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排序方式: 共有567条查询结果,搜索用时 8 毫秒
31.
Kim HS Lee SY Lee H Bae JY Park SJ Cheong M Lee JS Lee CH 《The Journal of organic chemistry》2006,71(3):911-914
The isomerization reaction of 5-vinyl-2-norbornene (VNB) to 5-ethylidene-2-norbornene (ENB) has been performed using a catalytic system consisting of an alkali metal hydride and an amine. Among various amines tested, only aliphatic 1,2-diamines exhibited the activity for the isomerization. The isomerization was also affected by the alkali metal hydride employed. The activity of the alkali metal hydride increased with the increasing size of alkali metal: KH > NaH > LiH. A series of electron paramagnetic resonance (EPR) and UV-vis experiments on the active species suggest that the isomerization of VNB proceeds through a radical mechanism. 相似文献
32.
Assessment of Joule heating and its effects on electroosmotic flow and electrophoretic transport of solutes in microfluidic channels 总被引:4,自引:0,他引:4
Joule heating is inevitable when an electric field is applied across a conducting medium. It would impose limitations on the performance of electrokinetic microfluidic devices. This article presents a 3-D mathematical model for Joule heating and its effects on the EOF and electrophoretic transport of solutes in microfluidic channels. The governing equations were numerically solved using the finite-volume method. Experiments were carried out to investigate the Joule heating associated phenomena and to verify the numerical models. A rhodamine B-based thermometry technique was employed to measure the solution temperature distributions in microfluidic channels. The microparticle image velocimetry technique was used to measure the velocity profiles of EOF under the influence of Joule heating. The numerical solutions were compared with experimental results, and reasonable agreement was found. It is found that with the presence of Joule heating, the EOF velocity deviates from its normal "plug-like" profile. The numerical simulations show that Joule heating not only accelerates the sample transport but also distorts the shape of the sample band. 相似文献
33.
Jae Chul Cheong Sung Ill Suh Beom Jun Ko Jin Young Kim Moon Kyo In Won Jo Cheong 《Journal of separation science》2010,33(12):1767-1778
A simple and rapid GC‐MS method has been developed for the screening and quantification of many illicit drugs and their metabolites in human urine by using automatic SPE and trimethylsilylation. Sixty illicit drugs, including parent drugs and their metabolites that are possibly abused in Korea, can be monitored by this method. Among them, 24 popularly abused illicit drugs were selected for quantification. Very delicate optimizations were carried out in SPE, trimethylsilylation derivatization, and GC/MS to enable such remarkable achievements. Trimethylsilylated analytes were well separated within 21 min by GC‐MS. In the validation results, the LOD of all the analytes were in the range of 2–75 ng/mL. The LOQ of the quantified analytes were in the range of 5–98 ng/mL. The linearity (r2) of the quantified analytes ranged 0.990–1.000 in each concentration range between 10 and 1000 ng/mL. The mean recoveries ranged from 62 to 126% at three different concentrations of each analyte. The inter‐day and inter‐person accuracies were within ?13.3~14.9%, and ?10.1~13.0%, respectively, and the inter‐day and inter‐person precisions were less than 12.9%. The method was reliable and efficient for the screening and quantification of abused illicit drugs in routine urine analysis. 相似文献
34.
Shen ZL Goh KK Cheong HL Wong CH Lai YC Yang YS Loh TP 《Journal of the American Chemical Society》2010,132(45):15852-15855
The first water-tolerant, ketone-type indium homoenolate was synthesized via the oxidative addition of In/InCl(3) to enones. The reaction proceeds exclusively in aqueous media. Both indium and indium(III) chloride are necessary for the smooth conversion of the reaction. Similar results were obtained when InCl or InCl(2) was used in place of In/InCl(3). The synthetic utility of the indium homoenolate was demonstrated through the synthesis of 1,4-dicarbonyl compounds via palladium-catalyzed coupling of indium homoenolate with acid chloride. 相似文献
35.
Pierce MD Johnston RC Mahapatra S Yang H Carter RG Ha-Yeon Cheong P 《Journal of the American Chemical Society》2012,134(33):13624-13631
Computational study of the mechanisms and stereoselectivities of a dual amino-catalyzed synthesis of cyclohexenones containing all-carbon γ-quaternary and ?-tertiary stereocenters is reported. Extensive conformational search with density functional theory optimizations, the high-accuracy SCS-MP2/cc-pV∞Z energies, and PCM solvation corrections were used to characterize all intermediates and transition states. Six mechanisms were considered, all consistent with available experiments. The reaction proceeds via sequential Michael and Mannich conjugate additions whereby the primary amine activates the aldehyde and the catalyst activates the pentenone. We have discovered a rare duumvirate stereocontrol: the Michael reaction sets the enantioselectivity, but both the Michael and the Mannich reactions control the diastereoselectivity. 相似文献
36.
Conrad T. Pfeiffer Justin D. Northrup Jae Eun Cheong Melody A. Pham Matthew F.L. Parker Christian E. Schafmeister 《Tetrahedron letters》2018,59(30):2884-2888
Spiroligomers are a class of peptidomimetics that connect interchangeable, stereochemically pure, cyclic monomers through pairs of amide bonds to form diketopiperazines between the monomers. This enables them to adopt predictable and programmable structure due to the rigidity of the final molecule. We present a new method for the solid phase synthesis of highly functionalized spiroligomers that incorporates the use of the p-nitrobenzyloxycarbonyl (pNZ) as a temporary amine protecting group and the pentafluorophenyl ester for monomer activation. This new method allows for the synthesis of spiroligomers with higher purity and increased yields when compared to previous methods. This improved method of synthesis of functionalized spiroligomers will facilitate the development of applications as catalysts, therapeutics and membrane channels. 相似文献
37.
This paper provides computable representations for the evaluation of the probability content of cones in isotropic random fields. A decomposition of quadratic forms in spherically symmetric random vectors is obtained and a representation of their moments is derived in terms of finite sums. These results are combined to obtain the distribution function of quadratic forms in spherically symmetric or central elliptically contoured random vectors. Some numerical examples involving the sample serial covariance are provided. Ratios of quadratic forms are also discussed. 相似文献
38.
Hexagonal HoMnO3 is a frustrated antiferromagnet (T(N)=72 K) ferroelectric (T(C)=875 K) in which these two order parameters are coupled. Our neutron measurements of the spin-wave dispersion for the S=2 Mn3+ on the layered triangular lattice are well described by a two-dimensional nearest-neighbor Heisenberg exchange J=2.44 meV, and an anisotropy D that is 0.28 meV above the spin-reorientation transition at 40 K and 0.38 meV below. For H parallel c the magnetic structures and phase diagram have been determined, and reveal additional transitions below 8 K where the ferroelectrically displaced Ho3+ ions are ordered magnetically. 相似文献
39.
Grenier S Hill JP Kiryukhin V Ku W Kim YJ Thomas KJ Cheong SW Tokura Y Tomioka Y Casa D Gog T 《Physical review letters》2005,94(4):047203
We report a study of electronic excitations in manganites exhibiting a range of ground states, using resonant inelastic x-ray scattering (RIXS) at the Mn K edge. Excitations with temperature dependent changes correlated with the magnetism were observed as high as 10 eV. By calculating Wannier functions, and finite-q response functions, we associate this dependence with intersite d-d excitations. The calculated dynamical structure factor is found to be similar to the RIXS spectra. 相似文献
40.
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