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   583088篇
  免费   5926篇
  国内免费   1761篇
化学   313789篇
晶体学   8677篇
力学   25136篇
综合类   19篇
数学   69403篇
物理学   173751篇
  2021年   4863篇
  2020年   5453篇
  2019年   5961篇
  2018年   7850篇
  2017年   7696篇
  2016年   11437篇
  2015年   7096篇
  2014年   10967篇
  2013年   26486篇
  2012年   20091篇
  2011年   24697篇
  2010年   17528篇
  2009年   17236篇
  2008年   22886篇
  2007年   22891篇
  2006年   21250篇
  2005年   19143篇
  2004年   17530篇
  2003年   15585篇
  2002年   15414篇
  2001年   16980篇
  2000年   13067篇
  1999年   10180篇
  1998年   8558篇
  1997年   8341篇
  1996年   8015篇
  1995年   7020篇
  1994年   7010篇
  1993年   6887篇
  1992年   7580篇
  1991年   7730篇
  1990年   7357篇
  1989年   7176篇
  1988年   7198篇
  1987年   7041篇
  1986年   6698篇
  1985年   8723篇
  1984年   8993篇
  1983年   7145篇
  1982年   7431篇
  1981年   7309篇
  1980年   6814篇
  1979年   7347篇
  1978年   7632篇
  1977年   7488篇
  1976年   7439篇
  1975年   6930篇
  1974年   6907篇
  1973年   7010篇
  1972年   4872篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
951.
952.
953.
Data were obtained on the composition of products and kinetics of the reaction of SO2 with Ph(R)CN2 (R=Ph, Me) in solutions of isooctane, 1,4-di-oxane, and ethanol at 295 K. A mechanism was proposed for the process in accordance with which the sulfene being formed participates in concurrent reactions with the starting reagents. The relative rate constants for the process stages were determined.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2496–2500, November, 1990.  相似文献   
954.
955.
An apparatus for the deuteration of solid samples and for recording their IR spectra in a current of D2O vapor, and an IR-spectroscopic method of determining the degree of deuteration (q) of the hydroxy groups of pectin substances (PSs) are described. It has been established that there are no zones of structurally different nature in films of derivatives of PSs at the carboxy group. The rate of deuterium-exchange of the water of hydration of PSs is higher than for the hydroxy groups of the pyranose rings. The rate of deuterium exchange depends on the density of packing of the polymer chains of the pectin derivatives.Center for the Automation of Scientific Investigations and Metrology, Academy of Sciences of the Moldavian SSR, Kishinev. Translated from Khimiya Priorodnykh Soedinenii, No. 3, pp. 343–348, May–June, 1987.  相似文献   
956.
On P? P Containing Cyclic Compounds. II. Reaction of N,N'-dimethylthiourea with phenyl dichlorophosphane yields 1.4-dimethyl-2. 3-diphenyl-1.4.2.3-diazadiphospholidine-5-thione-2-sulfide ( II ). The compound is characterized by its nmr, vibration and mass spectra as well as the results of an X-ray structural analysis. Byproduct of the reaction is N-(N,N'-dimethylcarbamimidoyl)-N,N'-dimethyl thiourea hydrochloride ( III ), which crystallizes with pyridinium chloride. Molecular and crystal structure are reported. With methyl dichlorophosphane N,N'-dimethylthiourea forms 1.2.3.4-tetramethyl-1.4.2.3-diazadiphospholidine-5-thione-2-sulfide ( IV ), which is characterized by its nmr and vibration spectra.  相似文献   
957.
958.
959.
Reaction of phenyl-ß-naphthylamines with formaldehyde and acetone in the presence of HCl and nitrobenzene in butanol solution leads to cyclization involving the naphthalene and phenyl rings, and formation of N-aryllepidine salts. 15 lepidine and 5, 6-benzoepidine salts are synthesized and characterized by their UV absorption spectra, as well as by their picrates.For Part LIII see [3].  相似文献   
960.
Chromatographic characteristics of urinary metabolites of theophylline were studied by two-dimensional thin-layer chromatography, high-performance liquid chromatography and gas chromatography--mass spectrometry. Quantitative date for the urinary metabolites of theophylline in asthmatic children are given. It was shown that 1,3-dimethyluric acid is the predominant excretory product. In addition, smaller amounts of 1-methyluric acid, 3-methylxanthine and unchanged theophylline were found. Excretory patterns after theophylline ingestion before and during the administration of allopurinol in asthma patients and in rats suggest the existence of three metabolic pathways of theophylline. The administration of this drug to a patient with xanthine oxidase of theophylline. The administration of this drug to a patient with xanthine oxidase deficiency resulted in the excretion of 1-methyluric acid in addition to 1,3-dimethyluric acid, 3-methylxanthine, 1-methylxanthine and unchanged theophylline. It was concluded that in man the oxidation of theophylline is not catalysed by xanthine oxidase.  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