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   456668篇
  免费   4293篇
  国内免费   1359篇
化学   231031篇
晶体学   7471篇
力学   21155篇
综合类   9篇
数学   55265篇
物理学   147389篇
  2021年   3932篇
  2020年   4292篇
  2019年   4818篇
  2018年   6472篇
  2017年   6778篇
  2016年   9552篇
  2015年   5495篇
  2014年   9179篇
  2013年   21510篇
  2012年   16355篇
  2011年   19655篇
  2010年   14493篇
  2009年   14401篇
  2008年   17864篇
  2007年   17712篇
  2006年   16518篇
  2005年   14461篇
  2004年   13453篇
  2003年   11997篇
  2002年   11878篇
  2001年   14018篇
  2000年   10440篇
  1999年   8150篇
  1998年   6776篇
  1997年   6575篇
  1996年   6198篇
  1995年   5493篇
  1994年   5447篇
  1993年   5232篇
  1992年   5879篇
  1991年   5999篇
  1990年   5737篇
  1989年   5620篇
  1988年   5369篇
  1987年   5556篇
  1986年   5178篇
  1985年   6697篇
  1984年   6813篇
  1983年   5782篇
  1982年   5926篇
  1981年   5531篇
  1980年   5457篇
  1979年   5857篇
  1978年   5893篇
  1977年   5808篇
  1976年   5806篇
  1975年   5555篇
  1974年   5493篇
  1973年   5675篇
  1972年   3920篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
991.
A series of 7-carboxy-3-R-1,5-dinitro-3-azabicyclo[3.3.1]non-6-enes was synthesized by reduction of 3,5-dinitrobenzoic acid with sodium borohydride followed by Mannich reaction with formaldehyde and primary amines. The mechanism of decomposition under electron impact of the 3-azabicyclo[3.3.1]nonane was established. Enthalpies of formation of compounds synthesized were calculated by semiempirical PM3 method.  相似文献   
992.
Kinetic regularities of hydrogenation, oxidation with oxygen, and condensation of allyl alcohol in the presence of colloid palladium in situ were revealed. A mechanism of the reactions was proposed.  相似文献   
993.
994.
995.
Carbamoylformhydroxymoyl chlorides were synthesized by the reactions of carbamoylformamide oximes with sodium nitrite in the presence of hydrochloric acid, and their reactivities were examined.  相似文献   
996.
Orlov  I. S.  Moiseeva  A. A.  Butin  E. P.  Egorov  M. P. 《Russian Chemical Bulletin》2002,51(10):1961-1962
An unusual reaction of diaminogermylene and diaminostannylene with sterically hindered phenols which leads to the formation of stable phenoxyl radicals in high concentrations was found. The reaction mechanism was proposed. Amides [(Me3Si)2N]2E (E = Ge, Sn) were investigated electrochemically.  相似文献   
997.
Russian Chemical Bulletin - The monomer [Cr3O(OCOCH=CH2)6]OH was produced by reaction of acrylic acid with fresh chromium hydroxide. This monomer was investigated by a number of physicochemical...  相似文献   
998.
999.
A polarization variant of an intracavity laser spectrometer based on the competition of beams in an argon ion laser, radiating a number of narrow lines in the 275 to 529 nm spectral range, is proposed and described for the spectral analysis of liquid samples, having broad absorption bands, mainly biomolecules in solutions. By careful optimization of the operating conditions, a procedure has been established that results in a minimal measurable absorbance decreased up to 10(-5) units of optical density. The examined spectrometer provides a wide dynamic range of analysis. A determination of trace contents of amino acids in methanol were carried out.  相似文献   
1000.
The molecular structures of the initial compounds and the products of photocyclization involving amino and azomethine groups in perchlorates ofN-amino (1) andN-azomethine (2) derivatives of 2,4,6-triphenylpyridinium cations were studied. Cations1 and2 have an essentially non-coplanar arrangement of the -Ph rings, and the product of photocyclization of2, cation3, is characterized by a flattened structure. It was found that the orientation of the lone electron pair of the N atom with respect to one of the planes of the -Ph rings in molecule1 is more favorable for monocyclization involving the amino group. The structure of molecule2 allows two routes of the photoreaction,viz., O N proton transfer and monocyclization with an -Ph ring. However, the structural features of molecule2 are more favorable for photocyclization.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 296–301, February, 1995.  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