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   439篇
  免费   21篇
化学   363篇
晶体学   3篇
力学   3篇
数学   20篇
物理学   71篇
  2021年   4篇
  2020年   6篇
  2019年   12篇
  2018年   3篇
  2017年   3篇
  2016年   12篇
  2015年   11篇
  2014年   7篇
  2013年   13篇
  2012年   24篇
  2011年   17篇
  2010年   9篇
  2009年   8篇
  2008年   16篇
  2007年   16篇
  2006年   27篇
  2005年   14篇
  2004年   14篇
  2003年   10篇
  2002年   7篇
  2001年   4篇
  2000年   4篇
  1999年   4篇
  1997年   5篇
  1996年   6篇
  1995年   3篇
  1994年   5篇
  1993年   6篇
  1991年   4篇
  1990年   7篇
  1989年   11篇
  1988年   10篇
  1987年   3篇
  1986年   3篇
  1985年   10篇
  1984年   18篇
  1983年   4篇
  1982年   11篇
  1981年   11篇
  1980年   8篇
  1979年   11篇
  1978年   14篇
  1977年   8篇
  1976年   8篇
  1975年   7篇
  1974年   12篇
  1973年   15篇
  1967年   2篇
  1956年   3篇
  1955年   2篇
排序方式: 共有460条查询结果,搜索用时 31 毫秒
111.
Homopolymerization of ethylene and 1-hexene and their copolymerizations were compared to investigate the influence of α-olefin on the enhancement of ethylene polymerization rate (Rp), which is often referred to as the “comonomer” effect. With the two homogeneous Ziegler–Natta catalysts, Et[Ind]2ZrCl2/MAO and (π-C5H5)2ZrCl2/MAO (MAO = methylaluminoxane), hexene causes reduction of Rp—in other words a negative “comonomer” effect. In the case of the high activity MgCl2 supported TiCl3 catalysts there is a slight positive “comonomer” effect; the Rp increases by 25 to 70% with the addition of 15 mol % of hexene. The “comonomer” effects in there catalyst systems are much smaller than that observed for the classical TiCl3 catalyst. © 1993 John Wiley & Sons, Inc.  相似文献   
112.
The reaction of terminal acetylenes with Bu3SnMgMe, Bu3SnA1Et2, or (Bu3Sn)2Zn in the presence of various transition-metal catalysts provides vinylstannanes in good yields. Whereas copper catalysed stannylmagnesation of 4-benzyloxy-1-butyne gives (E)-4-benzyloxy-1-tributylstannyl-1-butene exclusively, palladium catalysed stannylzincation affords 4-benzyloxy-2-tributylstannyl-1-butene preferentially.  相似文献   
113.
The novel, bridged bromocycloheptatriene (4) is obtained by the thermolysis of a tricyclo[6.4.1.01,8]tridecane derivative (6), and the observed anti stereochemistry of the Br atom on C(13) is accounted for by assuming the suprafacial [1,5] hydrogen shift in the intermediate 7. The mother hydrocarbon (8) and syn-methyltropylidene (9), as well as tropone (10), are derived from 4, and the structures of these products are examined spectrometrically. The covalent CBr bond of 4 remains intact under various reaction conditions of nucleophilic substitution, and this is ascribed to the conformational rigidity constrained by the short hexamethylene bridge. The attempted transformation of 4 or 10 to the corresponding tropyllium ion 12 or 14 fails to success and this is again attributed to the bridge effect.  相似文献   
114.
115.
116.
Treatment of lithium trialkylalkynylborates with acetyl chloride gave 2-oxa-3-borolenes (2). The enol borinates were oxidized by Jones reagent to highly substituted α,β-unsaturated ketones (4). The stability of 2 is ascribed to the steric hindrance of the B-alkyl groups and to the resonance contribution of the mesomeric structure 3.  相似文献   
117.
Three sets of dyads, in which a zinc-porphyrin (ZP) electron donor is connected to an aromatic diimide electron acceptor,either pyromellitimide (PI) or naphthalene-1,8:4,5-tetracarboxylic acid diimide (NI), via a boronate-ester bridge, a piperidine bridge, and a 1,3-dioxolane bridge, respectively, were prepared for the purpose of control of intramolecular electron transfer (ET) by acid-base reactions at the connecting bridge. Boronate-ester bridge is a Lewis acidic site and confers a chance to regulate intramolecular ET reaction upon base coordination. This has been demonstrated by suppression of photoinduced ET from ZP to PI or NI in highly electron-pair donating solvents or upon addition of a fluoride anion. To extend this strategy to control of ET-path selectivity, we prepared triad 18, which consists of a ZP donor bearing NI and PI acceptors at similar distances through a boronate-ester bridge and an acetal bridge, respectively. Photoexcitation of 18 in a free form led to intramolecular ET from (1)()ZP preferentially to NI, but the ET path was completely switched toward PI in F(-)-coordinated form without a serious drop in the rate, constituting a novel ET-switching molecular system.  相似文献   
118.
Trialkylaluminium-mediated alkylation of enol phosphates under the CO bond cleavage is performed stereospecifically in the presence of a catalytic amount of Pd(PPh3)4. Alkenylation and alkynylation are also described.  相似文献   
119.
Carrier- and salt-free42K milked from an42Ar–42K generator was utilized for isotope dilution analysis of potassium in ultra-pure water. Potassium in the sample marked with the42K was shown to be concentrated easily 60 times by crown-ether extraction and acid back-extraction to be determined by atomic absorption spectrophotometry. The same extractions proved to be effective for removing the reagent blank. Cerenkov radiation counting was often suitable for42K measurement. The42K was used also for studying the interaction of ultra-low concentrations of potassium with vessel walls.  相似文献   
120.
Abstract

Evidence for bulk superconductivity in Nb2SC0.90 carbosulfide is presented. Diamagnetic volume fraction of Nb2SC0.90 was precisely determined with low applied magnetic fields which were corrected by 6N lead rod as an internal standard. Based on magnetic field dependence of magnetic moment at several constant temperature's below TC = 5 K, Nb2SC0.90 is found to be a type II superconductor with relatively high H C2 and low HC1.  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