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951.
Cardillo G Fabbroni S Gentilucci L Perciaccante R Piccinelli F Tolomelli A 《Organic letters》2005,7(4):533-536
The stereoselective anti SN2' attack of NaN3 to 3-alkenyl-3-bromo-azetidin-2-ones gave a mixture of diastereomeric azides in fast equilibrium. The [3,3]-sigmatropic rearrangement of allylic azides occurred with complete stereocontrol, allowing the equilibrium to be directed preferentially toward the (E)- or (Z)-isomer, useful precursors of 3(2'-amino)-beta-lactams. [reaction: see text] 相似文献
952.
Bertolini D Cinacchi G De Gaetani L Tani A 《The journal of physical chemistry. B》2005,109(51):24480-24488
We report a molecular dynamics simulation study on the isotropic phase of an idealized calamitic liquid crystal model with a length-to-width ratio of approximately 5-6. The study focuses on the characterization of single-particle and collective orientational dynamics on approaching the phase transition to the nematic phase. Recent experimental and simulation works have suggested that a power law behavior exists at relatively short times in the decay of the time derivative of the orientational correlation functions. Qualitatively, our simulation data are consistent with these findings. Both single-particle and collective time correlation function derivatives possess, in their respective log-log plots, a linear region at very short times, whose slope is essentially independent from the thermodynamic state. Nevertheless, the single-particle orientational correlation functions are better described by a function which is the sum of a fast exponential, an intermediate stretched-exponential and a slow exponential, while the collective orientational correlation functions are satisfactorily described by a sum of two exponentials, at higher density, or by just one exponential, at lower density. 相似文献
953.
Luca Bernazzani Sergio Cabani Giovanni Conti Vincenzo Mollica 《Journal of solution chemistry》2000,29(4):389-404
Limiting enthalpies of solution solnH°(oct/w) of several monofunctionalcompounds (tetrahydropyran, diethylamine, pyrrolidine, piperidine,1-methylpiperidine) in water–octan-1-ol mixtures with water content ranging from zero tosaturation have been determined at 25°C. The observed phenomenology hasbeen interpreted by supposing that the solvent medium undergoes a microphasetransition at water mole fraction of about 0.08. In the water-poor region theexperimental behavior is consistent with the presence of water molecules inprevailingly monodispersed form, while in the water-rich region the formationof organized pseudomicellar water aggregates is suggested. Two different simplemodels have also been proposed to describe quantitatively the observed trendsof solnH°(oct/w) vs. mole fraction of water.Deceased while this research was in progress. The coauthors wish to dedicate this work to his memory 相似文献
954.
Florian Luca 《Rendiconti del Circolo Matematico di Palermo》2000,49(2):313-318
In this note, we show that the classical Fibonacci and Lucas sequence do not contain any perfect number. 相似文献
955.
We report electroreflectance spectra from a GaAs-Au Schottky barrier interface, in the region of the fundamental energy gap, at a temperature of ~ 1.8 K. Both the ground exciton level and the continuum of states are investigated separately, by operating in the small modulation and in the on-off field limit, respectively. The field profile is monitored by I–V, C-V, and photovoltage measurements. It is found that, even at ~ 1.8K, a high surface field is generally present. This moves the actual reflecting boundary of the exciton-polariton away from the surface and affects the ER lineshape through field-induced inhomogeneity effects and interference across the high field layer below the surface. The spectra corresponding to the states of the continuum are discussed in terms of the recent calculations of photon-assisted tunneling with coulomb interaction. 相似文献
956.
Functionalized 1-nitrohydrazones 5 react with sodium hydride in boiling benzene to afford different types of ring-closed products. The formation of the latter can be ascribed to competitive modes of evolution of first-formed nitrile imines. 相似文献
957.
958.
N Luca 《Journal of Mathematical Analysis and Applications》1979,67(2):323-339
Various stability types, analogous to those in the theory of ordinary differential equations, are defined for some classes of integrodifferential systems of equations with infinite delays. Then, on the basis of a variation of constants formula and some results in functional analysis and Laplace transform theory, some necessary conditions and necessary and sufficient conditions are established in order that the trivial solution of a class of systems may be classified as one of the stability types considered in the definitions. 相似文献
959.
M. Capizzi F. Evangelisti P. Fiorini A. Frova F. Patella 《Solid State Communications》1977,24(12):801-803
Wavelenght modulation spectra of the 1s indirect exciton in GaP are reported. Four lines are found which can be easily explained taking into account the “camel's back” structure of the conduction band. An exciton splitting of 0.9 meV- ±0.2 has been measured. 相似文献
960.
A novel pentacyclic saturated sesterpene condensed with hydroxyhydroquinone moiety, which we named disidein (1), has been isolated from the marine sponge Disidea pallescens as disulfate sodium calcium salt. A minor accompaining compound, which seems a likely precursor of disidein, has been identified as 2-pentaprenylbenzoquinone (7). 相似文献