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   1490篇
  免费   45篇
  国内免费   7篇
化学   1037篇
晶体学   14篇
力学   17篇
数学   158篇
物理学   316篇
  2022年   9篇
  2021年   7篇
  2020年   12篇
  2019年   13篇
  2018年   5篇
  2017年   20篇
  2016年   30篇
  2015年   24篇
  2014年   27篇
  2013年   77篇
  2012年   52篇
  2011年   88篇
  2010年   40篇
  2009年   60篇
  2008年   81篇
  2007年   101篇
  2006年   88篇
  2005年   107篇
  2004年   90篇
  2003年   69篇
  2002年   71篇
  2001年   24篇
  2000年   24篇
  1999年   24篇
  1998年   14篇
  1997年   18篇
  1996年   17篇
  1995年   8篇
  1994年   16篇
  1993年   14篇
  1992年   28篇
  1991年   11篇
  1990年   13篇
  1989年   12篇
  1988年   10篇
  1987年   11篇
  1986年   5篇
  1985年   22篇
  1984年   19篇
  1983年   14篇
  1982年   23篇
  1981年   13篇
  1980年   23篇
  1979年   20篇
  1978年   10篇
  1977年   12篇
  1976年   9篇
  1975年   15篇
  1974年   12篇
  1973年   9篇
排序方式: 共有1542条查询结果,搜索用时 31 毫秒
991.
A couple of radical carbonylations of gem-dihalocyclopropanes 1 using CO and Bu3SnH (formylation) or Bu3Sn(CH2CH=CH2) (allylacylation) successfully proceeded to give trans and cis adducts (2 and 3) with good to excellent stereoselectivity (trans/cis = >99/1-75/25 or 17/83-1/99). The formylation of 2,3-cis-disubstituted 1,1-dihalocyclopropanes enhanced trans selectivity (trans/cis = >99/1-95/5), whereas both 2,3-cis-disubstituted and 2-monosubstituted 1,1-dihalocyclopropanes underwent allylacylation with nearly complete trans selectivity (trans/cis = >99/1). Inherently less reactive gem-dichloro- and bromochlorocyclopropanes than gem-dibromocyclopropanes served as favorable substrates. [reaction: see text].  相似文献   
992.
993.
A distribution of Cu ions in polyelectrolyte film (Nafion) is directly observed with a small-angle X-ray scattering (SAXS) method utilizing an X-ray anomalous dispersion effect. A partial structure factor of the Cu ions, GAA(q), can be derived from the SAXS profiles obtained by scanning the incident X-ray energy around the Cu K absorption edge. GAA(q) has two peaks, indicating that the Cu ions hierarchically distribute in Nafion film. In addition, a standard SAXS also shows that Nafion film has a hierarchical structure. These results mean that the Cu ions locate in the domain where the hydrophilic bases aggregate.  相似文献   
994.
This article highlights Ru complexes, which effect catalytic hydrogenation of polar organic functionalities containing C-O bonds other than aldehydes or ketones. The unique ability of Ru complexes to undergo heterolytic dihydrogen cleavage seems to play a key role in these catalyses.  相似文献   
995.
996.
997.
Takao K  Ikeda Y 《Inorganic chemistry》2007,46(5):1550-1562
The molecular structures of UO2(salophen)L (L = DMF, DMSO) and a uranyl-salophen complex without any unidentate ligands (L) in solid and solution were investigated using single-crystal X-ray analysis and IR, 1H NMR, and UV-visible absorption spectroscopies. As a result, it was found that the uranyl-salophen complex without L is a racemic dimeric complex, [UO2(salophen)]2, in which the UO2(salophen) fragments are held together by bridging between one of the phenoxide oxygen atoms in salophen and the uranium in the other UO2(salophen) unit. Furthermore, it was spectrophotometrically demonstrated that [UO2(salophen)]2 retains its dimeric structure even in the noncoordinating solvents such as CH2Cl2 and CHCl3 and is in equilibrium with UO2(salophen)L {2UO2(salophen)L right arrow over left arrow [UO2(salophen)]2 + 2L}. The equilibrium constants and thermodynamic parameters of this equilibrium were evaluated from UV-visible absorption and 1H NMR spectral changes; log Kdim = -2.51 +/- 0.01 for L = DMF and solvent = CH2Cl2, log Kdim = -1.68 +/- 0.02 for L = DMF and solvent = CHCl3, log Kdim = -4.23 +/- 0.01 for L = DMSO and solvent = CH2Cl2, and log Kdim = -3.03 +/- 0.02 for L = DMSO and solvent = CHCl3. The kinetics of L-exchange reactions in UO2(salophen)L and enantiomer exchange of [UO2(salophen)]2 in noncoordinating solvents were also studied using NMR line-broadening method. As a consequence, it was suggested that the DMF-exchange reaction in UO2(salophen)DMF proceeds through two pathways (dissociative and associative paths) and that the predominant path of DMSO exchange in UO2(salophen)DMSO is the dissociative one. A sliding motion of the UO2(salophen) fragments was considered to be reasonable for the enantiomer-exchange mechanism of [UO2(salophen)]2. On the basis of the kinetic information for UO2(salophen)L and [UO2(salophen)]2, reaction mechanisms including the L-exchange reaction in UO2(salophen)L, the formation of [UO2(salophen)]2 from UO2(salophen)L, and the enantiomer exchange of [UO2(salophen)]2 are proposed.  相似文献   
998.
Dual fluorescence of 4,4(')-dimethylaminobenzonitrile (DMABN) was theoretically investigated on the basis of long-range corrected time-dependent density functional theory. Excited-state geometry optimization states and single-point energy calculations with and without solvent effect were carried out. It has been explained that DMABN emits dual fluorescence only in polar solvents through locally excited (LE) and charge transfer (CT) states. It was, however, concluded from this study that although the main spectrum of dual fluorescence in acetonitrile solvent is clearly due to twisted intramolecular CT fluorescence, small secondary fluorescence in acetonitrile may also emanate from CT fluorescence during the DMABN twisting process. This conclusion is supported by an experimental interpretation on polarization spectroscopy. It was also found that the optimized DMABN geometries have certain wagging angles for the CT state and no wagging angle for the LE state. This may support an early experimental hypothesis that the dual fluorescence of DMABN is induced by the wagging mode due to vibronic coupling between LE and CT states. Consequently, the authors propose a fluorescence mechanism of DMABN in gas phase and in acetonitrile solvent: the main absorption proceeds to the CT state in both situations. In gas phase, single fluorescence is chiefly emitted from the LE state through the internal conversion from CT to LE states. Dual fluorescence in acetonitrile solvent may only be emitted from the CT state.  相似文献   
999.
The single-stage separation factor for chlorine isotopes ((35)Cl and (37)Cl) was determined to be 1.00034 by anion exchange chromatography on a 4.5-m column operated in reverse breakthrough manner at 25 degrees C. This value is in good agreement with those obtained in our previous works. It was confirmed that the lighter isotope ((35)Cl) was preferentially fractionated into the resin phase, whereas the heavier isotope ((37)Cl) partitioned into the aqueous phase. This observation, however, contradicts the experimental results for Cl isotope fractionation during NaCl precipitation and the recent theoretical results on Cl isotope fractionation: the (37)Cl isotope selectively enriched into the solid phase and not into the aqueous phase. This discrepancy is discussed based on the theory of isotope distribution between two phases. It is suggested that the chromatographic results reflect an isotope effect accompanying hydration rather than an isotope effect due to a phase change, whereas the reverse is the case for the results in the NaCl precipitation study.  相似文献   
1000.
This investigation describes an optically transparent antistatic film composed of antimony-doped tin oxide (ATO) nanoparticles dispersed in a polymer matrix, with remarkably improved electrical and optical properties. The film is fabricated on the basis of a synergistic interaction between self-assembling nanoparticles and self-organizing matrix materials. The antistatic property of the film is obtained at ATO concentrations above a threshold value. A scaling analysis of the data yields an extremely low critical concentration (0.0020 volume fraction), which is considerably lower than the value predicted by percolation theory. Microscopic observations of the film have revealed a characteristic microstructure: "single-stranded" chainlike (linear form or fibrous) aggregates consisting of ATO nanoparticles and large ATO-depleted areas. The experiment results suggest that the high optical transparency and the low critical concentration are derived from the characteristic microstructures of the film.  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