首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   78篇
  免费   4篇
化学   56篇
力学   6篇
数学   1篇
物理学   19篇
  2022年   3篇
  2021年   3篇
  2020年   2篇
  2019年   5篇
  2018年   5篇
  2017年   5篇
  2016年   3篇
  2015年   4篇
  2014年   8篇
  2013年   9篇
  2012年   5篇
  2011年   5篇
  2010年   4篇
  2009年   4篇
  2008年   6篇
  2007年   3篇
  2006年   2篇
  2005年   2篇
  2004年   1篇
  2002年   2篇
  1989年   1篇
排序方式: 共有82条查询结果,搜索用时 17 毫秒
61.
Bimetallic nanocatalysts are key enablers of current chemical technologies, including car exhaust converters and fuel cells, and play a crucial role in industry to promote a wide range of chemical reactions. However, owing to significant characterization challenges, insights in the dynamic phenomena that shape and change the working state of the catalyst await further refinement. Herein, we discuss the atomic‐scale processes leading to mono‐ and bimetallic nanoparticle formation and highlight the dynamics and kinetics of lifetime changes in bimetallic catalysts with showcase examples for Pt‐based systems. We discuss how in situ and operando X‐ray spectroscopy, scattering, and diffraction can be used as a complementary toolbox to interrogate the working principles of today's and tomorrow's bimetallic nanocatalysts.  相似文献   
62.
Raju Ranjith Kumar 《Tetrahedron》2007,63(49):12220-12231
The 1,3-dipolar cycloaddition of an azomethine ylide to 1-methyl-3,5-bis[(E)-arylmethylidene]tetrahydro-4(1H)-pyridinones afforded novel spiro-pyrrolidines in good yields. Further cycloaddition of these spiro-pyrrolidines with nitrile oxide afforded mono-spiro-isoxazolines in moderate yields (45-56%), presumably via a di-spiro intermediate, which undergoes a spontaneous cycloreversion of the spiro-pyrrolidine unit. In contrast, the direct cycloaddition of nitrile oxide to 1-methyl-3,5-bis[(E)-arylmethylidene]tetrahydro-4(1H)-pyridinones gave the mono-spiro-isoxazoline as the minor product, while the bis-spiro-isoxazolines are formed predominantly.  相似文献   
63.
A four-dimensional (4-D) NMR study of Escherichia coli malate synthase G (MSG), a 723-residue monomeric enzyme (81.4 kDa), is described. Virtually complete backbone (1)HN, (15)N, (13)C, and (13)C(beta) chemical shift assignments of this largely alpha-helical protein are reported. The assignment strategy follows from our previously described approach based on TROSY triple resonance 4-D NMR spectroscopy [Yang, D.; Kay, L. E. J. Am. Chem. Soc. 1999, 121, 2571-2575. Konrat, R; Yang, D; Kay, L. E. J. Biomol. NMR 1999, 15, 309-313] with a number of modifications necessitated by the large size of the protein. A protocol for refolding deuterated MSG in vitro was developed to protonate the amides deeply buried in the protein core. Of interest, during the course of the assignment, an isoaspartyl linkage in the protein sequence was unambiguously identified. Chemical shift assignments of this system are a first step in the study of how the domains of the protein change in response to ligand binding and for characterizing the dynamical properties of the enzyme that are likely important for function.  相似文献   
64.
Pathak SM  Kumar AR  Musmade P  Udupa N 《Talanta》2008,76(2):338-346
A sensitive high performance liquid chromatographic (HPLC) method involving fluorescence detection was developed for the determination of fexofenadine (FEX), known to have low oral bioavailability, in rat plasma. In order to understand the effect of various chromatographic factors on the separation of analytes and to simultaneously optimize the resolution and analysis run time, a response surface method was used. The chromatographic separation was achieved using a Supelco C(18)-DB (250 mm x 4.6mm I.D./5 microm particle size) column with mobile phase comprising of ammonium acetate buffer and acetonitrile (63:37, v/v), delivered isocratically at a flow rate of 1.0 mL min(-1). Diphenhydramine was used as an internal standard (I.S.). The statistical evaluation of the method was examined and the method was found to be precise and accurate with a linearity range of 1-500 ng mL(-1) (r>0.9980). The intra- and inter-day precision studies showed good reproducibility with coefficients of variation (C.V.) less than 12.26%. The advantages of our method are small sample volume (100 microL), short time of analysis (13 min) and a simple sample extraction and clean-up as compared to the previously published methods. The established method provides a reliable bioanalytical methodology to carry out FEX pharmacokinetics in rat plasma.  相似文献   
65.
The 1,3-dipolar cycloaddition of azomethine ylides derived from acenaphthenequinone and α-amino acids viz. proline, phenylglycine and sarcosine to a series of 1-methyl-3,5-bis[(E)-arylmethylidene]tetrahydro-4(1H)-pyridinones afforded novel spiro-pyrido-pyrrolizines and pyrrolidines chemo-, regio- and stereoselectively in quantitative yields. These compounds were screened for their in vitro activity against Mycobacterium tuberculosis H37Rv (MTB), multi-drug resistant Mycobacterium tuberculosis (MDR-TB) and Mycobacterium smegmatis using agar dilution method. Among the synthesized compounds, spiro-[2.2″]acenaphthene-1″-one-spiro[3.3′]-5′-(2-chlorophenylmethylidene)-1′-methyltetrahydro-4′(1H)-pyridinone-4-(2-chlorophenyl)hexahydro-1H-pyrrolizine (3e) was found to be the most active with a minimum inhibitory concentration (MIC) of 0.4 μg/mL against MTB and MDR-TB.  相似文献   
66.
Silica-supported hierarchical graphitic carbon sheltering cobalt nanoparticles Co-HGC@SiO2 (1) were prepared by pyrolysis at 850 °C of [Co(phen)(H2O)4]SO4·2H2O complex with silica in the presence of pyrene as a carbon source under nitrogen atmosphere. Nanocomposites (2) and (3) were obtained by acid treatment of (1) with HCl and HF acid, respectively. The nanocomposites showed rough hierarchical carbon microstructures over silica support decorated with irregular cobalt nanospheres and nanorods 50 to 200 nm in diameter. The nanoparticles consist of graphitic shells and cobalt cores. SEM, EDAX and TEM elemental mapping indicate a noticeable loss of cobalt in the case of (2) and loss of cobalt and silica in the case of (3) with an increase in porosity. Nanocomposite (3) showed the highest BET surface area 217.5 m2g−1. Raman spectrum shows defect D-band and graphitic G-band as expected in carbon nanostructures. PXRD reveals the presence of cobalt(0) nanoparticles. XPS indicates the presence of Co(II) oxides and the successful doping of nitrogen in the nanocomposites. Moreover, TEM elemental mapping provides information about the abundance of Si, Co, C, N and S elements in zones. Nanocomposite (1) showed maximum uptake capacity of 192.3 and 224.5 mg/g for crystal violet CV and methyl orange MO dyes, respectively. Nanocomposite (2) showed a capacity of 94.1 and 225.5 mg/g for CV and MO dyes, respectively. Nanocomposite (4) obtained after treatment of (1) with crystal violet proved successful adsorption of CV. Co-HGC (5) prepared without addition of silica has a capacity for CV equal to 192 mg/g, while it is 769.2 mg/g with MO. Electrostatics and π–π interactions of graphite and cobalt species in the nanocomposites with aromatic rings of cationic and anionic dyes are responsible for the adsorption. Yan et al. was the best model to describe column kinetics. The thomas column adsorption model showed that the maximum uptake capacity of (1) was 44.42 mg/g for CV and 32.62 mg/g for MO. for a column packed with 0.5 gm of (1) and dye concentration of 100 mg/L at a flow rate of 1 mL/min. The column was recycled three times with no noticeable clogging or degradation of nanocomposites. Thus, Co-HGC@SiO2 adsorbents can be used efficiently to treat water contaminated with cationic and anionic dyes.  相似文献   
67.
The stereoselective syntheses of novel dispiro acenaphthylene–indolizine–pyridinone hybrid heterocycles have been achieved through one-pot four-component domino 1,3-dipolar cycloaddition–Michael addition–air oxidation sequence of reactions.  相似文献   
68.
A highly selective isophorone‐boronate ester based chemosensor, ( 1 ) , having a dicyanovinyl moiety as a convenient colorimetric probe, has been designed. Different types of anionic analyte such as CH3COO?, ClO4?, Cl?, F?, PF6?, Br? and HSO4? were tested and among them only highly nucleophilic F? anion displayed significant response towards the sensor. Addition of the fluoride anion across the boron atom disrupts the π‐conjugation thereby shifts the absorption wavelength towards the redshift region due to the decrease in the HOMO‐LUMO energy gap and a colour change from yellow to blue is observed under visible light condition. The detection limit of this probe was calculated to be 3.25 × 10—8 M for fluoride anion. The binding constants and the detection limits of the sensor were calculated using absorption titration studies. The silica gel TLC strips dip‐coated by the chemosensor ( 1 ) revealed a colour change from yellow to brick red to naked eye.  相似文献   
69.
70.

The structural, morphological, magnetic, dielectric, and gas analyzing properties are studied in CuFe2O4(Mn–CuFe2O4) substituted spinel ferrite nanoparticles synthesized via evaporation and automatic combustion. The obtained nanoparticles are established to possess a spherical shape. The smallest size of Mn–CuFe2O4 (~9 nm) nanoparticles is achieved at using automatic combustion. X-ray diffraction and Mössbauer spectroscopy reveal that the crystal lattice constant and the Mn–CuFe2O4 nanoparticle size are larger at augmenting the annealing temperature from 600 to 900°С. The dielectric permeability and losses of Mn–CuFe2O4 nanoparticles are studied at various synthesis conditions and temperatures of annealing. Various aspects of gas sensibility of synthesized Mn–CuFe2O4 nanoparticles are tested, as well. The maximum response to the presence of liquefied petroleum gas is 0.28 at the optimum working temperature of 300°C for Mn–CuFe2O4 nanoparticles obtained via automatic combustion and it is 0.23 at 250°C for deposited nanoparticles.

  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号