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91.
New perthiophosphonic acid anhydrides (R-PS2)n with R = 2,4,6-tri-iso-propylphenyl and 2,4-di-tert-butyl-6-methylphenyl have been prepared. For the latter case, the isolation of the monomer (n = 1), the 2,4-di-tertbutyl-6-methylphenyl-dithioxophosphorane, and both dimers (n = 2), cis- and trans-2,4-bis(2,4-di-tertbutyl-6-methylphenyl)-2, 4-dithioxo-1, 3, 2, 4-dithiadiphosphetane, has been determined. The new cis and trans dithiadiphosphetanes and dithioxophosphoranes as well as the known compounds (2,4,6-trimethylphenyl-PS2)2 and 2,4,6-tri-tert-butylphenyl-PS2 are characterized by solution and high resolution solid-state 31P and 13C NMR spectroscopy. The existence of a dimer-monomer equilibrium is directly proved by 2D exchange 31P NMR spectroscopy. It is shown that the reaction of the monomer with methanol is faster than the reaction of the dimer with methanol. 相似文献
92.
Fabio RD Alvaro G Bertani B Donati D Giacobbe S Marchioro C Palma C Lynn SM 《The Journal of organic chemistry》2002,67(21):7319-7328
To prepare in multigram scale new antagonists of the glycine binding site associated to the NMDA receptor, an efficient distereoselective route was set up. The addition of suitable allyltin reagents to chiral N-aryl alpha-imino esters (R-(+)-tert-butyl lactate used as chiral auxiliary), gave the corresponding alpha amino acid-type derivative in high chemical yield and optical purity. This allylation reaction represents a novel example of efficient long-range stereodifferentiation process. In the last part of the synthesis, a regioselective Heck-type cyclization reaction enabled preparation of the target tetrasubstituted exocycle and trisubtituted endocycle double bond derivatives. 相似文献
93.
94.
Prof. M. J. Beckmann 《International Journal of Game Theory》1973,2(1):263-268
Equilibrium in spatial markets has been studied as a problem of monopolistic competition. The typical assumption is that firms charge mill prices and believe that other firms either keep prices constant or preserve their market areas. In this paper somewhat different assumptions are made. First, a system of delivered or c.i.f. prices is assumed. A second model considers different production costs. A third model examines the case of more than two firms, but only for the special case where all firms are spaced at equal distance. Throughout this paper it is assumed that collusion is illegal, the game is a noncooperative one. 相似文献
95.
96.
The mean residence times τ of lithium particles on oxygen covered tungsten surfaces were measured accurately over a wide temperature range (1200 < T < 1900 K) by the beam modulation technique with a lock-in analyzer. A predominant monocrystalline W(100) structure was obtained by recrystallization of a polycrystalline tungsten ribbon. The residence time was determined as a function of the oxygen coverage θ and the temperatureT of the surface. The desorption energy l and the preexponential factor τ0, calculated from the Arrhenius equation, are not only dependent on the amount of adsorbed oxygen but also on the oxygen structure. Apparently the desorption parameters l and τ0 are correlated. An increasing desorption energy l is connected to a decreasing “vibration period” τ0 whereby the influence on the residence time is partly compensated. 相似文献
97.
We have measured the nonlocal resistance of aluminum-iron spin-valve structures fabricated by e-beam lithography and shadow evaporation. The sample geometry consists of an aluminum bar with two or more ferromagnetic wires forming point contacts to the aluminum at varying distances from each other. In the normal state of aluminum, we observe a spin-valve signal which allows us to control the relative orientation of the magnetizations of the ferromagnetic contacts. In the superconducting state, at low temperatures and excitation voltages well below the gap, we observe a spin-dependent nonlocal resistance which decays on a smaller length scale than the normal-state spin-valve signal. The sign, magnitude, and decay length of this signal are consistent with predictions made for crossed Andreev reflection. 相似文献
98.
Sol-Gel Synthesis of Transparent Alumina Gel and Pure Gamma Alumina by Urea Hydrolysis of Aluminum Nitrate 总被引:2,自引:0,他引:2
Maria I. F. Macêdo Carla C. Osawa Celso A. Bertran 《Journal of Sol-Gel Science and Technology》2004,30(3):135-140
-Alumina was synthesized by a sol-gel method with the aluminum ion hydrolysis control performed by urea. The initial saturated Al3+/urea solution presented urea coordinated with the aluminum ion, as shown in the 13C NMR and 27Al NMR spectra and longitudinal relaxation times, T
1, from the latter. The substitution of water molecules in the Al3+ coordination shell by urea controlled the hydrolysis process and provided an extensive nucleation during the initial steps of the aluminum hydroxide formation due to urea thermolysis at 90°C. The resulting sol, composed of Al(OH)3 nanoparticles, coalesced and became a transparent gel permeated by a solution of urea and the polycation ion [Al13O4(OH)24(H2O)12]7+. The freshly prepared gel was transformed, under heating at 300°C, directly to -alumina, characterized by FTIR, 27Al-MAS-NMR and SBET techniques, without - or -phases, as a consequence of the high degree of homogeneity of the -alumina precursor. 相似文献
99.
Ferreri C Samadi A Sassatelli F Landi L Chatgilialoglu C 《Journal of the American Chemical Society》2004,126(4):1063-1072
Trans unsaturated fatty acids in humans may be originated by two different contributions. The exogenous track is due to dietary supplementation of trans fats and the endogenous path deals with free-radical-catalyzed cis-trans isomerization of fatty acids. Arachidonic acid residue (5c,8c,11c,14c-20:4), which has only two out of the four double bonds deriving from the diet, was used to differentiate the two paths and to assess the importance of a radical reaction. A detailed study on the formation of trans phospholipids catalyzed by the HOCH2CH2S* radical was carried out on L-alpha-phosphatidylcholine from egg lecithin and 1-stearoyl-2-arachidonoyl-L-alpha-phosphatidylcholine (SAPC) in homogeneous solution or in large unilamellar vesicles (LUVET). Thiyl radicals were generated from the corresponding thiol by either gamma-irradiation or UV photolysis, and the reaction course was followed by GC, Ag/TLC, and 13C NMR analyses. The isomerization was found to be independent of cis double bond location (random process) in i-PrOH solution. In the case of vesicles, the supramolecular organization of lipids produced a dramatic change of the isomerization outcome: (i) in egg lecithin, the reactivity of arachidonate moieties is higher than that of oleate and linoleate residues, (ii) in the linoleate residues of egg lecithin, the 9t,12c-18:2 isomer prevailed on the 9c,12t-18:2 isomer (3:1 ratio), and (iii) a regioselective isomerization of SAPC arachidonate residues occurred in the 5 and 8 positions. This effect of "positional preference" indicates that thiyl radicals entering the hydrophobic region of the membrane bilayer start to isomerize polyunsaturated fatty acid residues having the double bonds nearest to the membrane surfaces. We propose that arachidonic acid and its trans isomers can function as biomarkers in membranes for distinguishing the two trans fatty acid-forming pathways. 相似文献
100.
Rodolphe AbdayemGraziano Baccolini Carla BogaMagda Monari Simona Selva 《Tetrahedron letters》2003,44(13):2649-2653
The reaction between 4-nitroalkoxybenzenes 7 and PCl3/AlCl3, when carried out in appropriate molar ratio, gives a prevalent formation of diazenes 8 and 2,7-dialkoxyphenazines 9 with their new chlorinated derivatives 10-13. These compounds are obtained, in satisfactory yield, in a one-pot procedure, in mild conditions, from commercially available and safe starting materials. In this reaction both the reagents PCl3 and AlCl3 play a fundamental role in obtaining the products, and this method might be applicable to other 4-alkoxynitrobenzenes. 相似文献