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   1554篇
  免费   57篇
  国内免费   11篇
化学   1038篇
晶体学   44篇
力学   41篇
数学   191篇
物理学   308篇
  2023年   11篇
  2022年   27篇
  2021年   38篇
  2020年   47篇
  2019年   34篇
  2018年   33篇
  2017年   36篇
  2016年   54篇
  2015年   36篇
  2014年   53篇
  2013年   106篇
  2012年   98篇
  2011年   109篇
  2010年   67篇
  2009年   55篇
  2008年   62篇
  2007年   73篇
  2006年   79篇
  2005年   58篇
  2004年   50篇
  2003年   44篇
  2002年   33篇
  2001年   14篇
  2000年   15篇
  1999年   17篇
  1998年   8篇
  1997年   10篇
  1996年   13篇
  1995年   9篇
  1994年   14篇
  1993年   12篇
  1992年   12篇
  1991年   10篇
  1990年   13篇
  1989年   14篇
  1988年   10篇
  1987年   13篇
  1986年   9篇
  1985年   13篇
  1984年   16篇
  1983年   10篇
  1982年   9篇
  1981年   16篇
  1980年   29篇
  1979年   19篇
  1978年   17篇
  1977年   16篇
  1976年   17篇
  1975年   7篇
  1957年   6篇
排序方式: 共有1622条查询结果,搜索用时 109 毫秒
31.
Dimers of the pyrrole amino acid (Paa), 5-(aminomethyl)pyrrole-2-carboxylic acid, and its derivatives having Lys anchored on N- and C-termini bind in the minor groove of DNA with considerable apparent binding affinities. When the Lys unit is attached to the C-terminus, the resulting ligand binds to ds-DNA with twice the affinity, of the order of 105, than the one carrying two positive charges at the same end.  相似文献   
32.
Acetals are obtained in good yields by treatment of aldehydes and ketones with trialkyl orthoformate and the corresponding alcohol in the presence of 0.1 mol % Bi(OTf)3.4H2O. A simple procedure for the formation of acetals of diaryl ketones has also been developed. The conversion of carbonyl compounds to the corresponding 1,3-dioxolane using ethylene glycol is also catalyzed by Bi(OTf)3.4H2O (1 mol %). Two methods, both of which avoid the use of benzene, have been developed.  相似文献   
33.
A reaction condition is established which determines the nature of the products in the molybdate-hydroxylamine-cyanide reaction. With hydroxylamine always used in excess, it is the hydroxyl ion concentration of the reaction mixture which plays a vital role in determining whether K4[Mo(NO)(CN)5] or K2[Mo(NO)(CN)5] is obtained exclusively. The latter product is hereby reported for the first time. Its powder diffractogram being typical of a cubic system, a gross structural characterization has been made possible. The former under aqueous, aerobic conditions yields a new product, (NMe4)2[Mo(NO)(CN)4].  相似文献   
34.
Water quality data set from the alluvial region in the Gangetic plain in northern India, which is known for high fluoride levels in soil and groundwater, has been analysed by chemometric techniques, such as principal component analysis (PCA), discriminant analysis (DA) and partial least squares (PLS) in order to investigate the compositional differences between surface and groundwater samples, spatial variations in groundwater composition and influence of natural and anthropogenic factors. Trilinear plots of major ions showed that the groundwater in this region is mainly of Na/K-bicarbonate type. PCA performed on complete data matrix yielded six significant PCs explaining 65% of the data variance. Although, PCA rendered considerable data reduction, it could not clearly group and distinguish the sample types (dug well, hand-pump and surface water). However, a visible differentiation between the water samples pertaining to two watersheds (Khar and Loni) was obtained. DA identified six discriminating variables between surface and groundwater and also between different types of samples (dug well, hand pump and surface water). Distinct grouping of the surface and groundwater samples was achieved using the PLS technique. It further showed that the groundwater samples are dominated by variables having origin both in natural and anthropogenic sources in the region, whereas, variables of industrial origin dominate the surface water samples. It also suggested that the groundwater sources are contaminated with various industrial contaminants in the region.  相似文献   
35.
[reaction: see text] A novel, mild, ecofriendly protocol for the deoxygenation of epoxides to alkenes using indium metal and indium(I) chloride or ammonium chloride in alcohol has been developed. It was necessary for the presence of good radical-stabilizing groups adjacent to the oxirane ring for the deoxygenation reaction to occur. It is proposed that this reaction occurs through an SET process with indium as an electron donor.  相似文献   
36.
Journal of Theoretical Probability - In this work, we consider the stochastic generalized Burgers–Huxley equation perturbed by space–time white noise and discuss the global solvability...  相似文献   
37.
38.
Kidwai  M.  Mohan  R.  Saxena  S. 《Russian Chemical Bulletin》2003,52(11):2457-2460
The microwave-assisted dry media technology was utilized for the synthesis of some novel derivatives in the Hantzsch—Biginelli reaction. In this eco-friendly approach, the usage of organic solvents and hazardous reagents resulting in corrossion of materials was minimized, giving high yields of products within a short reaction time.  相似文献   
39.
From the seeds of Phytolacca americana two new lignans, Americanin B and D, have been isolated and their structures elucidated mainly by spectroscopic methods  相似文献   
40.
A new class of five membered heterocycles, thiadiazoles, triazoles and oxadiazoles were prepared from sulfonyl acetic acids via acid hydrazides.  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