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51.
Jorge Reyes‐González Rosa María Gómez Fernando Cortés‐Guzmán 《Journal of Physical Organic Chemistry》2012,25(3):230-238
This work describes the conformational behavior and the activation mechanism of timoprazole and substituted prazoles from the most stable conformation to the sulphenic acid. The stability of the conformers can be explained by the presence of hydrogen bonds, stereoelectronic effect because of the lone pair of sulfur atom and the N…C and N…S interactions. The first step of the Smile rearrangement is a nucleophilic addition to benzimidazole by pyridine moiety, which depends on the difference of the electron population of the atoms involved in the attack. The second step produces sulphenic acid by a concerted reaction where breaking of the S–C bond goes along with a proton migration, and is determined by the electron population of the sulfur atom. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
52.
53.
José R. Mora Rosa M. Domínguez Armando Herize María Tosta Gabriel Chuchani 《Journal of Physical Organic Chemistry》2008,21(5):359-364
The gas‐phase elimination kinetics of 2,2‐diethoxyethyl amine and 2,2‐diethoxy‐N,N‐diethylethanamine (320–380 °C; 40–150 Torr) in a seasoned reaction vessel are homogeneous, unimolecular and obey a first‐order rate law. These elimination processes involve two parallel reactions. The first gives ethanol and the corresponding 2‐ethoxyethenamine. The latter compound further decomposes to ethylene, CO and the corresponding amine. The second parallel reaction produce ethane and the corresponding ethyl ester of an α‐amino acid. The following Arrhenius expressions are given as: For 2,2‐diethoxyethyl amine For 2,2‐diethoxy‐N,N‐diethylethanamine Comparative kinetic and thermodynamic parameters of the overall, the parallel and the consecutive reactions lead to consider two types of mechanisms in terms of a concerted polar cyclic transition state structures. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
54.
We have investigated in detail the mechanism of infrared emission and upconversion emission of Er3+ in tellurite glass as a function of the dopant concentration. Both the infrared and upconversion emissions are competing processes and the efficiency of infrared emission at 1534 nm is 100% at the lowest Er content (0.5 mol%) and reduces to 50% at higher dopant concentration (>2 mol%). The green upconversion emission at 548 nm is mainly due to the excited state absorption (ESA) from 4I11/2, which populate the 4F7/2 level. In addition to this, the possible energy transfer (ET) through Er3+(4I11/2) + Er3+(4I11/2) → Er3+(4F7/2) + Er3+(4I15/2) can also results in the green emission as is noticed from the concentration dependent efficiency change of the green emission. The fluorescence quenching of green emission with Er concentration may be related with the cross relaxation (CR) process 2H11/2 + 4I15/2 → 4I9/2 + 4I13/2. The red emission is due to the combined effect of the ESA from level 4I13/2 to 4F9/2, the energy transfer process described by Er3+(4I13/2) + Er3+(4I11/2) → Er3+(4F9/2) + Er3+(4I15/2) and the cross relaxation process. 相似文献
55.
A three-core polarization splitter based on a square-lattice photonic-crystal fiber is presented. The component separates the input field into two orthogonally polarized beams that are coupled to the horizontal and vertical output ports. The splitter has been designed through modal and beam propagation analysis by employing high-performance codes based on the finite-element method. Results obtained for a device length of 20 mm show extinction ratios as low as -23 dB with bandwidths as great as 90 nm. 相似文献
56.
José Edgar Madriz Aguilar Carlos Romero Adriano Barros 《General Relativity and Gravitation》2008,40(1):117-130
We investigate, in the context of five-dimensional (5D) Brans–Dicke theory of gravity, the idea that macroscopic matter configurations
can be generated from pure vacuum in five dimensions, an approach first proposed by Wesson and collaborators in the framework
of 5D general relativity. We show that the 5D Brans–Dicke vacuum equations when reduced to four dimensions (4D) lead to a
modified version of Brans–Dicke theory in 4D. As an application of the formalism, we obtain two 5D extensions of 4D O’Hanlon
and Tupper vacuum solution and show that they lead two different cosmological scenarios in 4D. 相似文献
57.
Sarma DD Topwal D Manju U Krishnakumar SR Bertolo M La Rosa S Cautero G Koo TY Sharma PA Cheong SW Fujimori A 《Physical review letters》2004,93(9):097202
We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional sensitivity to chemical compositions to investigate high-quality single crystal samples of La(1/4)Pr(3/8)Ca(3/8)MnO3, establishing the formation of distinct insulating domains embedded in the metallic host at low temperatures. These domains are found to be at least an order of magnitude larger in size compared to previous estimates and exhibit memory effects on temperature cycling in the absence of any perceptible chemical inhomogeneity, suggesting long-range strains as the probable origin. 相似文献
58.
Maria Rosa M. L. Albuquerque Gilcarlos R. Medeiros Sandro J. F. Oliveira Adaildo G. dAssunção 《International Journal of Infrared and Millimeter Waves》2005,26(6):855-866
The design of nonuniform bilateral finlines on anisotropic substrates for millimeter wave applications is presented. The taper consists of a planar circuit with smooth variation of the slot width profile along the structure. The spectral domain method and Galerkin procedure are combined with the transmission line theory for studying the behavior of tapered bilateral finlines and evaluation of their design parameters. The influence of the substrate anisotropy on the propagation characteristics of these structures is also examined. This technique is general and can be applied to investigate a broad class of planar transmission line tapers. 相似文献
59.
Renata Monteiro-Maia Maria B Ortigão-de-Sampaio Rosa T Pinho Luiz RR Castello-Branco 《Journal of immune based therapies and vaccines》2006,4(1):1-6
We hypothesize that the energy strategy of a cell is a key factor for determining how, or if, the immune system interacts
with that cell. Cells have a limited number of metabolic states, in part, depending on the type of fuels the cell consumes.
Cellular fuels include glucose (carbohydrates), lipids (fats), and proteins. We propose that the cell's ability to switch
to, and efficiently use, fat for fuel confers immune privilege. Additionally, because uncoupling proteins are involved in
the fat burning process and reportedly in protection from free radicals, we hypothesize that uncoupling proteins play an important
role in immune privilege. Thus, changes in metabolism (caused by oxidative stresses, fuel availability, age, hormones, radiation,
or drugs) will dictate and initiate changes in immune recognition and in the nature of the immune response. This has profound
implications for controlling the symptoms of autoimmune diseases, for preventing graft rejection, and for targeting tumor
cells for destruction. 相似文献
60.