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   139123篇
  免费   12034篇
  国内免费   9312篇
化学   83295篇
晶体学   1876篇
力学   8353篇
综合类   658篇
数学   16664篇
物理学   49623篇
  2023年   1512篇
  2022年   3034篇
  2021年   3486篇
  2020年   3836篇
  2019年   3963篇
  2018年   4202篇
  2017年   4093篇
  2016年   5332篇
  2015年   4257篇
  2014年   5721篇
  2013年   7743篇
  2012年   8479篇
  2011年   8974篇
  2010年   6702篇
  2009年   6719篇
  2008年   7074篇
  2007年   6519篇
  2006年   6026篇
  2005年   5123篇
  2004年   4452篇
  2003年   3892篇
  2002年   3948篇
  2001年   3487篇
  2000年   2897篇
  1999年   2459篇
  1998年   2299篇
  1997年   2036篇
  1996年   2011篇
  1995年   1712篇
  1994年   1727篇
  1993年   1521篇
  1992年   1488篇
  1991年   1457篇
  1990年   1414篇
  1989年   1281篇
  1988年   1122篇
  1987年   1080篇
  1986年   1039篇
  1985年   1072篇
  1984年   1014篇
  1983年   892篇
  1982年   865篇
  1979年   811篇
  1978年   812篇
  1977年   808篇
  1976年   919篇
  1975年   813篇
  1974年   844篇
  1973年   847篇
  1972年   752篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
3,4-Bis(hydroxymethyl)furan was electrolytically methoxylated during intense cooling. 4,5-Bis(hydroxymethyl)pyridazine was obtained by reaction of the methoxylation product — 2,5-dimethoxy-3,4-bis (hydroxymethyl)-2,5-dihydrofuran — with hydrazine hydrate.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1306–1307, October, 1970.  相似文献   
994.
Summary For the primary alkyl halides (R=from C2H5 to n-C9H19; halogen=C1, Br, and I) and the simplest aryl halides (R=C6H5 and p-CH3C6H4) it was shown that the organomagnesium synthesis in absence of catalysts for the Grignard reaction can be applied with success to the preparation of heteroorganic compounds of elements of Groups II–IV (mercury, boron, aluminum, silicon, germanium, tin, phosphorus, arsenic, antimony).  相似文献   
995.
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   
996.
A novel cyclic ammonium salt, N,N-dimethylpyrrolidinium tetrafluoroborate (P11-BF4), was successfully synthesized for the first time. The smallest cyclic structure of P11-BF4 induced high solubility and conductivity in PC, which can easier enter the micropores of activated carbon and occupy more surface area during charge/discharge process.  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.
An investigation was carried out of the spectral-luminescent and acid-base properties at the stage of formation of the hydrogen bond and during the protonation of compounds with annelated aromatic and heterocyclic rings, obtained by introduction of bridging groups -CH=CH- (I), -CH=N- (II) and -CO-N(CH3)-(III) in the 4- and 2-positions of the molecules of 4-substituted derivatives of 2,5-diphenyloxazole. The nature of the reaction center was estabished, and an analysis was carried out of its sensitivity to the electronic influence of the substituents, and the reasons for the observed differences in the properties of compounds of various reaction series are discussed. A comparative analysis of the fluorescent properties of the neutral and protonated forms of the oxazole derivatives studied was made and the conclusion that the generation is possible of laser emission by the protonated forms of compounds of the reaction series (I) was confirmed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 4, pp. 343–349, July–August, 1992.  相似文献   
999.
9-Methyl-3,3a,4,5-tetrahydro-6H-thieno[2,3-b]thiocino[4,5-c]isoxazole and 9-hydroxyimino-2-methyl-7-methylene-6, 7,8,9-tetrahydro-5H-thieno[2,3-b]thiocine have been prepared for the first time by the oxidation of 2--alkenylthio-3-thiophenecarbaldoximes with NaOCl. X-ray structural investigations of thieno[2,3-b]thiocine and 2-homomethallylthio-5-methyl-3-thiophenecarbaldoxime have been carried out.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1262–1266, July, 1993.  相似文献   
1000.
A 11-complex of trinitrophenol with benzyldimethylamine (1) and a 111-complex, the product of the interaction of benzyldimethylamine, glycidyl phenyl ether, and phenol (2), have been synthesized and characterized by means of X-ray analysis. Complex1 is a precursor for the catalysts of selective isocyanate cyclotrimerization, while complex2 is one of those catalysts. In the crystal structure complex1 forms H-bonded cationic-anionic aggregates with proton transfer from phenol to the N-atom of the tertiary amine. Complex2 crystallizes as a monohydrate (2a) with a strong H-bond between the quaternary ammonium and phenolic components, and exists in crystals in the form of globular H-bonded dimeric hydrates. Possible pathways of dissociation of complex2a are discussed. The data obtained may be used as a basis for further interpretation of the regularities of cyclotrimerization of isocyanates catalyzed by complexes of this type.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No 1, pp. 92–97, January, 1993.  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