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   1326931篇
  免费   29633篇
  国内免费   7668篇
化学   662476篇
晶体学   20197篇
力学   74315篇
综合类   101篇
数学   240382篇
物理学   366761篇
  2021年   13475篇
  2020年   15909篇
  2019年   16038篇
  2016年   27521篇
  2015年   20652篇
  2014年   30357篇
  2013年   74257篇
  2012年   37318篇
  2011年   33512篇
  2010年   36363篇
  2009年   38807篇
  2008年   33145篇
  2007年   28496篇
  2006年   35615篇
  2005年   27358篇
  2004年   29032篇
  2003年   27689篇
  2002年   28923篇
  2001年   28038篇
  2000年   24539篇
  1999年   22420篇
  1998年   20943篇
  1997年   21002篇
  1996年   21078篇
  1995年   19179篇
  1994年   18613篇
  1993年   18161篇
  1992年   18062篇
  1991年   18378篇
  1990年   17592篇
  1989年   17679篇
  1988年   17210篇
  1987年   17168篇
  1986年   16112篇
  1985年   22518篇
  1984年   23780篇
  1983年   19968篇
  1982年   21657篇
  1981年   20882篇
  1980年   20238篇
  1979年   20417篇
  1978年   21704篇
  1977年   21319篇
  1976年   21097篇
  1975年   19747篇
  1974年   19480篇
  1973年   19950篇
  1972年   14405篇
  1968年   12245篇
  1967年   12581篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
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   
892.
893.
We have measured the PMR spectra of 5, 7-diaryl-1, 4-diazabicyclo[4. ].O]hept-4-enes. We have studied the structure of the 5-(4-nitrophenyl)-7-phenyl derivative by x-ray diffraction. We show that the six-membered heterocycle has the half-chair form. The 6-H and 7-H protons of the aziridine ring are trans, while the 7-aryl is exo relative to the six-membered ring. Introduction of substituents into the aromatic ring has practically no effect on the geometry of the bicycle.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1117–1124, August, 1994.For Communication 11, see [1].  相似文献   
894.
895.
896.
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   
897.
Halogen in positions 3 and 5 of the furan ring in ftirfurylidenedioxanediones is readily replaced by dithiocarbonate, dithiocarbatnate, dithiophosphate, thiocyanate, and azide anions, and by thiourea, and secondary and tertiary amines. The conformation of the initial halogen derivative is retained in the substitution products.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 327–334, March, 1994.  相似文献   
898.
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   
899.
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   
900.
The yields and fatty acid compositions of the free lipids and lipids weakly and strongly bound to proteins from seeds of cottonplant varieties resistant and susceptible to verticillium wilt have been determined and the changes in these indices in the seeds of the infected plants have been measured. It has been established that the seeds of healthy plants of the two varieties are similar in the qualitative compositions of the lipids and fatty acids but differ with respect the amounts of lipids in them bound with protein in different ways and with respect to the amounts of the individual fatty acids. The observed changes in the lipid and fatty-acid compositions depend on the degree of resistance of the variety to wilt.  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