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51.
Vyskocil S Meca L Tislerová I Císarová I Polásek M Harutyunyan SR Belokon YN Stead RM Farrugia L Lockhart SC Mitchell WL Kocovský P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(20):4633-4648
The title binaphthyls 19 and 26, which are the positional isomers of 2-methoxy-2'-(diphenylphosphino)-1,1'-binaphthyl (MOP, 19) and 2-amino-2'-hydroxy-1,1'-binaphthyl (NOBIN, 26), have been synthesized by Suzuki coupling as the key step (10 + 15-->18), followed by functional group transformations, involving C-P and C-N bond formation (18-->19 and 18-->23). Racemic intermediate 22 was resolved by co-crystallization with N-benzylcinchonidinium chloride and the absolute configuration determined by X-ray crystallography. These novel binaphthyls are configurationally stable and, as such, potentially usable as chiral ligands in asymmetric reactions. Michael addition of the glycine-derived enolate 40 to methyl acrylate, carried out in the presence of (R)-(-)-27 as the chiral phase-transfer catalyst, afforded L-glutamic acid (S)-(+)-43 of 92% ee (after hydrolysis of the primary product). 相似文献
52.
Thermodynamical approach to the traveling salesman problem: An efficient simulation algorithm 总被引:9,自引:0,他引:9
V. Černý 《Journal of Optimization Theory and Applications》1985,45(1):41-51
We present a Monte Carlo algorithm to find approximate solutions of the traveling salesman problem. The algorithm generates randomly the permutations of the stations of the traveling salesman trip, with probability depending on the length of the corresponding route. Reasoning by analogy with statistical thermodynamics, we use the probability given by the Boltzmann-Gibbs distribution. Surprisingly enough, using this simple algorithm, one can get very close to the optimal solution of the problem or even find the true optimum. We demonstrate this on several examples.We conjecture that the analogy with thermodynamics can offer a new insight into optimization problems and can suggest efficient algorithms for solving them.The author acknowledges stimulating discussions with J. Piút concerning the main ideas of the present paper. The author is also indebted to P. Brunovský, J. erný, M. Hamala, . Peko, . Znám, and R. Zajac for useful comments. 相似文献
53.
54.
The image of growth striations in Si formed by the double crystal X-ray topography in the Laue case (DCTL) is investigated. The results of the dynamical diffraction theory for crystals with small microdefects have been compared with the contrast behaviour determined experimentally. It has been found that the theory explains qualitatively the contrast on the striations and it has been demonstrated that the DCTL method is suitable for detection of the presence of small microdefects in growth striations. Since the paper completes a series of papers on X-ray topography of growth striations, some general conclusions are formulated concerning the applicability of the X-ray topographical methods to the investigation of the structure of the growth striations 相似文献
55.
We determine the electronic structure of a semiinfinite crystal with a general coverage by adatoms, if there exists a partial order described by the long-range order parameter5. The tight-binding formalism is used, the presence of the surface is included via the surface Green function method, and the effect of disorder is described by a version of the coherent potential approximation. The local densities of states at/near the surface as well as the interaction energy of adatoms are calculated as functions of the coverage, and the long-range order parameterS. The numerical examples modelling the cases of a gas adsorption on the transition metal surfaces as well as the transition metal atoms on the transition metal surfaces, are investigated. 相似文献
56.
Kucera R Zizkovský V Sochor J Klimes J Dohnal J 《Journal of separation science》2005,28(12):1307-1314
Zirconia-based stationary phases represent an interesting alternative to silica-based materials. Two zirconia-based stationary phases were studied as an option for use in drug analysis. The different properties of zirconia material, distinct from RP silica-columns, were employed for the development of a novel and rapid stability monitoring HPLC method. This method enables simultaneous control of possible degradation processes of active substance (ibuprofen) as well as antimicrobial excipients (methyl-and propylparaben). The separation of ibuprofen, its two main degradation products 2-(4-isobutyrylphenyl)propionic acid and 4-isobutylacetophenone, parabens, and 4-hydroxybenzoic acid as their degradation product was successfully accomplished on a Zr-CarbonC18 column using a mobile phase consisting of acetonitrile-phosphate buffer (pH 4.8)-propan-2-ol (27:56:17, v/v/v). Detection was performed at 258 nm and the analysis was completed within 17 minutes. 相似文献
57.
Wallace WT Wyrwas RB Whetten RL Mitrić R Bonacić-Koutecký V 《Journal of the American Chemical Society》2003,125(27):8408-8414
The discovery that supported gold clusters act as highly efficient catalysts for low-temperature oxidation reactions has led to a great deal of work aimed at understanding the origins of the catalytic activity. Several studies have shown that the presence of trace moisture is required for the catalysts to function. Using near-atmospheric pressure flow reactor techniques, we have studied humidity and temperature effects on the reactivity of gas-phase gold cluster anions with O2. Near room temperature, the humid source produces abundant gold-hydroxy cluster anions, Au(N)OH(-), and these have a reversed O2 adsorption activity: Nonreactive bare gold clusters become active when in the form Au(N)OH(-), while active bare clusters are inactive when -OH is bound. The binding energies for the stable structures obtained from density functional calculations confirm fully these findings. Moreover, the theory provides evidence that electron-transfer induced by the binding of a OH group enhances the reactivity toward molecular oxygen for odd anionic gold clusters and suppresses the reactivity for the even ones. The temperature dependence of O2 addition to Au(3)OH(-) and Au(4)(-) indicates deviations from equilibrium control at temperatures below room temperature. The effects of humidity on gold cluster adsorption activity support the conclusion drawn for the mechanism of O2 adsorption on "dry" gold cluster anions and provides insight into the possible role of water in the enhanced activity of supported gold cluster catalysts. 相似文献
58.
A potentiometric reductimetric method for the determination of platinum (Pt(IV)Pt(II)) with a standard Fe(II) solution in an alkaline medium of mannitol is described. The method, the error of which does not exceed 2%, can be used in the presence of palladium, iridium, and rhodium. 相似文献
59.
P. Vaňura V. Jedináková-Křížová M. Vobecký 《Journal of Radioanalytical and Nuclear Chemistry》2006,267(2):501-503
Summary An extraction technique for the separation of 90Sr from a high excess of90Y has been developed. This procedure can be used for the determination of trace amounts of90Sr in90Y prepared by a radiochemical90Y/90Sr generator by liquid scintillation. 相似文献
60.
Zusammenfassung Die bei 1850, 2000 bzw. 2300°C durch die Reduktion von Scandiumoxid mittels Kohlenstoff in dem der Scandiumdicarbidbildung entsprechenden Molverhältnis hergestellten Produkte wurden mit Wasser zersetzt und mit Hilfe der gleichzeitigen gaschromatographischen und massenspektrometrischen Analyse untersucht. Aus den Ergebnissen geht hervor, daß bei der Scandiumdicarbidbildung gleichzeitig ein weiteres Scandiumcarbid, höchstwahrscheinlich ein Sesquicarbid, entsteht.
Herrn Prof. Dr.H. Nowotny gewidmet.
7. Mitt.: Coll. Czech. Chem. Commun., im Druck. 相似文献
Scandium(III) dicarbide problem. Further experimental results
Products obtained by the reduction of Sc2O3 with carbon at 1850, 2000 and 2300°C, resp., in the molar ratio corresponding to the scandium dicarbide were hydrolysed with water. Gaseous reaction products were analysed using gas chromatography combined with mass spectrometry. Results show, that during dicarbide formation another scandium carbide, probably sesquicarbide, is also formed.
Herrn Prof. Dr.H. Nowotny gewidmet.
7. Mitt.: Coll. Czech. Chem. Commun., im Druck. 相似文献