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971.
Horiuchi Y Gnanadesikan V Ohshima T Masu H Katagiri K Sei Y Yamaguchi K Shibasaki M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(18):5195-5204
The development of a direct catalytic asymmetric aldol-Tishchenko reaction and the nature of its catalyst are described. An aldol-Tishchenko reaction of various propiophenone derivatives with aromatic aldehydes was promoted by [LaLi3(binol)3] (LLB), and reactivity and enantioselectivity were dramatically enhanced by the addition of lithium trifluoromethanesulfonate (LiOTf). First, we observed a dynamic structural change of LLB by the addition of LiOTf using 13C NMR spectroscopy, electronspray ionization mass spectrometry (ESI-MS), and cold-spray ionization mass spectrometry (CSI-MS). X-ray crystallography revealed that the structure of the newly generated self-assembled complex was a binuclear [La2Li4(binaphthoxide)5] complex 6. A reverse structural change of complex 6 to LLB by the addition of one equivalent of Li2(binol) was also confirmed by ESI-MS and experimental results. The drastic concentration effects on the direct catalytic asymmetric aldol-Tishchenko reaction suggested that the addition of LiOTf to LLB generated an active oligomeric catalyst species. 相似文献
972.
973.
Michael D. Shultz Scott Calvin Panos P. Fatouros Shannon A. Morrison Everett E. Carpenter 《Journal of magnetism and magnetic materials》2007
Enhanced ferrite nanoparticles are a new class of contrast agents for magnetic resonance imaging (MRI). The enhanced ferrites are synthesized by reverse micelles technique to form iron core and oxide or ferrite shell preventing further oxidation of the nanoparticles. The nanoparticles are further functionalized using dopamine and PEG-600 to increase the solubility of the high magnetic moment nanoparticles. 1H relaxation measurements of aqueous solutions of the nanoparticles were conducted at 2.4 T. The relaxivities r1 and r2, representing the slopes of these curves, are 7.19 and 9.96 s−1 mM−1, respectively. These values should be compared with relaxivities of 4–5 s−1 mM−1 corresponding to commonly used commercial contrast agents in human MR examinations. 相似文献
974.
BaFe12−x (Mn0.5Cu0.5Zr)x/2O19 hexaferrites with x=1, 2 and 3 were prepared by sol–gel process. The ferrite powders possess hexagonal shape and are well separated from one another. The powders of these ferrites were mixed with polyvinylchloride (PVC) plasticizer to be converted into a microwave absorbing composite ferrite with a thickness of 1.8 mm. X-ray diffractometer (XRD), scanning electron microscope (SEM), ac susceptometer, vibrating sample magnetometer and vector network analyzer were used to analyze its structure, electromagnetic and microwave absorption properties. The results showed that magnetoplumbite structures for all samples were formed. The sample with higher magnetic susceptibility and coercivity exhibits a larger microwave absorbing ability. Also the present investigation demonstrates that a microwave absorber using BaFe12−x(Mn0.5Cu0.5Zr)x/2O19 (x=2 and 3)/PVC with a matching thickness of 1.8 mm can be fabricated for applications over 15 GHz, with reflection loss more than −25 dB for specific frequencies, by controlling the molar ratio of the substituted ions. 相似文献
975.
Co1-xZnxFe2O4 nanoparticles, prepared by the polyvinyl alcohol sol-gel method, have been investigated by x-ray diffraction and MSssbauer spectroscopy. These results are compared with those for the bulk material. The lattice parameters of CoZn ferrite nanoparticles are larger than those of the bulk material. Thermal scanning of MSssbauer measurement shows that the transition temperatures for nanoparticles are higher than those of the bulk material except for the sample CoFe2O4. 相似文献
976.
Nardello V Barbillat J Marko J Witte PT Alsters PL Aubry JM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(2):435-441
The decomposition of hydrogen peroxide into singlet molecular oxygen-(1)O(2) ((1)Delta(g))-in the presence of lanthanum(iii) salts was studied by monitoring its characteristic IR luminescence at 1270 nm. The process was found to be heterogeneously catalyzed by La(III), provided that the heterogeneous catalyst is generated in situ. The yield of (1)O(2) generation was assessed as 45+/-5 % both in water and in methanol. The pH-dependence on the rate of (1)O(2) generation corresponds to a bell-shaped curve from pH 4.5 to 13 with a maximum around pH 8. The study of the influence of H(2)O(2) showed that the formation of (1)O(2) begins as soon as one equivalent of H(2)O(2) is introduced. It then increases drastically up to two equivalents and more smoothly above. Unlike all other metal salt catalyst systems known to date for H(2)O(2) disproportionation, this chemical source of (1)O(2) is able to generate (1)O(2) not only in basic media, but also under neutral and slightly acidic conditions. In addition, this La-based catalyst system has a very low tendency to induce unwanted oxygenating side reactions, such as epoxidation of alkenes. These two characteristics of the heterogeneous lanthanum catalyst system allow non-photochemical (i.e., "dark") singlet oxygenation of substrate classes that cannot be peroxidized successfully with conventional molybdate catalysts, such as allylic alcohols and alkenyl amines. 相似文献
977.
Performance of radar absorbing materials by waveguide measurements for X- and Ku-band frequencies 总被引:1,自引:0,他引:1
Magali Silveira Pinho Maria Luisa GregoriRegina Célia Reis Nunes Bluma Guenther Soares 《European Polymer Journal》2002,38(11):2321-2327
In this work, samples consisting of an elastomeric matrix containing different kinds of active materials in particulate form were prepared for reflectivity measurements. The materials individually used in weight contents of 80% in polychloroprene matrix were: carbonyl-iron (CI) and doped ferrite powders. The microwave reflectivity levels were determined from the magnetic and dielectric properties of the elastomeric composites obtained from scattering data, by fitting the samples in a waveguide, for measurements in the frequency range from 8 to 16 GHz. Better microwave absorption for X-band was obtained for CI while doped ferrite absorbed at higher frequencies (Ku-band). 相似文献
978.
Formation of solid solution has been explored in the valence compensated perovskite oxide system Sr1−xLaxTi1−xFexO3 (x ≤ 0.50). XRD studies indicate the formation of solid solution for the whole range investigated. All the compositions synthesised
have cubic structure similar to undoped SrTiO3. Study of dielectric behaviour of these materials show that orientational polarisation and space charge polarisation contribute
significantly to it.
Dedicated to Professor C N R Rao on his 70th birthday 相似文献
979.
稀土化合物中稀土离子和配位体的成键性质一直是人们注意研究的课题之一,一般认为属于离子键,但具有少量的共价性,然而,具体的数量级尚不清楚。本文利用Phillips,VanVechten和Levine理论,对LaX_3(X=F、Cl、Br、OH)晶体的键性和有关参数进行了具体计算,定量指出了镧离子和配体成键的离子性和共价性程度,比较了各晶体中有关的键参数,发现了和宏观物理现象相关的规律。1 理论公式 相似文献
980.
CrystalStructureofTris-(dimethoxyphosphato)Lanthanum(Ⅲ)CoordinationPolymer{La〔PO_2(OCH_3)_2〕_3}_n¥ZengGuang-Fu;GuoXin;WangCui-Y?.. 相似文献