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排序方式: 共有679条查询结果,搜索用时 15 毫秒
661.
662.
R. F. Rodríguez J. A. Reyes J. Fujioka E. Cortés J. A. Olivares F. L.S. Cuppo 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(4):411-418
We investigate the effects produced on the diffraction pattern of a dyed
nematic thin film under the action of an optical field and a low frequency
AC electric field. For a homeotropically aligned mixture of the nematic E7
doped with a dichroic dye, a sequence of dynamical regimes of the far field
diffraction pattern is observed. For specific values of the beam's power,
frequency and amplitude of the AC field, a uniform steady rotational
motion (SR) of the pattern sets in with a measured angular velocity
νexp =2.58 Hz. To account for this and other observed features of the
diffraction pattern an analytical model is proposed. This allows us to
describe quantitatively the reorientation of the film, to calculate some
specific structural features of the diffraction pattern, as well as its
angular velocity. We find that the predicted angular velocity νtheor=5.7 Hz, is in quite good agreement with the measured value. 相似文献
663.
Sandra Rodriguez Uxue Uria Efraim Reyes Luisa Carrillo Toms Tejero Pedro Merino Jose L. Vicario 《Angewandte Chemie (International ed. in English)》2020,59(17):6780-6784
The enantioselective synthesis of tropanols has been accomplished through chiral phosphoric acid catalyzed pseudotransannular ring opening of 1‐aminocyclohept‐4‐ene‐derived epoxides. The reaction proceeds together with the desymmetrization of the starting material and leads to the direct formation of the 8‐azabicyclo[3.2.1]octane scaffold with excellent stereoselectivity. The synthetic applicability of the reaction was demonstrated by the enantioselective synthesis of the two natural products (?)‐α‐tropanol and (+)‐ferruginine. 相似文献
664.
Sandra Rodriguez Uxue Uria Efraim Reyes Luisa Carrillo Tomás Tejero Pedro Merino Prof. Dr. Jose L. Vicario 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(17):6846-6850
The enantioselective synthesis of tropanols has been accomplished through chiral phosphoric acid catalyzed pseudotransannular ring opening of 1-aminocyclohept-4-ene-derived epoxides. The reaction proceeds together with the desymmetrization of the starting material and leads to the direct formation of the 8-azabicyclo[3.2.1]octane scaffold with excellent stereoselectivity. The synthetic applicability of the reaction was demonstrated by the enantioselective synthesis of the two natural products (−)-α-tropanol and (+)-ferruginine. 相似文献
665.
We investigate the Pauli energy in atoms and molecules as a measure of electron localisation. Our results indicate that the Pauli energy has an exponential dependence on the number of localised electrons. This relationship yields to a kinetic energy density expression that depends on the electron density ρ(r) and the pair density ρ2(r, r′). The proposed equation shows certain advantages over a similar orbital-free kinetic energy functional recently proposed by Delle Site and co-workers. The methodology introduced here is a novel approach for exploring electronic quantities with a partition scheme that might be useful for research in density functional theory. 相似文献
666.
Cantillo D Ávalos M Babiano R Cintas P Jiménez JL Palacios JC 《Organic & biomolecular chemistry》2011,9(8):2952-2958
The archetypal Cu(I)-catalyzed alkyne-azide click cycloaddition (CuAAC) has been explored thoroughly via density functional calculations, modeling copper nuclei with the LANL2DZ basis set and aqueous environments with CPCM solvation. All the mechanistic proposals, ranging from the intermediacy of copper acetylides to π-complexes and multinuclear clusters have been compared. The known features of the CuAAC reaction such as the observed second order kinetics for the Cu(I) species and the marked regioselectivity have been taken into account. The calculated energy barriers point to the intermediacy of copper(i) acetylides with two metal centers, in agreement with the observed kinetics, which exhibit barriers of 10.1 kcal mol(-1) and 13.7 kcal mol(-1) for the 1,4- and 1,5-regiochemistries, respectively, thus accounting for the marked regioselectivity of the copper catalyzed azide-alkyne cycloaddition. The copper acetylide versus π-complexes dilemma has also been experimentally addressed through the click reaction of benzyl azide and isotopically labeled phenylacetylene. The total proton/deuterium exchange in the afforded triazole demonstrates the formation of a copper acetylide intermediate during the transformation. 相似文献
667.
Martínez RF Ávalos M Babiano R Cintas P Jiménez JL Palacios JC Pérez EM 《Organic & biomolecular chemistry》2011,9(9):3279-3289
The conformational population of Z and E isomers of the amide bond in N-acetyl oxazolidines is dictated by the electronic nature of the vicinal aryl ring. Experimental and theoretical data support a rationale based on a strong and stereodirecting charge-charge interaction that should be added to the arsenal of non-covalent interactions and whose influence can be more important than once thought. 相似文献
668.
669.
The Newton diagram and, in particular, the lowest-degree quasi-homogeneous terms of an analytic planar vector field allow us to determine the existence of characteristic orbits and separatrices of an isolated singular point. We give an easy algorithm for obtaining the local phase portrait near the origin of a bi-dimensional differential system and we provide several examples. 相似文献
670.
We study a one-dimensional twisted photonic liquid crystal (TPLC), consisting of various nematic liquid crystal cells adopting a twisted configuration intercalated by isotropic dielectric layers, submitted to a dc electric field (Edc ) aligned along the periodicity axis. We write the corresponding Euler–Lagrange equations describing the nematic layer configuration. By assuming arbitrary anchoring quasi-planar boundary conditions, we calculate the equilibrium textures for the nematic, parametrized by the two types of strength of its interaction (polar and azimuthal) with the plane walls. We write the electromagnetic equations in a 4?×?4 matrix representation and using the transfer matrix formalism, we obtain the transmittance and reflectance coefficients for normal incidence as functions of the external electric field and anchoring strengths. We have observed a remarkable dependence of the electric field on the transmission and reflection spectra in opening and closing band gaps. 相似文献