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排序方式: 共有272条查询结果,搜索用时 31 毫秒
61.
In this paper,we proposed an innovation diffusion model with three compartments to investigate the diffusion of an innovation(product) in a particular region.The model exhibits two equilibria,namely,the adopter-free and an interior equilibrium.The existence and local stability of the adopter-free and interior equilibria are explored in terms of the effective Basic Influence Number(BIN) RA.It is investigated that the adopter free steady-state is stable if RA <1.By conside... 相似文献
62.
Qiu L Kwong FY Wu J Lam WH Chan S Yu WY Li YM Guo R Zhou Z Chan AS 《Journal of the American Chemical Society》2006,128(17):5955-5965
A series of chiral diphosphine ligands denoted as PQ-Phos was prepared by atropdiastereoselective Ullmann coupling and ring-closure reactions. The Ullmann coupling reaction of the biaryl diphosphine dioxides is featured by highly efficient central-to-axial chirality transfer with diastereomeric excess >99%. This substrate-directed diastereomeric biaryl coupling reaction is unprecedented for the preparation of chiral diphosphine dioxides, and our method precludes the tedious resolution procedures usually required for preparing enantiomerically pure diphosphine ligands. The effect of chiral recognition was also revealed in a relevant asymmetric ring-closure reaction. The chiral tether bridging the two aryl units creates a conformationally rigid scaffold essential for enantiofacial differentiation; fine-tuning of the ligand scaffold (e.g., dihedral angles) can be achieved by varying the chain length of the chiral tether. The enantiomerically pure Ru- and Ir-PQ-Phos complexes have been prepared and applied to the catalytic enantioselective hydrogenations of alpha- and beta-ketoesters (C=O bond reduction), 2-(6'-methoxy-2'-naphthyl)propenoic acid, alkyl-substituted beta-dehydroamino acids (C=C bond reduction), and N-heteroaromatic compounds (C=N bond reduction). An excellent level of enantioselection (up to 99.9% ee) has been attained for the catalytic reactions. In addition, the significant ligand dihedral angle effects on the Ir-catalyzed asymmetric hydrogenation of N-heteroaromatic compounds were also revealed. 相似文献
63.
Ragupathi G Koide F Livingston PO Cho YS Endo A Wan Q Spassova MK Keding SJ Allen J Ouerfelli O Wilson RM Danishefsky SJ 《Journal of the American Chemical Society》2006,128(8):2715-2725
Several novel, fully synthetic, carbohydrate-based antitumor vaccines have been assembled. Each construct consists of multiple cancer-related antigens displayed on a single polypeptide backbone. Recent advances in synthetic methodology have allowed for the incorporation of a complex oligosaccharide terminating in a sialic acid residue (i.e., GM2) as one of the carbohydrate antigens. Details of the vaccine synthesis as well as the results of preliminary immunological investigations are described herein. 相似文献
64.
Surendra S. More Ram H. Kadam Ankush B. Kadam Dhanraj R. Mane Govind K. Bichile 《Central European Journal of Chemistry》2010,8(2):419-425
A series of CoAlxCrxFe2−2xO4 systems (x = 0.1 to 0.5 in steps of x = 0.1) spinel ferrites have been synthesized successfully using wet chemical co-precipitation
technique. The samples were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR) and magnetization measurements.
The powder XRD patterns confirm the single phase spinel structure for the materials synthesized. X -ray diffraction measurements
were performed to yield the lattice constant as function of the amount x corresponding to Al-Cr substitution. Lattice parameters,
X-ray density, bulk density and particle size decrease whereas porosity increases with the increase in Al-Cr content, ‘x’.
Infrared studies show two absorption bands at about 400 cm−1 and 600 cm−1 for octahedral and tetrahedral sites, respectively. Saturation magnetization decreases with the increase in Al-Cr content
x. AC magnetic susceptibility measurements were carried out as a function of temperature to measure the Curie temperature,
which was found to decrease with Al-Cr content x. The decrease of Curie temperature has been explained by A-B interaction.
相似文献
65.
We investigate the propagation characteristics of ultrafast pulses inside silicon waveguides considering frequency chirp associated with each input pulse. Effects of linear losses, two-photon absorption, and free-carrier dynamics are included analytically within the framework of a modified variational formalism and the results are validated by comparing them with full numerical simulations. It is found that an initial chirp helps in maintaining the pulse shape and spectrum in the anomalous-dispersion regime, thereby resulting in soliton-like propagation of ultrashort optical pulses. 相似文献
66.
Mahavir Prashad Bin Hu Hong-Yong Kim Denis Har Oljan Repi? Thomas J. Blacklock 《Tetrahedron letters》2007,48(12):2087-2089
A new reaction of N-aryl-2-pyrimidinamines with triphosgene to afford N-aryl-N-(2-pyrimidinyl)-2-pyrimidinamine is described. 相似文献
67.
Kim KC Ahn HJ Kim JB Hwang BH Baik HK 《Langmuir : the ACS journal of surfaces and colloids》2005,21(24):11079-11084
The mechanism of liquid crystal (LC) alignment has been investigated during the last few decades for inorganic materials as well as for organic materials; however, it has not been clearly confirmed for some alignment materials. Inorganic alignment materials such as amorphous silicon oxide (a-SiOx) and hydrogenated amorphous silicon oxide (a-SiOx:H) are deposited on indium tin oxide (ITO) films on glass by reactive sputtering deposition. After deposition, the inorganic alignment materials are irradiated using an Ar+ ion beam (IB) for LC alignment. On the basis of the experimental results, a-SiOx films deposited by the sputtering do not align the LC, but a-SiOx:H films treated with varying IB energies, IB incident angles, IB doses, and IB irradiation times have excellent alignment properties and electrooptical properties, identical to those of polyimide (PI). These results imply that inorganic alignment layers irradiated by IB can be adopted as an LC alignment layer instead of rubbed PI. Additionally, hydrogen plays an important role in LC alignment because of the difference in alignment properties between a-SiOx films and a-SiOx:H films. We investigate the mechanism of IB-treated inorganic alignment layers and suggest that LCs are aligned by chemical effects, such as van der Waals interaction, more than by physical effects, such as morphology effects, in the inorganic alignment layer irradiated by IB. 相似文献
68.
Kwong FY Li YM Lam WH Qiu L Lee HW Yeung CH Chan KS Chan AS 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(13):3872-3880
An interesting rhodium-catalyzed asymmetric aqueous Pauson-Khand-type reaction was developed. A chiral atropisomeric dipyridyldiphosphane ligand was found to be highly effective in this system. This operationally simple protocol allows both catalyst and reactants to be handled under air without precautions. Various enynes were transformed to the corresponding bicyclic cyclopentenones in good yield and enantiomeric excess (up to 95 % ee). A study of the electronic effects of the enyne substrates revealed a correlation between the electronic properties of the substrates and the ee value obtained in the product of the Pauson-Khand-type reaction. A linear free-energy relationship was observed from a Hammett study. 相似文献
69.
Abdullah Alqudami S. Annapoorni Govind S. M. Shivaprasad 《Journal of nanoparticle research》2008,10(6):1027-1036
Homogenous Ag–Au alloy nanoparticles having an average size of 12 ± 2 nm were successfully prepared by the exploding wire
technique comprising of a wire–plate system and using 12 V batteries. The X-ray photoelectron spectroscopy data reveal the
formation of alloy nanoparticles with Ag80-Au20 composition, which agrees with the absorption data, obtained using UV-Visible
spectroscopy. XPS also reveals a thin metal-oxide shell on the metallic alloy core. These alloy nanoparticles show visible
fluorescence emission that was compared with the observed fluorescence from pure Ag nanoparticles. A mechanism for the observed
fluorescence is also provided. 相似文献
70.
Govind P. Agrawal 《Optics Communications》1981,39(4):272-276
The theory of nondegenerate four-wave mixing is described for the case wherein the counter-propagating pump waves are in two-photon resonance with the nonlinear medium. In contrast to the one-photon case, the reflection bandpass is found to be mainly determined by the phase-matching requirement. The variation of the phase-conjugate reflectivity with the pump intensity and pump detuning is considered. The effect of the intensity-dependent Stark shift on the phase-conjugation efficiency is discussed in some detail. 相似文献