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   566692篇
  免费   4425篇
  国内免费   1629篇
化学   284343篇
晶体学   7968篇
力学   27869篇
综合类   29篇
数学   78789篇
物理学   173748篇
  2021年   5137篇
  2020年   5590篇
  2019年   6330篇
  2018年   9920篇
  2017年   10308篇
  2016年   12548篇
  2015年   6913篇
  2014年   11119篇
  2013年   24533篇
  2012年   21805篇
  2011年   24554篇
  2010年   18113篇
  2009年   17786篇
  2008年   21712篇
  2007年   21564篇
  2006年   19704篇
  2005年   22314篇
  2004年   20913篇
  2003年   17217篇
  2002年   14691篇
  2001年   15009篇
  2000年   11527篇
  1999年   8966篇
  1998年   7804篇
  1997年   7658篇
  1996年   7094篇
  1995年   6368篇
  1994年   6384篇
  1993年   6118篇
  1992年   6565篇
  1991年   7046篇
  1990年   6709篇
  1989年   6612篇
  1988年   6435篇
  1987年   6220篇
  1986年   5938篇
  1985年   7501篇
  1984年   7853篇
  1983年   6591篇
  1982年   6884篇
  1981年   6390篇
  1980年   6077篇
  1979年   6574篇
  1978年   6782篇
  1977年   6682篇
  1976年   6676篇
  1975年   6356篇
  1974年   6182篇
  1973年   6493篇
  1972年   4755篇
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
41.
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   
42.
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   
43.
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   
44.
45.
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   
46.
6-Halogeno-1,3-benzodioxane analogs of chalcones have been synthesized by various methods. Flavanone and flavone analogs of silandrin and hydnocarpin have been obtained from them. The structures of the new flavonoids have been shown by their PMR spectra.  相似文献   
47.
The electron-impact mass spectra of a number of phenylpropanoids and iridoid glycosides containing a phenylethanoid fragment in their structure have been studied. This has revealed features of the formation of fragments of 4-hydroxyphenylethyl and 3,4-dihydroxyphenylethyl alcohols and also of fragments of phenylethylamine and indolylethylamine derivatives present in the structures of the compounds investigated.  相似文献   
48.
Some features of the kinetics of the hydrolysis of apple protopectin have been studied on the basis of an analysis of monosaccharide residues of the pectin substances and determinations of characteristic viscosity. It has been shown that the fine regulation of the physicochemical parameters of the hydrolysis of protopectin leads to the formation of pectin substances with different properties.  相似文献   
49.
50.
The phsicochemical indices and the fatty-acid and triacylglycerol compositions of cottonseed and palm oils, of cottonseed oil hydrogenate, and of compositions based on them have been determined with the aid of IR and UV spectroscopies, GLC analysis, and chemical transformations. The compositions and amounts of the main classes of lipids in the initial oils and the hydrogenate have been studied. It has been found that in the process of hydrogenation the amount of oxygenated components falls. The work was conducted with the financial support of the Uzbek Fund for Fundamental Investigations.  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