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   529篇
  免费   10篇
  国内免费   2篇
化学   440篇
晶体学   8篇
力学   2篇
数学   39篇
物理学   52篇
  2023年   3篇
  2021年   7篇
  2020年   10篇
  2019年   11篇
  2018年   6篇
  2017年   7篇
  2016年   16篇
  2015年   15篇
  2014年   12篇
  2013年   16篇
  2012年   35篇
  2011年   41篇
  2010年   19篇
  2009年   23篇
  2008年   34篇
  2007年   32篇
  2006年   29篇
  2005年   29篇
  2004年   27篇
  2003年   23篇
  2002年   23篇
  2001年   4篇
  2000年   5篇
  1999年   4篇
  1998年   10篇
  1997年   4篇
  1996年   9篇
  1994年   2篇
  1993年   1篇
  1992年   5篇
  1991年   3篇
  1990年   7篇
  1989年   4篇
  1988年   7篇
  1987年   6篇
  1986年   7篇
  1985年   7篇
  1984年   5篇
  1983年   1篇
  1982年   3篇
  1981年   2篇
  1980年   4篇
  1979年   5篇
  1978年   3篇
  1975年   2篇
  1974年   1篇
  1973年   2篇
  1971年   1篇
  1969年   2篇
  1954年   4篇
排序方式: 共有541条查询结果,搜索用时 234 毫秒
21.
The methanol extract of the seeds of Phytolacca americana was reinvestigated to yield three new 1,4-benzodioxane-type compounds, americanoic acid methyl ester (1), isoamericanoic acid A methyl ester (2), and 9'-O-methylamericanol A (3) along with the previously isolated neolignans 6-9. The structures of 1-3 were characterized by 2D NMR and long-range selective proton-decoupling (LSPD) techniques. The neuritogenic effects of compounds 1-3, and dicarboxilic acids 4 and 5, which had been previously synthesized with horseradish peroxidase-catalyzed oxidative coupling of caffeic acid, were examined in primary cultured rat cortical neurons. Americanoic acid A methyl ester (1) exhibited neurite outgrowth-promoting activity at concentration of 0.01-1.0 microM, whereas dicarboxilic acids 4 and 5 were found to induce neuritogenesis dose dependently at the concentrations from 0.1 microM to 10 microM.  相似文献   
22.
The simplest chiral portion of sparteine, N,N'-dimethyl-2-endo-methylbispidine, was prepared and evaluated in the asymmetric lithiation-substitution of N-Boc-pyrrolidine. The results indicate that the complete A-ring of sparteine is essential for high levels of asymmetric induction. DFT-QSSR analyses of the diamine/Li(+) complexes and DFT calculations of the pertinent i-PrLi/diamine/N-Boc-pyrrolidine complexes are predictive and provide complementary pictures of the stereochemical features critical to this transformation.  相似文献   
23.
Long chain alkanethiols self-assembled monolayers (SAMs) formed on Au microelectrodes showed higher sensitivity towards defects than the same monolayers on macroelectrodes. The analysis of cyclic voltammetry and electrochemical impedance spectroscopy (EIS) experiments performed on covered microelectrodes were consistent with the formation of pinholes of about 10 nm in diameter. Moreover, the EIS data exhibited a specific behavior that was interpreted invoking the short circuiting of the pinholes impedance by the surrounding surface of the microelectrode in the high frequency domain, whereas in the low frequencies, the surface covered by the SAM was assume to act as an insulator.  相似文献   
24.
25.
1,3-Dipolar cycloaddition between aromatic selenoaldehydes, generated by thermal retro Diels-Alder reaction of anthracene cycloadducts, and nitrile oxides or nitrile imines proceeded efficiently to give the corresponding [3+2] cycloadducts as a single isomer in good yields, being 1,4,2-oxaselenazoles or 1,3,4-selenadiazoles, respectively.  相似文献   
26.
A novel pressure-driven sample injection method was developed as an alternative to electrokinetic injection, and electrophoretic separation was carried out on a microfabricated device employing this method. This method enables a defined volume of liquid dispensing, followed by instantaneous injection driven by pneumatic pressure, greatly simplifying the injection procedure. A particular microstructure, called a "metering chamber", has been designed for the quantitative dispensing of an ultra-low volume of sample liquid; a "hydrophobic passive valve" equipped with an air vent channel is employed for injecting a dispensed sample into the separation channel. The reproducibility of dispensing was 3.3% (n = 15), expressed by the variation of dispensed volumes. The electrophoretic separation of DNA fragments was performed using this injection method, varying the injection volumes from 0.45 to 4.0 nL, and the separation efficiencies were compared. This precise injection method, easily variable in injection volumes, is highly suitable for quantitative as well as qualitative electrophoretic analyses.  相似文献   
27.
28.
29.
New β-fused uracil-porphyrin conjugates were synthesized by the tetramerization of uracil-pyrroles under acidic conditions. Two different synthetic approaches were systematically studied in order to evaluate their efficiency, as well as the possibility to obtain a single regioisomer. Metallation effects were studied for aggregation in solution, and preliminary photophysical experiments were also performed in order to evaluate the potential of these new compounds.  相似文献   
30.
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