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   5980篇
  免费   165篇
  国内免费   23篇
化学   4143篇
晶体学   37篇
力学   98篇
数学   796篇
物理学   1094篇
  2021年   47篇
  2020年   50篇
  2019年   64篇
  2017年   30篇
  2016年   108篇
  2015年   117篇
  2014年   110篇
  2013年   215篇
  2012年   197篇
  2011年   210篇
  2010年   129篇
  2009年   137篇
  2008年   185篇
  2007年   206篇
  2006年   212篇
  2005年   206篇
  2004年   197篇
  2003年   159篇
  2002年   194篇
  2001年   121篇
  2000年   100篇
  1999年   95篇
  1998年   95篇
  1997年   87篇
  1996年   110篇
  1995年   86篇
  1994年   99篇
  1993年   97篇
  1992年   102篇
  1991年   110篇
  1990年   91篇
  1989年   99篇
  1988年   109篇
  1987年   127篇
  1986年   85篇
  1985年   135篇
  1984年   125篇
  1983年   83篇
  1982年   113篇
  1981年   115篇
  1980年   102篇
  1979年   118篇
  1978年   90篇
  1977年   91篇
  1976年   83篇
  1975年   74篇
  1974年   88篇
  1973年   62篇
  1972年   31篇
  1971年   49篇
排序方式: 共有6168条查询结果,搜索用时 93 毫秒
101.
N,N-Dialkyl-N′-ferrocenoylthioureas and Ferrocene-1,1′ -dicarbonic-acid-di-N,N-dialkylthioureides as Ligands for Transition Metals: Synthesis, Acid-Dissociation Constants, X-Ray Structure Determination, and EPR-Single Crystal Investigation Reaction of ferrocenoylisothiocyanate or ferrocene-1,1′ -dicarbonic acid -diisothiocyanate with secondary amines gives N,N-dialkyl-N′ -ferrocenoylthioureas or the respective ferrocene-1,1′-dicarbonic acid-di-N,N-dialkyl-thioureides. The former yield neutral complexes with transition metal ions (Ni2+, Cu2+, Co3+, Pt2+, Fe3+). The acid dissociation constants of the ligands were determined. The EPR spectra of a bis-(N,N-dinbutyl-N′-ferrocenoylthioureato)copper(II) single crystal are discussed. The X-ray structure determination of ferrocene-1,1′-dicarbonic acid-di-N,N-diethyl-thioureide and its different behaviour against Ni2+ and Cu2+ is presented.  相似文献   
102.
A new synthetic route for the preparation of the betain-like compound (CH3N)6P4F8 from (CH3NPF3)2,N-methyl-hexamethyldisilazane andN,N-dimethyl-urea has been found. The steps of this multi-stage reaction could be rationalized to a far extent.
10. Mitteilung:Kubjacek M., Utvary K., Mh. Chem.112, 305 (1981).  相似文献   
103.
Tristannanes used as photochemical precursors for the synthesis of organic compounds react via triplet states followed by homolytic CSn and SnSn bond scission.  相似文献   
104.
α-Diazo phosphonates with a conjugated 1,3-diene unit are synthesised by the Bamford-Stevens reaction (243). They undergo [4+2]-cycloadditions with the dienophile 5 to form the tetrahydrotriazolopyridazines 8, which possess an unchanged diazo group. In contrast, dimethyl acetylenedicarboxylate (9) reacts exclusively with the diazo dipole of 3 to yield the 3H-pyrazoles 10, which rearrange to 11 by sigmatropic PO-shifts and hydrolyse to form 13. The diazo compound 3b isomerises to the pyrazole 16 when heated in benzene.  相似文献   
105.
Loss of vinyl alcohol from 1-amino-3-aryloxy-2-propanols under electron impact Under electron impact compounds of type 1 (see Scheme 1) split off 44 mass units from the molecular ion. This unusual reaction was studied using derivatives and deuterium labelled compounds. It could be demonstrated that for this fragmentation reaction 16 is the important structural feature from which H2(C3)?C(2)HOH (44 mass units) is lost. The preferred reaction mechanism involves a transition state in which four members of the side chain are involved (Scheme 2, mechanism 2).  相似文献   
106.
Contribution to the mass spectrometry of substituted α,ω-alkane diamines The main mass spectral fragmentation pattern of compounds of types 1 to 4 is discussed. After loss of C6H5 · CH2 · from the molecular ion the acid correspondin to the N,N-disubstituted residue is splitted off. The mechanism of this fragmentation reaction depends on the member of CH2-groups between the two nitrogen atoms (Schemes 1 and 3) and on the substitution pattern of both nitrogens (Scheme 2).  相似文献   
107.
Zusammenfassung Durch den Einsatz zweiwertiger Ionen in der wäßrigen Subphase kann die Packungsdichte gespreiteter Monoschichten wasserunlöslicher Tenside pH-abhän-gig erhöht werden. Ergebnisse für Octadecylamin auf einer Lösung von Natriumhydrogensphosphat werden angegeben.
Summary The packing density of spread monolayers of waterinsoluble surface-active agents can be increased by use of bivalent ions in the aqueous subphase. Results for octadecyl amine spread on a solution of sodium hydrogen phosphate are given.


Mit 2 Abbildungen  相似文献   
108.
Photoelectron spectroscopy combined with the liquid microjet technique enables the direct probing of the electronic structure of aqueous solutions. We report measured and calculated lowest vertical electron binding energies of aqueous alkali cations and halide anions. In some cases, ejection from deeper electronic levels of the solute could be observed. Electron binding energies of a given aqueous ion are found to be independent of the counterion and the salt concentration. The experimental results are complemented by ab initio calculations, at the MP2 and CCSD(T) level, of the ionization energies of these prototype ions in the aqueous phase. The solvent effect was accounted for in the electronic structure calculations in two ways. An explicit inclusion of discrete water molecules using a set of snapshots from an equilibrium classical molecular dynamics simulations and a fractional charge representation of solvent molecules give good results for halide ions. The electron binding energies of alkali cations computed with this approach tend to be overestimated. On the other hand, the polarizable continuum model, which strictly provides adiabatic binding energies, performs well for the alkali cations but fails for the halides. Photon energies in the experiment were in the EUV region (typically 100 eV) for which the technique is probing the top layers of the liquid sample. Hence, the reported energies of aqueous ions are closely connected with both structures and chemical reactivity at the liquid interface, for example, in atmospheric aerosol particles, as well as fundamental bulk solvation properties.  相似文献   
109.
110.
This paper reports the observation of the far infrared absorption spectrum of a single crystal of N2, measured over absorption paths of 4 cm (lengthwise) and 2 cm (across the crystal). The crystal chamber, with quartz windows, was immersed in a flow of cold helium gas. The spectrum from 20 to 120 cm–1 was recorded in the liquid phase, the-phase, and over the full temperature range of the-phase (35.6–2.0 K) with a Fourier transform spectrometer. The spectral resolution, which was not instrument-limited, and the large path allowed the observation of more detailed multiphonon-transition structure in the spectrum of the-phase than has previously been observed.  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