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   474篇
  免费   9篇
  国内免费   3篇
化学   226篇
晶体学   2篇
力学   12篇
数学   134篇
物理学   112篇
  2020年   5篇
  2018年   5篇
  2016年   8篇
  2015年   8篇
  2014年   10篇
  2013年   20篇
  2012年   11篇
  2011年   20篇
  2010年   13篇
  2009年   13篇
  2008年   10篇
  2007年   17篇
  2006年   18篇
  2005年   7篇
  2004年   9篇
  2003年   8篇
  2002年   9篇
  1999年   5篇
  1994年   6篇
  1993年   6篇
  1989年   6篇
  1988年   5篇
  1987年   4篇
  1986年   5篇
  1985年   5篇
  1983年   4篇
  1982年   4篇
  1980年   4篇
  1979年   6篇
  1978年   5篇
  1977年   4篇
  1974年   4篇
  1973年   4篇
  1959年   5篇
  1933年   4篇
  1932年   12篇
  1931年   6篇
  1930年   5篇
  1928年   9篇
  1927年   6篇
  1926年   5篇
  1915年   4篇
  1913年   4篇
  1901年   4篇
  1900年   4篇
  1898年   4篇
  1896年   4篇
  1895年   4篇
  1893年   13篇
  1892年   4篇
排序方式: 共有486条查询结果,搜索用时 15 毫秒
11.
Ethyl carbamate (EC), a potentially toxic compound, is found in alcoholic beverages and fermented foodstuff. A combined experimental and theoretical study of Raman on EC is reported in this work for the first time. The Raman bands observed for EC in solid phase are characteristic for the carbonyl group, C―C, C―H and N―H stretching and deformation vibrations. These spectral features coupled with a pKa study allowed establishing the neutral species of EC present in the aqueous solutions experimentally tested at different concentrations. In addition, by performing a density functional theory study in the gas phase, the calculated geometry, the harmonic vibrational modes, and the Raman scattering activities of EC were found to be in good agreement with our experimental data and helped establish the surface‐enhanced Raman scattering (SERS) behavior and EC adsorption geometry on the silver surfaces. The Raman peak at 1006 cm−1, assigned to the υs(CC) + ω(CH) modes, the strongest and best reproducible peak in the SERS spectra, was used for a quantitative evaluation of EC. The limit of detection, which corresponds to a signal‐to‐noise ratio equal to 3, was found to be 2 × 10−7 M (17.8 µg l−1). SERS spectra obtained by using hydroxylamine hydrochloride‐reduced silver nanoparticles provide a fast and reproducible qualitative and quantitative determination of EC in aqueous solution. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
12.
13.
A gravitational law is proposed on a De Sitter covariant space. Dirac's De Sitter covariant spinning electron equation is generalized to the presence of gravitational fields. The resulting equation differs from the generally covariant Dirac equation by a mass renormalization. The last result is a generalization of that of Gürsey and Lee [2] in case of the homogenous De Sitter metric, and it gives a wider outlook on the significance of this result from the point of view of gauge theory.Dedicated to Achille Papapetrou on the occasion of his retirement.  相似文献   
14.
Abstract

Thioäther, Sulfoxide, Sulfone, organische Selenide und Telluride (R – Y – R': R, R', = Alkyl, Aryl, Y=S, SO, SO2, Se, Te) werden an der Grenzfläche von Raney-Nickel hydrogenolysiert. Als Spaltprodukte entstehen die Verbindungen RH und R'H sowie eine Grenzflächenverbindung (Raney-Nickel-Y) (Y=S, Se, Te; S auch aus Sulfoxiden und Sulfonen. Letztere entbinden ebenfalls mit Mineralsäuren YH2).

Die Größe und ‘Struktur’ der Oberfläche des Raney-Nickels entscheidet über die Hydrogenolysekapazität. Bei einer technischen Raney-Nickel-Probe liegt bei Diphenylsulfid die Sättigungsausbeute an Benzol bei 1,1 mmol/g Raney-Nickel. Die Sättigungskapazität hängt ab von der Struktur von R bzw. R' und dem Heteroatom Y. (Auch Benzylether können–wenn auch langsamer als Thioether–zu Toluol aufgespalten werden). Auf Grund bekannter Daten werden Vorstellungen entwickelt, die ein Bild über den topochemischen Verlauf der Hydrogenolyse von R–Y–R' vermitteln sollen.

Vergiftetes Raney-Nickel ist im Gegensatz zum aktivierten Raney-Nickel nicht mehr oder nur stark vermindert zur Isomerisierung, Disproportionierung und zum H-D-Austausch befähigt. Bei diesen Prozessen spielt offenbar der im Nickel strukturgebundene Wasserstoff eine wichtige Rolle.

Some thioethers, sulphoxides, sulphones, organoselenium and tellurium compounds (R–Y–R' R, R'= Alkyl, Aryl, Y?S, SO, SO2, Se, Te) have been reductively cleaved on the surface of Raney-nickel. The products comprise RH and R'H, and a surface bound material (Raney-nickel-Y) (Y?S, also from sulphoxides and sulphones, Se, Te. The latter may be released as YH2 on treatment with mineral acids).

The area and ‘structure’ of the surface determines the hydrogenolysis capacity of the catalyst. For technical quality Raney-nickel, the limiting yield of benzene from diphenylsulphide is 1.1 mmol/g. The limiting yield is dependent on the structure of R and R' and also the nature of Y. (Benzyl ethers may also be reduced to toluene, if somewhat slower than the corresponding thioether).

Proposal for the processes possibly involved at the surface during hydrogenolysis of R–Y–R' are put forward in the basis of consideration of the available experimental data.

Poisoned Raney-nickel (in contrast to the activated catalyst) is practically inactive as agent for isomerization, disproportionation, or H–D exchange, and hence surface bound hydrogen clearly plays an important role in these processes.  相似文献   
15.
16.
17.
The synthesis and detailed characterization of a new Ru polypyridine complex containing a heteroditopic bridging ligand with previously unexplored metal-metal distances is presented. Due to the twisted geometry of the novel ligand, the resultant division of the ligand in two distinct subunits leads to steady state as well as excited state properties of the corresponding mononuclear Ru(II) polypyridine complex resembling those of prototype [Ru(bpy)3]2+ (bpy=2,2'-bipyridine). The localization of the initially optically excited and the nature of the long-lived excited states on the Ru-facing ligand spheres is evaluated by resonance Raman and fs-TA spectroscopy, respectively, and supported by DFT and TDDFT calculations. Coordination of a second metal (Zn or Rh) to the available bis-pyrimidyl-like coordination sphere strongly influences the frontier orbitals, apparent by, for example, luminescence quenching. Thus, the new bridging ligand motif offers electronic properties, which can be adjusted by the nature of the second metal center. Using the heterodinuclear Ru−Rh complex, visible light-driven reduction of NAD+ to NADH was achieved, highlighting the potential of this system for photocatalytic applications.  相似文献   
18.
The isoforms distribution of the glycoprotein antithrombin III (ATIII) derived from human plasma was investigated by means of isoelectric focusing (IEF) in polyacrylamide gels with immobilized pH gradients (IPG) and two-dimensional gel electrophoresis (2-DE) as well as capillary electrophoretic methods. It turned out that the presence of high concentrations of chaotropics (urea, thiourea) and zwitterionic detergents (3-[(3-cholamidepropyl)dimethylammonio]-1-propanesulfonate (CHAPS)) was decisive for attaining good resolution of the protein isoforms. Resolution by IPG-IEF was obtained with excellent reproducibility and pI differences down to 0.01 pH units could be distinguished. ATIII-alpha and ATIII-beta-fractions preseparated by heparin affinity chromatography showed an analogous but shifted spot pattern consisting each of one major and three minor isoforms. The main isoforms of ATIII-alpha and ATIII-beta exhibit pI values of 5.18 and 5.32, respectively, both values determined in the presence of high concentrations of urea. The pI difference of 0.14 pH units correspond to the effect of two sialic acids absent in ATIII-beta. The formation and occurrence of ATIII dimers and trimers turned out to be dependent on the sample preparation. The results obtained by 2-DE were compared with those of capillary zone electrophoresis (CZE) and capillary IEF (CIEF). Quantitative analysis regarding the CZE separated isoforms of plasma derived ATIII yielded a content of about 70% ATIII-alpha main isoform and about 6.6% of ATIII-beta. The pI values of ATIII determined by CIEF with internal calibration were in fair agreement with the pI values of the main isoforms achieved with 2-DE.  相似文献   
19.
The Stark effect has been observed in the rotational spectra of several gas-phase amine-hydrogen halide complexes and the following electric dipole moments have been determined: H(3)(15)N-H(35)Cl (4.05865 +/- 0.00095 D), (CH(3))(3)(15)N-H(35)Cl (7.128 +/- 0.012 D), H(3)(15)N-H(79)Br (4.2577 +/- 0.0022 D), and (CH(3))(3)(15)N-H(79)Br (8.397 +/- 0.014 D). Calculations of the binding energies and electric dipole moments for the full set of complexes R(n)()(CH(3))(3)(-)(n)()N-HX (n = 0-3; X = F, Cl, Br) at the MP2/aug-cc-pVDZ level are also reported. The block localized wave function (BLW) energy decomposition method has been used to partition the binding energies into contributions from electrostatic, exchange, distortion, polarization, and charge-transfer terms. Similarly, the calculated dipole moments have been decomposed into distortion, polarization, and charge-transfer components. The complexes studied range from hydrogen-bonded systems to proton-transferred ion pairs, and the total interaction energies vary from 7 to 17 kcal/mol across the series. The individual energy components show a much wider variation than this, but cancellation of terms accounts for the relatively narrow range of net binding energies. For both the hydrogen-bonded complexes and the proton-transferred ion pairs, the electrostatic and exchange terms have magnitudes that increase with the degree of proton transfer but are of opposite sign, leaving most of the net stabilization to arise from polarization and charge transfer. In all of the systems studied, the polarization terms contribute the most to the induced dipole moment, followed by smaller but still significant contributions from charge transfer. A significant contribution to the induced moment of the ion pairs also arises from distortion of the HX monomer.  相似文献   
20.
The surface-enhanced Raman scattering (SERS) spectra of Eriochrome Black T (EBT) and its Cu(II), Fe(III), Mn(II) and Pb(II) complexes were recorded using a hydroxylamine reduced silver colloid. Molecular geometry optimization, molecular electrostatic potential (MEP) distribution and vibrational frequencies calculation were performed at B3LYP/6-31G(d) level of theory for the EBT molecule and its Cu(EBT), Fe(EBT) and Mn(EBT) metal complexes. Differentiation between EBT complexes of Cu(II), Fe(III), Mn(II) and Pb(II) is shown by the SERS spectral features of each complex.  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