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   276297篇
  免费   1193篇
  国内免费   583篇
化学   122342篇
晶体学   3834篇
力学   16953篇
综合类   24篇
数学   45816篇
物理学   89104篇
  2019年   2521篇
  2018年   11009篇
  2017年   11735篇
  2016年   8001篇
  2015年   2731篇
  2014年   3985篇
  2013年   7543篇
  2012年   9693篇
  2011年   17909篇
  2010年   11962篇
  2009年   12305篇
  2008年   15968篇
  2007年   19162篇
  2006年   6124篇
  2005年   10614篇
  2004年   7765篇
  2003年   7699篇
  2002年   6174篇
  2001年   5452篇
  2000年   4230篇
  1999年   3038篇
  1998年   2792篇
  1997年   2631篇
  1996年   2593篇
  1995年   2299篇
  1994年   2209篇
  1993年   2261篇
  1992年   2294篇
  1991年   2620篇
  1990年   2544篇
  1989年   2637篇
  1988年   2495篇
  1987年   2543篇
  1986年   2352篇
  1985年   2926篇
  1984年   2958篇
  1983年   2572篇
  1982年   2639篇
  1981年   2512篇
  1980年   2330篇
  1979年   2683篇
  1978年   2865篇
  1977年   2875篇
  1976年   3056篇
  1975年   2793篇
  1974年   2753篇
  1973年   2977篇
  1972年   2439篇
  1971年   2209篇
  1970年   2081篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
171.
In a solution of DMF, 2-carbethoxymethyl-4H-3,1-benzoxazin-4-one reacts with primarily aromatic amines basically with the formation of the corresponding 2-carbethoxymetlzyl-3-arylquinazolin-4(3H)-ones. Possible mechanisms of these chemical transformations are reported and discussed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 225–228, February, 1994. Original article submitted June 28, 1993.  相似文献   
172.
The mass spectral behavior of 1, 2-di (mesoetioporphyr inyl-1)- and 1, 2-di (mesooctaethylporphyrlnyl) ethanes and ethylenes, their Cu and Ni complexes, as well as Ni Schiff bases of 1, 2-di (mesooctaethylporphyrinyl)ethane complexes, under ionization by electron impact and bombardment by accelerated atoms, was studied. Peaks of molecular ions, for which the basic decomposition pathways were established on the basis of an analysis of the parent and daughter ions, were observed in the electron impact mass spectra of all these compounds.For Communication 29, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 24–29, January, 1994.  相似文献   
173.
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   
174.
175.
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   
176.
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   
177.
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   
178.
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   
179.
180.
The main directions of fragmentation and the EI of six pseudoguaianolides related to carpesiolin have been studied. It has been established that acylation of the OH group at C-6 suppresses the selective breakdown of the guaiane skeleton, while the simultaneous presence of AcO groups at C-2 and C-4 favors the formation of a number of characteristic ions. The hydrogenation of the exomethylene group has no appreciable influence on direction of fragmentation.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 530–535, July–August, 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