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   336637篇
  免费   3640篇
  国内免费   1067篇
化学   162460篇
晶体学   4388篇
力学   18259篇
综合类   14篇
数学   63163篇
物理学   93060篇
  2021年   2519篇
  2020年   2748篇
  2019年   3215篇
  2018年   14371篇
  2017年   14171篇
  2016年   12049篇
  2015年   4331篇
  2014年   5886篇
  2013年   11933篇
  2012年   13113篇
  2011年   21044篇
  2010年   13654篇
  2009年   14035篇
  2008年   16634篇
  2007年   18610篇
  2006年   9545篇
  2005年   9585篇
  2004年   9204篇
  2003年   8806篇
  2002年   7683篇
  2001年   6922篇
  2000年   5309篇
  1999年   3949篇
  1998年   3537篇
  1997年   3565篇
  1996年   3534篇
  1995年   2987篇
  1994年   3134篇
  1993年   3052篇
  1992年   3326篇
  1991年   3292篇
  1990年   3158篇
  1989年   3232篇
  1988年   3128篇
  1987年   3134篇
  1986年   2956篇
  1985年   3843篇
  1984年   3922篇
  1983年   3225篇
  1982年   3366篇
  1981年   3284篇
  1980年   3083篇
  1979年   3455篇
  1978年   3390篇
  1977年   3548篇
  1976年   3504篇
  1975年   3258篇
  1974年   3180篇
  1973年   3237篇
  1972年   2384篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
12.
Seven new copper(II) complexes of type [Cu(A)(L)]?H2O (A = sparfloxacin, ciprofloxacin, levofloxacin, gatifloxacin, pefloxacin, ofloxacin, norfloxacin; L = 5‐[(3‐chlorophenyl)diazenyl]‐4‐hydroxy‐1,3‐thiazole‐2(3H)‐thione) were synthesized and characterized using elemental and thermogravimetric analyses, and electronic, electron paramagnetic resonance (EPR), Fourier transform infrared and liquid chromatography–mass spectroscopies. Tetrahedral geometry around copper is assigned in all complexes using EPR and electronic spectral analyses. All complexes were investigated for their interaction with herring sperm DNA utilizing absorption titration (Kb = 1.27–3.13 × 105 M?1) and hydrodynamic volume measurement studies. The studies suggest the classical intercalative mode of DNA binding. The cleavage reaction on pUC19 DNA was monitored by agarose gel electrophoresis. The results indicate that the Cu(II) complexes can more effectively promote the cleavage of plasmid DNA. The superoxide dismutase mimic activity of the complexes was evaluated by nitroblue tetrazolium assay, and the complexes catalysed the dismutation of superoxide at pH = 7.8 with IC50 values in the range 0.597–0.900 μM. The complexes were screened for their in vitro antibacterial activity against five pathogenic bacteria. All the complexes are good cytotoxic agents and show LC50 values ranging from 5.559 to 11.912 µg ml?1. All newly synthesized Cu(II) complexes were also evaluated for their in vitro antimalarial activity against Plasmodium falciparum strain (IC50 = 0.62–2.0 µg ml?1). Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   
13.
14.
Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis.  相似文献   
15.
16.
17.
The results of investigation of optical image detectors designed for the largest problem, near-VUV, range of the spectrum are presented. The possibility of using a dual-stage image detection system to appreciably lower the sensitivity threshold and make computer data processing feasible is considered. The integration of a UV module into a wideband image detector is studied.  相似文献   
18.
19.
20.
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