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   269583篇
  免费   2098篇
  国内免费   624篇
化学   143300篇
晶体学   4195篇
力学   11980篇
综合类   9篇
数学   29641篇
物理学   83180篇
  2019年   2359篇
  2018年   3160篇
  2017年   3154篇
  2016年   4543篇
  2015年   2696篇
  2014年   4342篇
  2013年   10810篇
  2012年   8188篇
  2011年   10027篇
  2010年   7155篇
  2009年   6928篇
  2008年   9405篇
  2007年   9603篇
  2006年   8960篇
  2005年   8149篇
  2004年   7508篇
  2003年   6779篇
  2002年   6691篇
  2001年   7343篇
  2000年   5673篇
  1999年   4377篇
  1998年   3866篇
  1997年   3851篇
  1996年   3591篇
  1995年   3347篇
  1994年   3497篇
  1993年   3202篇
  1992年   3676篇
  1991年   3673篇
  1990年   3608篇
  1989年   3474篇
  1988年   3575篇
  1987年   3474篇
  1986年   3294篇
  1985年   4371篇
  1984年   4644篇
  1983年   3885篇
  1982年   4014篇
  1981年   3937篇
  1980年   3929篇
  1979年   3886篇
  1978年   4199篇
  1977年   4128篇
  1976年   4186篇
  1975年   3861篇
  1974年   3897篇
  1973年   4056篇
  1972年   2851篇
  1971年   2385篇
  1970年   2182篇
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
971.
A method is suggested for isomerization of -acetylenic ketones with migration of the oxo group from position 1 to position 3 and of the triple bond from position 2 to position 1 of the ketoacetylenic fragment; it consists of addition of Et2NH at the triple bond of the ketone, conversion of the adduct to the imminium salt with POCl3, followed by alkaline hydrolysis. The isomerization is achieved in one preparative stage without isolation of intermediate compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2094–2098, September, 1990.  相似文献   
972.
The reaction of the acid chlorides of alkylenebisphosphonic acids with ethylene glycol and ethylene chlorohydrin gave ethylene glycol and -chloroethyl esters of alkylenebisphosphonic acids. The quaternization of these products leads to choline esters of alkylenebisphosphonic acids.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1424–1429, June, 1990.  相似文献   
973.
Unlike 3-acylindoles, 3-acyl-2-aminoindoles display high selectivity on being electrophilically attacked in the benzene ring, and are substituted at the 6-position. At the same time, an unco-subsitution of the acyl group takes place (to the greatest extent — the formyl group). Direct bromination of 3-acyl- and 3-cyano-2-aminoindoles provides the 6-bromo- and 6,4-dibromo-2-aminoindoles and their derivatives.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 486–490, April, 1990.  相似文献   
974.
In this article we derive a segment-level formula for the matrix elements of the U(2n) generators in a basis symmetry adapted to the subgroup U(n) × U(2) (i.e., spin-orbit basis), for the representations appropriate to many-electron systems. This enables the direct evaluation of the matrix elements of spin-dependent Hamiltonians.  相似文献   
975.
The reactions of thionyl and sulphuryl chlorides on isoflavones have been found to give a number of new chloroisoflavones and isoflavanones, whose structures have been elucidated on the basis of their spectral analytical data.  相似文献   
976.
Dimethyl α-bromo-o-benzenediacetate ( 1 ) condensed with hydrazine and acetylhydrazine to give respectively 1-carbomethoxy-2-amino-1,4-dihydro-3-(2H)isoquinolinone (2) and its N-acetyl derivative ( 9 ). Replacement of the bromine atom of 1 with the N-1-methylhydrazino ( 3 ) and the N-1-phenylhydrazino ( 5 ) groups occurred by allowing 1 to react respectively with methylhydrazine and phenylhydrazine. In the latter case alkylation by 1 at the N-2 also occurred which led to the formation of the 2-phenylaminoisoquinolinone ( 8 ). Derivatives 3 and 5 smoothly cyclized to the 1-earbomethoxy-5(H)-1,2,3,4-tetrahydro-2,3-benzodiazepin-4-ones 4 and 6 . A series of derivatives of 2 were also pharmacologically tested as antiinflammatory and CNS depressant agents.  相似文献   
977.
The phase diagram of the acrylic acid–acrylamide system as determined by DTA and the supporting x-ray diffraction data are presented. This diagram shows that the system forms a 1 : 1 addition compound which decomposes above 0°C. There is also one eutectic point at a 67 mole-% acrylic acid composition.  相似文献   
978.
Zusammenfassung Das in der vorangehenden Mitt. vonE. Späth undF. Gandini-Kesztler beschriebene Nebenalkaloid wird auf synthetischem Wege mit dem 6-Hydroxy-peganin identifiziert. Die phenolische OH-Gruppe steht also, bezogen auf Formel III, nicht an der Stelle, welche dem CH3O des Harmins entspricht.  相似文献   
979.
Using simple physical arguments, a local spin-polarized exchange potential, Vxσ, is constructed from the single-particle Hartree–Fock (HF ) potentials (generalized Slater method) that reduces to the usual Kohn–Sham (KS ) result in the uniform gas limit. Numerical results for 10 closed subshell atoms demonstrate that the total energy calculated employing this Vxσ is closer to the exact KS results than those of other standard exchange approximations with electron densities and highest occupied orbital eigenvalues that closely approximate the HF results.  相似文献   
980.
Taking perfluoro-1,1-dialkylbenzocyclobutenes as examples, it has been shown to be possible to enlarge the four-membered ring in polyfluorobenzocyclobutenes to five-membered in the presence of SbF5, by cleavage of the four-membered ring followed by cyclization of the resulting polyfluorostyrene to the polyfluoroindane, which then undergoes further reactions. Perfluoro-1-methyl-1-ethylbenzocyclobutene isomerizes at 50°C in the presence of SbF5 to perfluoro-,-o -trimethylstyrene, which is reversibly converted at 130°C into perfluoro-1,2-dimethylindane. Perfluoro-l,l-diethylbenzocyclobutene isomerizes at 130°C in the presence of SbF5 to give perfluoro- -ethyl-,o -dimethylstyrene, which at 170°C gives perfluoro-2-methyl-3-ethylindene- and perfluoro-2-methyl-3-ethyl-4,5,6,7-tetrahydroindene. The last two compounds, together with perfluoro-o-dipropylbenzene, are obtained from per fluoro-1,2-diethylbenzocyclobutene-with SbF5 at 170°C. From perfluoro-1-methyl-2-ethylbenzocyclobutene with SbF5 at 95°C there is obtained perfluoro-1-ethylindane, while at 130°C, in addition to the latter compound, there are obtained perfluorinated 1,1-dimethylindane, 1,2-dimethylindane, ,,o-trimethylstyrene, 2,3-dimethylindene, and 2,3-dimethyl-4,5,6,7-tetrahydroindene.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1114–1120, May, 1990.  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