an
interval of time which can be estimated in terms of the initial deviation from a columnar configuration and the magnitude
of the initial data in H
m
, the gradient of the bottom topography in H
m+1
, and the aspect ratio of the basin. In particular, any solution to the lake equations remains close to some solution of the
rigid-lid equations for an interval of time that can be made arbitrarily large by choosing the aspect ratio of the basin small.
Received 10 October 1996 and accepted 15 May 1997 相似文献
3.
Marcel Crabbé 《Comptes Rendus Mathematique》2004,338(6):433-436
We investigate the notion of substitution in an abstract way, without defining it explicitly. We single out the essential features of the operation of performing a substitution in order to define a concept of substitutive structure, called logos. We then prove a completeness theorem making precise and justifying the intuition that formulas true for the usual substitution can be proved from the logos axioms only. To cite this article: M. Crabbé, C. R. Acad. Sci. Paris, Ser. I 338 (2004). 相似文献
4.
Marcel Erné 《Order》1991,8(2):159-173
We introduce a special type of order-preserving maps between quasiordered sets, the so-called cut-stable maps. These form the largest morphism class such that the corresponding category of quasiordered sets contains the category of complete lattices and complete homomorphisms as a full reflective subcategory, the reflector being given by the Dedekind-MacNeille completion (alias normal completion or completion by cuts). Suitable restriction of the object class leads to the category of separated quasiordered sets and its full reflective subcategory of completely distributive lattices. Similar reflections are obtained for continuous lattices, algebraic lattices, etc. 相似文献
5.
Eggenhffner R. Masini R. Bellingeri E. Cimberle M. R. Ferdeghini C. Costa G. A. Kaiser E. Signorelli E. 《Il Nuovo Cimento D》1994,16(10):1659-1664
Il Nuovo Cimento D - The formation of Hg-based HTSC materials was investigated through the development of non-conventional techniques. The former was based on a singlestep synthesis at temperatures... 相似文献
6.
Data about the existence of native HO? CH2? CH2? OH (MEG) in natural wines and champagne has led to delicate questions because of legal decisions already made to declare wine or champagne illegal for sale if the MEG concentrations found are above 10 mg/liter. Action has been taken because of the DEG (diethylene glycol) disaster in European wines of 1985…86. An incorrect legal decision was made due to the belief that MEG cannot be produced biochemically by grape vines. A further reason may be lack of correct data on native MEG trace concentrations, as a result of the special behavior of this diol. As first member of a homologous series whose higher members (C4) are normally found in all wines at quite high concentration levels, MEG shows extremely adsorptive behavior. The solution of the chromatographic problems is summarized in this paper. MEG concentrations in Riesling as example are in the range of 2 to 6 mg/liter and can easily be increased by biotechnological steps to a level of around 10 to 60 mg/liter. This is again due to the specific adsorptive behavior of MEG, which can be enriched on filter surfaces and displaced when the wine acidity changes with changing types. In order to control and guarantee the qualitative and quantitative results of MEG analyses we used a combination of Deans heart cutting on-line with Multi-Chromatography. It was easy to produce false data by many otherwise useful single column or two-dimensional separation processes, etc. 相似文献
7.
The syntheses of two rigid organic molecular rods bearing a nitronyl-nitroxide radical and a terminal nitrogen-based functionality like a pyridine or a cyano group are reported. Both new paramagnetic molecules are fully characterized, including crystal structure analysis. Furthermore their magnetic behaviours in the crystalline state are investigated and their spin concentration corroborate their excellent purity. While the pyridine functionalized rod is synthesized by converting the corresponding benzaldehyde to the phenyl-nitronyl-nitroxide radical, the synthesis of the cyano functionalized rod demonstrates the accessibility of highly sophisticated spin-labelled molecules via cross-coupling reaction with a meta-iodo-phenyl-nitronyl-nitroxide moiety. 相似文献
8.
9.
Marek Pękała Jan Mucha Benedicte Vertruyen Rudi Cloots Marcel Ausloos 《Journal of magnetism and magnetic materials》2006
Samples of La0.7Ca0.3Mn1−xGaxO3 with x=0, 0.025, 0.05 and 0.10 were prepared by standard solid-state reaction. They were first characterized chemically, including the microstructure. The magnetic properties and various transport properties, i.e. the electrical resistivity, magnetoresistivity (for a field below 8 T), thermoelectric power and thermal conductivity measured each time on the same sample, are reported. The markedly different behaviour of the x=0.1 sample from those with a smaller Ga content, is discussed. The dilution of the Mn3+/Mn4+ interactions with Ga doping considerably reduces the ferromagnetic double exchange interaction within the manganese lattice leading to a decrease of the Curie temperature. The polaron binding energy varies from 224 to 243 meV with increased Ga doping. 相似文献
10.
The 29th PME research forum on theories included only one European perspective on mathematics education. In order to convey trends in theory usage in Europe we compile, survey and analyze a large subset of the research papers from the 4th European Congress on Mathematics Education (CERME4). That is, this paper includes a discussion of trends seen within CERME4 reports1 on theory usage by European researchers inseven of the fourteen working groups and (a) Outlines similarities and differences in theory usage and (b) takes a futuristic stance on ways in which researchers from different traditions could understand each other. Such an enterprise would further Hans-Georg Steiner's vision of bridging theoretical traditions which are independently formulated in different regions of the world. 相似文献