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   298024篇
  免费   3524篇
  国内免费   757篇
化学   158892篇
晶体学   4319篇
力学   12849篇
综合类   14篇
数学   34667篇
物理学   91564篇
  2020年   2463篇
  2019年   2600篇
  2018年   3509篇
  2017年   3512篇
  2016年   5330篇
  2015年   3308篇
  2014年   5019篇
  2013年   12152篇
  2012年   9484篇
  2011年   11415篇
  2010年   8111篇
  2009年   7753篇
  2008年   10744篇
  2007年   10870篇
  2006年   10097篇
  2005年   9163篇
  2004年   8379篇
  2003年   7517篇
  2002年   7336篇
  2001年   7922篇
  2000年   6147篇
  1999年   4716篇
  1998年   4189篇
  1997年   4141篇
  1996年   4009篇
  1995年   3708篇
  1994年   3891篇
  1993年   3592篇
  1992年   4118篇
  1991年   3950篇
  1990年   3909篇
  1989年   3766篇
  1988年   3832篇
  1987年   3762篇
  1986年   3557篇
  1985年   4735篇
  1984年   5003篇
  1983年   4186篇
  1982年   4322篇
  1981年   4259篇
  1980年   4249篇
  1979年   4189篇
  1978年   4489篇
  1977年   4424篇
  1976年   4447篇
  1975年   4134篇
  1974年   4143篇
  1973年   4300篇
  1972年   2997篇
  1971年   2542篇
排序方式: 共有10000条查询结果,搜索用时 656 毫秒
161.
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   
162.
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   
163.
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   
164.
On the basis of the results of acid hydrolysis, chromatographic analysis of the hydrolysates, and a study of IR spectra, it has been established that the suspended particles liberated under the action of electrohydraulic shocks on heterodisperse water—plant systems consist of small fragments of the lignocarbohydrate complex of the plant tissue.  相似文献   
165.
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   
166.
167.
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   
168.
169.
Literature information on the formation, chemical composition, and structural features of lignins of herbaceous plants for 1980–1992 is generalized and analyzed. Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, Fax (3712) 62 73 48. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 160–177, March–April, 1994.  相似文献   
170.
The synthesis has been effected of alkyl anabasinoprop-2-yl and alkyl piperidinoprop-2-yl methylphosphonothionates. It has been shown by1H,13C and31P NMR spectroscopies that the anabasine derivatives exist in solution as four diastereomeric forms. The kinetics of the interaction of the compounds synthesized with two types of cholinesterases has been studied.A. S. Sadykov Institute of Bioorganic Chemistry, Academy of Sciences of the Republic of Uzbekistan, Tashkent, Fax 627071. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 70–74, January–February, 1994.  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