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941.
The syntheses of new chiral cyclic 1,2-diacetals from (2R, 3R)-( )-tartaric acid are described. C(2)-symmetrical diamines were prepared via direct amidation of the tartrate or from the corresponding bismesylate via reaction with sodium azide. For C1-symmetrical compounds, the Appel reaction was used to form the key intermediate, a monochlorocarbinol, from the diol. Some of the new chiral compounds, produced in good to high yields, may be potentially useful as asymmetric organocatalysts or as nitrogen and sulfur chelating ligands for asymmetric metal catalyzed reactions. Thus, a bis-N-methyl-methanamine derivative, used in substoichiometric amounts, was found to catalyze the enantioselective addition of cyclohexanone to (E)-beta-nitrostyrene with high diastereoselectivity (syn / anti = 92:8), albeit giving moderate optical purity (syn: 30 %). 相似文献
942.
943.
On the basis of the irradiance-moment formalism for describing general partially coherent beams, we investigate the relation between the spatial orientation of the transverse beam profile upon propagation and the asymmetrical part of its orbital angular momentum (OAM). More specifically, a necessary and sufficient condition (one-to-one correspondence) is shown between freely propagating non-rotating beams and vanishing asymmetrical OAM. As a corollary, it is obtained that any beam emerging from optical systems that transform a rotating field into a non-rotating beam exhibits vortex OAM only. 相似文献
944.
Patricia Hurd Teresa Cusati Maurizio Persico 《Journal of computational physics》2010,229(6):2109-2116
Many approximate methods of quantum chemistry yield potential energy surfaces with discontinuities. While clearly unphysical, such features often fall within the typical error bounds of the method, and cannot be easily eliminated. The integration of nuclear trajectories when the potential energy is locally discontinuous is obviously problematic. We propose a method to smooth out the discontinuities that are detected along a trajectory, based on the definition of a continuous function that fits locally the computed potential, and is used to integrate the trajectory across the discontinuity. With this correction, the energy conservation error can be reduced by about one order of magnitude, and a considerable improvement is obtained in the energy distribution among the internal coordinates. 相似文献
945.
M. Salazar-Ramírez D. Martínez V. D. Granados R. D. Mota 《International Journal of Theoretical Physics》2010,49(5):967-973
We apply the Schrödinger factorization to construct the generators of the dynamical algebra su(1,1) for the radial equation of the generalized MICZ-Kepler problem. 相似文献
946.
Martínez-Martín D Jaafar M Pérez R Gómez-Herrero J Asenjo A 《Physical review letters》2010,105(25):257203
In this work we investigate possible ferromagnetic order on the graphite surface by using magnetic force microscopy (MFM). Our data show that the tip-sample interaction along the steps is independent of an external magnetic field. Moreover, by combining kelvin probe force microscopy and MFM, we are able to separate the electrostatic and magnetic interactions along the steps obtaining an upper bound for the magnetic force gradient of 16 μN/m. Our experiments suggest the absence of ferromagnetic signal in graphite at room temperature. 相似文献
947.
M. Díaz L. Martínez E. Román A. Redondo-Cubero M.A. Mu?oz-Márquez G. van der Laan N.D. Telling P. Bailey T.C.Q. Noakes Y. Huttel 《Surface science》2010,604(23-24):2177-2183
The impact of annealing at 300 °C on the elemental composition and the atomic structure of the Co/V interface in the 2.5 Å Co/70 Å V/MgO (100) system has been investigated by medium energy ion scattering (MEIS) using 100 keV He+ ions. By combining the experimental MEIS results with simulations we show that, while the Co/V interface is abrupt for the system kept at room temperature, annealing at 300 °C induces a strong interdiffusion leading to a Co0.5V0.5 surface bcc alloy with a high degree of disorder. Additionally, the MEIS data suggest that the surface of the annealed system is slightly rumpled by ~ 0.2 Å. 相似文献
948.
Hydrogen adsorption in gold nanocontact electrodes in electrochemical solution is experimentally discerned. This is performed with gold nanocontact conductance histograms in an electrochemical environment in which both the electrochemical potential and the electrolyte type are varied. Different salts, acids, and hydrogen peroxide electrolytes are studied. Salts and acids exhibit at negative electrochemical potentials different fractional quantum conductance histograms peaks associated to extra stable structures due to H adsorption while these peaks do not appear for H2O2 where electron transfer between solution and electrodes occurs without hydrogen formation or hydrogen adsorption on the gold electrode. 相似文献
949.
950.
Martín Szyld 《Journal of Pure and Applied Algebra》2018,222(8):2359-2373
We give a definition of weak morphism of T-algebras, for a 2-monad T, with respect to an arbitrary family Ω of 2-cells of the base 2-category. By considering particular choices of Ω, we recover the concepts of lax, pseudo and strict morphisms of T-algebras. We give a general notion of weak limit, and define what it means for such a limit to be compatible with another family of 2-cells. These concepts allow us to prove a limit lifting theorem which unifies and generalizes three different previously known results of 2-dimensional monad theory. Explicitly, by considering the three choices of Ω above our theorem has as corollaries the lifting of oplax (resp. σ, which generalizes lax and pseudo, resp. strict) limits to the 2-categories of lax (resp. pseudo, resp. strict) morphisms of T-algebras. 相似文献