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   1776篇
  免费   78篇
  国内免费   6篇
化学   1319篇
晶体学   21篇
力学   39篇
数学   280篇
物理学   201篇
  2023年   19篇
  2022年   11篇
  2021年   47篇
  2020年   55篇
  2019年   41篇
  2018年   26篇
  2017年   32篇
  2016年   59篇
  2015年   45篇
  2014年   55篇
  2013年   118篇
  2012年   104篇
  2011年   119篇
  2010年   67篇
  2009年   70篇
  2008年   105篇
  2007年   75篇
  2006年   76篇
  2005年   84篇
  2004年   63篇
  2003年   58篇
  2002年   50篇
  2001年   28篇
  2000年   22篇
  1999年   17篇
  1998年   23篇
  1997年   23篇
  1996年   8篇
  1995年   13篇
  1994年   12篇
  1993年   17篇
  1992年   17篇
  1991年   17篇
  1990年   15篇
  1988年   7篇
  1985年   12篇
  1984年   12篇
  1983年   16篇
  1982年   12篇
  1981年   25篇
  1980年   21篇
  1979年   11篇
  1978年   16篇
  1977年   11篇
  1976年   15篇
  1975年   16篇
  1974年   15篇
  1973年   14篇
  1968年   6篇
  1962年   4篇
排序方式: 共有1860条查询结果,搜索用时 15 毫秒
11.
The primary amino group of 3-(2-aminoethyl)-3-(4-chlorophenyl)-2,3-dihydro-1H-isoindol-1-one ( 1 ) was acylated with acetyl chloride, benzoyl chloride and phenyl acetyl chloride to form the amides 2a -c, respectively. These were cyclized in phosphorus oxychloride to give the 1-substituted-4a-(4-chlorophenyl)-4,4a-dihydropyrimido [6,1-a]isoindol-9(3H)-ones 3a-c . Heating of 1 in formic acid lead to the formation of 4a-(4-chlorophenyl)-4,4a-dihydropyrimido[6,1-a]isoindol-9(3H) -one (3d) . Heating of 1 in the presence of phosgene lead to the formation of 4a-(4-chlorophenyl)-2,3,4,4a-tetrahydropyrimido[6,1-a]isoindole-1, 9-dione (4).  相似文献   
12.
The fragmentation mechanisms of oxazole have been studied in detail on using various experimental techniques (refocusing of metastable ions, deuteration, AP measurements) and by theoretical calculations.  相似文献   
13.
The temperature dependence of the 35Cl nuclear quadrupole resonance spectra of tetrachloro-o-benzoquinone (TOB) is reported. An analysis of the change in line multiplicities suggests that TOB undergoes three phase changes in going from 77 to 330 K. The transition temperatures have been determined.  相似文献   
14.
Real-time optical emission spectroscopy (OES) was used to monitor the deposition of TiN both from mixtures of tetrakis(dimethylamino)titanium (TDMATi)-N2 and TiCl4-H2-N2 in an electron cyclotron resonance chemical vapor deposition system. The accurate control of the ratio of the emission intensities of ionized nitrogen at 391.4 nm and molecular nitrogen at 357.7 nm (N 2 + /N2) led to low temperature deposition of stoichiometric TiN (Ti/N ≈ 1) and very low resistivity in both cases. It was found that high ion density plasmas are crucial for a considerable reduction of the deposition temperature while maintaining good film quality. OES shows that the abundance of certain excited plasma species is not only dependent on the gas mixture and the deposition parameters, such as total pressure and microwave power, but also is strongly affected by the magnetic field configuration. The deposition rate and the film resistivity can be related to the emission intensity ratio, I(N 2 + )/I(N2). Finally, the two processes are compared in terms of the quality of as-deposited and heat-treated films. The comparison shows that the films obtained with TDMATi exhibit lower resistivity and are thermally more stable than with TiCl4.  相似文献   
15.
The synthesis of a series of novel analogues of the 3-substituted-2,3-dihydro-1,4-dioxino[2,3-b]pyridine core, modified in the non-aromatic ring, is described. Due to the presence of a versatile hydroxymethyl group in their structure, these novel scaffolds are attractive intermediates for the preparation of potential new therapeutic agents.  相似文献   
16.
The authors describe the synthesis of decahydro-4-quinolinones and N-substituted 4-alkynyl-, 4-allyl- and 4-alkyldecahydro-4-quinolinols and discuss the stereochemical problems of this compounds.  相似文献   
17.
The two isomeric tris(trichlorothienyl)carbinols have been synthesized. Upon treatment with acid the tris(trichlorothienyl)carbenium ions were formed, which upon reduction with iodide gave the tris(trichlorothienyl)methyl radicals, which were studied by epr-spectroscopy. The β-isomer, the tris(2,4,5-trichloro-3-thienyl)methyl radical, was much more stable than the α-isomer and showed great similarity to the perchlorotrityl radical.  相似文献   
18.
By cyclization in aqueous media, 2-aminomethyl 1-alcynyl 1-cyclohexanols lead to perhydroisoindolinium or perhydroisoquinolinium salts. The stereochemistry of isolated diastereoisomers is elucidated. A stereoselectivity favouring trans junction of the rings is pointed out.  相似文献   
19.
An original one-pot ayntheais of 4,7-diazaindoles is achieved by metalation of methylpyrazines or methylquinoxaline wich are then condensed with an aromatic nitrile: an intermediate imine-enamine is formed which leads, by intramolecular cyclization, to diazaindoles.  相似文献   
20.
The present paper deals with the experimental and theoretical determination of energies of conjugation of organic molecules.A computation process has been applied, first to simple molecules comprising an hydrocarbon (styrene), an oxygen function (benzaldehyde) and a nitrogen function (aniline). The results are compared with those obtained from the literature.The application of this computation process to naphthoic acids permits the determination of energies of conjugation and isomerization of these molecules.  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