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排序方式: 共有180条查询结果,搜索用时 15 毫秒
31.
Y. K. Kim E. Y. Kim C. M. Whang Y. H. Kim W. I. Lee 《Journal of Sol-Gel Science and Technology》2005,33(1):87-91
Nanophase silica-titania particles were prepared by two different synthetic routes, namely, sol–gel and hydrothermal processing. The crystallinity and crystallographic phases, particle size and surface area of the materials were controlled by varying the calcination temperature, and/or the ratio of Si to Ti. It was determined by XRD that the crystallite sizes of SiO2-TiO2 prepared by sol–gel and hydrothermal processing decreased from 11 to 6 nm and 12 to 9 nm, respectively, as the mole fraction of silica was increased from 0.1 to 0.4. It is proposed that the presence of the amorphous silica suppresses the growth of anatase TiO2 grains and their phase transformation to rutile. The photocatalytic decomposition rate of 1,4-dichlorobenzene (DCB) in aqueous solution with the sol–gel derived SiO2-TiO2 powder prepared at 750 °C was about 10 ± 5% higher than that observed with Degussa P25, whereas the SiO2-TiO2 samples prepared by hydrothermal processing at 250 °C showed a slightly lower decomposition rate than P25. 相似文献
32.
Park KM Whang D Lee E Heo J Kim K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(2):498-508
This paper reports a synthetic strategy to construct one- and two-dimensional (1D and 2D) polyrotaxanes, in which a number of rings are threaded onto a coordination polymer, by the combination of self-assembly and coordination chemistry. Our approach to construct polyrotaxanes with high structural regularity involves threading a cucurbituril (CB) "bead" with a short "string" to form a stable pseudorotaxane, followed by linking the pseudorotaxanes with metal ions as "linkers" to organize into a 1D or 2D polyrotaxane. A 4- or 3-pyridylmethyl group is attached to each end of 1,4-diaminobutane or 1,5-diaminopentane to produce the short "strings", which then react with the cucurbituril "bead" to form stable pseudorotaxanes. The reaction of the pseudorotaxanes with various transition metal ions including CuII, CoII, NiII, AgI, and CdII produces 1D or 2D polyrotaxanes, in which many molecular "beads" are threaded onto 1D or 2D coordination polymers as confirmed by X-ray crystallography. The overall structure of a polyrotaxane is the result of interplay among various factors that include the coordination preferences of the metal ion, spatial disposition of the donor atoms with respect to the CB beads in the pseudorotaxane, and the size and coordination ability of the counteranion. 相似文献
33.
Kinetic studies on the pyridinolysis of aryl furan-2-carbodithioates 1 are carried out at 60.0 degrees C in acetonitrile. The biphasic rate dependence on the pyridine basicity with a breakpoint at pK(a) degrees = 5.2 is interpreted to indicate a change of the rate-limiting step from breakdown (beta(X) = 0.7-0.8) to formation (beta(X) = 0.2) of the tetrahedral intermediate, T(+/-), at the breakpoint as the basicity of the pyridine nucleophile is increased. Observation of the breakpoint is possible with pyridines since the expulsion rate of the pyridine (k(-)(a)) from T(+/-) is sufficiently low, with the low k(-)(a)/k(b) ratio leading to a low breakpoint, pK(a) degrees. The clear-cut change in the cross-interaction constants, rho(XZ), from a positive (rho(XZ) = +0.86) to a small negative (rho(XZ) = -0.11) value at the breakpoint supports the mechanistic change proposed. The magnitudes of rho(Z) and activation parameters are also consistent with the proposed mechanism. 相似文献
34.
Quantitative determination of 15 bioactive triterpenoid saponins in different parts of Acanthopanax henryi by HPLC with charged aerosol detection and confirmation by LC–ESI‐TOF‐MS
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Guo‐zhu Liu Xiang Wang Ok‐kyoung Kwon Hyeong‐kyu Lee Wan‐kyunn Whang Xiang‐qian Liu 《Journal of separation science》2016,39(12):2252-2262
Triterpenoid saponins are difficult to analyze using high‐performance liquid chromatography coupled to UV/vis spectrophotometry due to their lack of chromophores. This study describes the first analytical method for the determination of 15 triterpenoid saponins from the leaves, stems, root bark, and fruits of Acanthopanax henryi, using a high‐performance liquid chromatography with charged aerosol detection coupled with electrospray ionization mass spectrometry method. The separation was carried out on a Kinetex XB‐C18 column with an acetonitrile/water gradient as the mobile phase, followed by charged aerosol detection. The operating conditions of charged aerosol detection were set at 24 kPa for nitrogen pressure and 100 pA for the detection range. Liquid chromatography with electrospray ionization mass spectrometry is described for the identification of compounds in plant samples. The electrospray ionization mass spectrometry method involved the use of the [M + Na]+ and [M + NH4]+ ions for compounds 1 – 15 in the positive ion mode with an extracted ion chromatogram. The developed method was fully validated in terms of linearity, sensitivity, precision, repeatability, and recovery, then subsequently applied to evaluate the quality of A. henryi. 相似文献
35.
Non-linear calibration is one major drawback of suppressed conductivity ion chromatography and thus, it has been subjected to intensive investigations over the last two decades. The investigations are mostly confined to anion chromatography using weak base eluents like carbonate because the extent of non-linearity is proportional to dissociation constant, Ka, of the suppressed eluate. 相似文献
36.
Kim CK Han IS Ryu WS Lee HW Lee BS Kim CK 《The journal of physical chemistry. A》2006,110(7):2500-2504
The pi-donating effects of pi-accepting X-substituents in substituted benzylic cations, X-C(6)H(5)-CHR(+) where R = CF(3), H and OCH(3), and X = p-NH(2), p-OCH(3), p-CH(3), H, p-F, p-Cl, p-CHO, m-CN, p-CN, m-NO(2) or p-NO(2), have been studied theoretically by using isodesmic hydride transfer reactions at various levels of theory. It might be difficult to determine the pi-donating effects of pi-acceptors using the simple Hammett-type linear equation, because it is not sensitive enough to include small pi-donating effects. Therefore, this effect was estimated using the NBO deletion energy (DeltaE(D)) of the second-order charge-transfer interaction (DeltaE(ct)) between the pi-orbitals (or lone pair orbitals) of the X-substituent and the pi-orbitals of phenyl ring. The extents of pi-donating effects increased in the order X = p-NO(2) < p-CHO < p-CN < p-Cl for both neutral and cationic species, and these effects were found to be more important for para- than for meta-substituents. Moreover, this could represent a general trend for pi-donation by pi-acceptors. On the other hand, the effects of R-substituents on this pi-donating effect were found to be in the order R = OCH(3) < H congruent with CF(3), as predicted by natural resonance theory (NRT) analyses. 相似文献
37.
Thou-Jen Whang Mu-Tao HsiehJia-Ming Tsai Shyan-Jer Lee 《Applied Surface Science》2011,257(22):9539-9545
Compact and homogeneous c-axis preferred orientation of zinc oxide (ZnO) films on indium tin oxide (ITO) coated glass have been prepared electrochemically at −1.2 V vs. Ag|AgCl in a weak acidic condition from 0.06 M Zn(NO3)2 with 3 mM lactic acid (LA) added. LA was found having strong influence on the electrodeposition of c-axis preferred orientation of zinc oxide films. Other experimental variables such as deposition temperature, potential, and precursor concentration were also conducted in this article. Among these variables, it was found that precursor concentration of zinc nitrate influenced significantly on growth direction and crystal diameter of zinc oxide. Cyclic voltammetry was used to observe the electrochemistry of the deposition. Crystallinities of the films were examined by X-ray diffractometer. The morphologies of zinc oxide films were observed with a field emitting scanning electron microscope. Optical characteristics of zinc oxide layers were measured with UV-vis spectrophotometer. The band gap of the deposited zinc oxide thin films was evaluated from the Tauc relationship of (αhν)2 vs. hν, which was found to be 3.31 eV. 相似文献
38.
39.
Heui Seob Jeong Jeong Hyun Seo Cha Keun Yoon Joong Kyun Kim Ki-Woong Whang 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1999,27(1):12-13
A direct observation of two-dimensional, time dependent behavior of 147 nm and 173 nm vacuum ultra violet emission from a single surface discharge type AC plasma display panel cell were obtained using a gated, image intensified charge-coupled device (ICCD). The images on the ICCD are formed by a pinhole. The two lines show similar spatial profile but quite different temporal behavior 相似文献
40.
Based on data representing standard Korean male adults, a mathematical anthropomorphic phantom was developed to calculate the specific absorbed fraction for several organs. The results were then compared against values calculated with other phantoms (OTOKO of Japan, MIRD of the U.S. and GOLEM of Germany). We compared the specific absorbed fractions of six organs (the adrenals, kidneys, liver, lungs, pancreas and spleen) for two cases: when the source and target organs are the same, and when they are different. When both the source and target organs are the same the organ with the highest specific absorbed fraction regardless of energy level was found to be the adrenals in all phantoms. In comparing phantoms, the phantom developed on the basis of standard Koreans produced the highest specific absorbed fraction. Also, as a result of conducting an assessment by separating source organs from target organs, the phantom developed on the basis of standard Koreans displayed relatively high specific absorbed fractions although there were slight differences per organ. 相似文献