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991.
A series of Ce(1-x)M(x)O(2-δ) (M = Gd, Zr, La, Sm, Y, Lu, and Pr) samples were characterized by Raman spectroscopy to investigate the evolution of defect sites (oxygen vacancies and MO(8)-type complex) and their distributions in the samples. It was found that the evolution of oxygen vacancies was due to the different ionic valence state of dopant from that of Ce(4+), while the evolution of the MO(8)-type complex was due to the different ionic radius of dopant from that of Ce(4+). The distributions of defect sites were investigated using 325 and 514 nm excitation laser lines, indicating that the defect sites were surface enriched. Moreover, the increasing ordering level of the sample led to a decline in the concentration of the MO(8)-type complex in the sample but the constant concentration of oxygen vacancies, implying that the metastable MO(8)-type complex species were more disordered compared to the oxygen vacancies. 相似文献
992.
Zhu B Yuan F Li R Li Y Wei Q Ma Z Du B Zhang X 《Chemical communications (Cambridge, England)》2011,47(25):7098-7100
A simple but highly selective colorimetric and ratiometric fluorescent chemodosimeter was designed and synthesized to detect fluoride ions (F(-)) in aqueous solution and living cells by virtue of the strong affinity of F(-) toward silicon. 相似文献
993.
Using scanning tunneling microscopy (STM) together with density functional theory (DFT) the growth behavior of perylene on the Cu(100) substrate has been investigated. As revealed by STM images, perylene molecules prefer to adopt lying configuration with their molecular plane parallel to the substrate, and two symmetrically equivalent ordered domains were observed. DFT calculations show that perylene molecule prefers to adsorb on the top site of substrate Cu atoms with its long molecular axis aligning along the [011] or [01-1] azimuth of the substrate which is the most stable adsorption geometry according to its highest binding energy. Consequently, two adsorption structures of c(8×4) and c(8×6), each containing two perylene molecules per unit cell, are proposed based on our STM images. The growth mechanism for ordered perylene domains on Cu(100) can be attributed to the balance between weak adsorbate-adsorbate interaction and comparable adsorbate-substrate interaction. 相似文献
994.
Zhao L Li H Lu G Huang F Zhang C Wang ZX 《Dalton transactions (Cambridge, England : 2003)》2011,40(9):1929-1937
This study extends our previous work of using π-FLP strategy to develop metal-free hydrogenation catalysts. Using small MeN=CMe(2) imine (im1) as a model, we previously designed cat1 and cat2 catalysts. But it is unclear whether they are capable of catalyzing the hydrogenations of bulky imines. Using tBuN=C(H)Ph (im2) as a representative of large imines, we assessed the energetics of the cat1- and cat2-catalyzed im2 hydrogenations. The predicted energetics indicates that they can still catalyze large imine hydrogenations with experimentally accessible kinetic barriers, although the energetics becomes less favorable. To improve the catalysis, we proposed new catalysts (cat3 and cat4) by tailoring cat1 and cat2. The study indicates that cat3 and cat4 could have better performance for the hydrogenation of the bulky im2 than cat1 and cat2. Remarkably, cat3 and cat4 are also found suitable for small imine (im1) hydrogenation. Examining the hydrogen transfer substeps in the eight hydrogenations involved in this study, we observed that the mechanism for the hydrogen transfer step in the catalytic cycles depends on the steric effect between catalyst and substrate. The mechanism can be switched from stepwise one in the case of large steric effect (e.g.im2/cat2) to the concerted one in the case of small steric effect (e.g.im1/cat3). The new catalysts could be better targets for experimental realization because of their simpler constructions. 相似文献
995.
A simple, sensitive and selective method of high-performance liquid chromatography with electrochemical detection (HPLC-ECD) has been developed for simultaneous determination of bavachin and isobavachalcone in Fructus Psoraleae. At optimized conditions, bavachin and isobavachalcone could be well separated within 15 min at a detection potential of +0.80 V with 0.03 mol/L acetate buffer solution (pH 5.17)/acetonitrile (2:3, v/v) as the mobile phase. The relationships between peak areas and concentrations were linear from 8.26 × 10(-7) to 1.21 × 10(-4) mol/L for bavachin, and from 1.01 × 10(-8) to 1.61 × 10(-4) mol/L for isobavachalcone, respectively. The method offered excellent linearity with regression coefficient R(2) >0.995. The method presented detection limits (S/N = 3) of 8.81 × 10(-9) mol/L for bavachin and 1.17 × 10(-10) mol/L for isobavachalcone. It indicates that the sensitivity of electrochemical detection is ten times higher than that of diode array detection (DAD). The mean recoveries around 98% with a relative standard deviation less than 3.1% for the two analytes have been obtained. The proposed separation and detection procedures were successfully applied to the simultaneous determination of bavachin and isobavachalcone in traditional Chinese medicine. 相似文献
996.
Zhao Y Zhang J Wang B Kim SH Fang A Bogdanov B Zhou Z McClain C Zhang X 《Journal of chromatography. A》2011,1218(18):2577-2583
A method was developed to calculate the second dimension retention index of comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC/TOF-MS) data using n-alkanes as reference compounds. The retention times of the C(7)-C(31) alkanes acquired during 24 isothermal experiments cover the 0-6s retention time area in the second dimension retention time space, which makes it possible to calculate the retention indices of target compounds from the corresponding retention time values without the extension of the retention space of the reference compounds. An empirical function was proposed to show the relationship among the second dimension retention time, the temperature of the second dimension column, and the carbon number of the n-alkanes. The proposed function is able to extend the second dimension retention time beyond the reference n-alkanes by increasing the carbon number. The extension of carbon numbers in reference n-alkanes up to two more carbon atoms introduces <10 retention index units (iu) of deviation. The effectiveness of using the proposed method was demonstrated by analyzing a mixture of compound standards in temperature programmed experiments using 6 different initial column temperatures. The standard deviation of the calculated retention index values of the compound standards fluctuated from 1 to 12 iu with a mean standard deviation of 5 iu. 相似文献
997.
998.
With a microfluidic droplet-based liquid/liquid extraction setup, we demonstrate that the extraction of an ionic analyte from complex matrices can be modulated by the interfacial Galvani potential difference and the extraction equilibrium follows the classical Nernst equation. 相似文献
999.
Health or environmental issue caused by abnormal level of metal ions like Zn2+ or Cd2+ is a worldwide concern. Developing an inexpensive and facile detection method for Zn2+ and Cd2+ is in urgent demand. Due to their super optical properties, fluorescent quantum dots (QDs) have been developed as a promising alternative for organic dyes in fluorescence analysis. In this study, a CdTe QDs-based sensitive and selective probe for Zn2+ and Cd2+ in aqueous media was reported. The proposed probe worked in fluorescence “turn-on” mode. The initial bright fluorescence of CdTe QDs was effectively quenched by sulfur anions (S2−). The presence of Zn2+ (or Cd2+) can “turn-on” the weak fluorescence of QDs quenched by S2− due to the formation of ZnS (or CdS) passivation shell. Under optimal conditions, a good linear relationship between the fluorescence response and concentration of Zn2+ (or Cd2+) could be obtained in the range from 1.6 to 35 μM (1.3–25 μM for Cd2+). The limit of detection (LOD) for Zn2+ and Cd2+ were found to be 1.2 and 0.5 μM, respectively. Furthermore, the present probe exhibited a high selectivity for Zn2+ and Cd2+ over other metal ions and was successfully used in the detection of Zn2+ or Cd2+ in real water samples. 相似文献
1000.
In the present paper, different electronic structure methods have been used to determine stationary and intersection structures on the ground (S(0)) and (1)ππ? (S(2)) states of 4-methylpyridine, which is followed by adiabatic and nonadiabatic dynamics simulations to explore the mechanistic photoisomerization of 4-methylpyridine. Photoisomerization starts from the S(2)((1)ππ?) state and overcomes a small barrier, leading to formation of the prefulvene isomer in the S(0) state via a S(2)∕S(0) conical intersection. The ultrafast S(2) → S(0) nonradiative decay and low quantum yield for the photoisomerization reaction were well reproduced by the combined electronic structure calculation and dynamics simulation. The prefulvene isomer was assigned as a long-lived intermediate and suggested to isomerize to 4-methylpyridine directly in the previous study, which is not supported by the present calculation. The nonadiabatic dynamics simulation and electronic structure calculation reveal that the prefulvene isomer is a short-lived intermediate and isomerizes to benzvalene form very easily. The benzvalene form was predicted as the stable isomer in the present study and is probably the long-lived intermediate observed experimentally. A consecutive light and thermal isomerization cycle via Dewar isomer was determined and this cycle mechanism is different from that reported in the previous study. It should be pointed out that formation of Dewar isomer from the S(2)((1)ππ?) state is not in competition with the isomerization to the prefulvene form. The Dewar structure observed experimentally may originate from other excited states. 相似文献