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   1398701篇
  免费   39998篇
  国内免费   19293篇
化学   707922篇
晶体学   21114篇
力学   79424篇
综合类   762篇
数学   251001篇
物理学   397769篇
  2021年   16788篇
  2020年   19048篇
  2019年   19442篇
  2018年   14927篇
  2017年   13012篇
  2016年   31511篇
  2015年   24674篇
  2014年   35477篇
  2013年   80806篇
  2012年   44549篇
  2011年   40433篇
  2010年   41791篇
  2009年   44215篇
  2008年   37856篇
  2007年   32393篇
  2006年   39057篇
  2005年   30239篇
  2004年   31154篇
  2003年   29286篇
  2002年   30292篇
  2001年   28532篇
  2000年   25255篇
  1999年   23751篇
  1998年   22347篇
  1997年   22187篇
  1996年   22412篇
  1995年   20321篇
  1994年   19658篇
  1993年   18965篇
  1992年   18606篇
  1991年   18840篇
  1990年   17949篇
  1989年   17839篇
  1988年   17289篇
  1987年   17278篇
  1986年   16172篇
  1985年   22534篇
  1984年   23793篇
  1983年   20044篇
  1982年   21682篇
  1981年   20891篇
  1980年   20209篇
  1979年   20278篇
  1978年   21652篇
  1977年   21201篇
  1976年   20884篇
  1975年   19595篇
  1974年   19246篇
  1973年   19717篇
  1972年   14212篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Molecular n-v complexes of quinoline N-oxides with BF3 have been studied by electronic and IR spectroscopy. In most case the donor centre is the oxygen atom of the N-oxide group.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1087–1092, August, 1994.  相似文献   
102.
We review literature data on synthesis and conversions of heterocyclic compounds using ultrasonic treatment.We analyze the effect of ultrasound on organic reactions involving participation of heterocyclic compounds.A. N. Kosygin State Technical Academy, Moscow 117918. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 723–755, June 1994. Original article submitted December 22, 1993.  相似文献   
103.
We have developed a method for synthesis of N-(3-clzloro-2-quinoxalyl)sulfonamides by reaction of 2,3-dichloroquinoxaline with substituted arylsufonamides. Based on the IR spectra, we have established that in the solid state, the synthesized compounds exist in the form of amide tautomers. Alkylation of these compounds leads to N-metliyl-N-(3-chloro-2-quinoxalyl)arylsulfonamides. We demonstrate the possibility of nucleophilic substitution of the halogen upon treatment with O- and N-nucleophiles. The use of bifunctional nucleophiles leads to condensed quinoxalines.Translated from Khimiya Geterotsiklicheskikh Soedinenii, Vol. 30, No. 3, pp. 387–392, March, 1994.  相似文献   
104.
The reaction of 2-(2 pyridytcarbonyl)benzoic acid with thionyl chloride affords an unexpected product of the intramolecular acylation of the pyridine nitrogen atom, namely, 6,11-dioxo-6,11-dihydrobenzo[blquinotizinium chloride. At the same time, 2-(2-quinotylcarbonyl)benzoic acid forms the expected cyclic acid chloride, namely, 3-(2-gitinotyl)-3-chlorophthalide in this reaction. Both compounds acylate ammonia and primary amines, including those with bulky alkyl groups (tert-butyl, 1-adamantyl, and 1,1,3,3-tetramethylbutyl) with the formation of 2-R-3-hydroxy-3-(2pyridyl- or 2-quinolyl)isoindolines. The protonation of the pyridine nitrogen atom of N-(1,1,3,3-tetramethylbutyl)-2-(2pyridylcarbonyl)benzamide, obtained in the open amide form, is accompanied by the closing of the isoindotinone ring; the deprotonation is accompanied by ring opening.Riga Technical University, Riga LV-1048. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 499–504, April, 1994. Original article submitted March 17, 1994.  相似文献   
105.
106.
Alkylation of 2 furanyl- and 2-thienylalkyl ketoximes with alkyl, allyl, and propargyl halides in the two phase catalytic system sol. K2CO3/C6H6/18-crown-6 at room temperature causes the formation of mixtures of E- and Z-isomers of the corresponding O-ethers with yields of 32–74%. Partial E/Z-isomerization takes place during the reaction.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 18–23, January, 1994.  相似文献   
107.
We have synthesized 5-alkyl(alkoxy)-2-(o-lzydroxy-p-alkoxyphenyl)pyrimidines and considered the effect of the OH group on the inesomorphism. We have found that in binuclear o-hydroxyarylpyrimidines, an ortho OH group forming an intramolecular hydrogen bond increases Tmp, inhibits the appearance of nematic properties, and promotes the appearance of a smectic A.Translated from Khimiya Geterotsiklicheskikh Soedinenii, Vol. 30, No. 1, pp. 90–92, January, 1994.  相似文献   
108.
By changing the conditions of the electrohydraulic treatment of aqueous plant heterodisperse systems comparatively high degradation of the lignin component of plant tissue has been achieved. In addition to free monosaccharides, their reduced forms have been detected in the aqueous phase. Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, fax (3712) 89 14 75. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 795–798, November–December, 1994.  相似文献   
109.
110.
The influence of sunflowerseed and cottonseed oils and also of fractions of normal and oxidized triacylglycerols isolated from them on cytochrome P-450 from rat liver microsomes has been investigated.  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