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   965篇
  免费   9篇
  国内免费   7篇
化学   563篇
晶体学   9篇
力学   34篇
数学   85篇
物理学   290篇
  2021年   13篇
  2020年   19篇
  2019年   10篇
  2018年   7篇
  2017年   7篇
  2016年   11篇
  2014年   7篇
  2013年   42篇
  2012年   33篇
  2011年   41篇
  2010年   15篇
  2009年   27篇
  2008年   34篇
  2007年   38篇
  2006年   41篇
  2005年   28篇
  2004年   29篇
  2003年   35篇
  2002年   21篇
  2001年   20篇
  2000年   35篇
  1999年   22篇
  1998年   20篇
  1997年   21篇
  1996年   16篇
  1995年   14篇
  1994年   17篇
  1993年   16篇
  1992年   16篇
  1991年   12篇
  1990年   12篇
  1989年   6篇
  1988年   11篇
  1987年   8篇
  1986年   21篇
  1985年   22篇
  1984年   13篇
  1983年   10篇
  1982年   12篇
  1981年   12篇
  1980年   10篇
  1979年   15篇
  1978年   6篇
  1977年   22篇
  1976年   9篇
  1975年   14篇
  1974年   9篇
  1973年   10篇
  1970年   5篇
  1917年   7篇
排序方式: 共有981条查询结果,搜索用时 15 毫秒
241.
Abstract

The reactions of (Me3SiNR)PPh2 (NSiMe3) (R=H, SiMe3) with (NSCl)3 or SxCl2 (x=1, 2) provide improved preparative routes to mixed phosphazene-thia-zene ring systems, e.g. (Ph2PN) (SN)2, 1,3- and 1,5-(Ph2PN)2(SN)2. The interaction of these heterocycles with protonic and Lewis acids, e.g. HBF4, BCl3, MeSO3CF3, has been investigated in order to determine the site of attack and the effect of adduct formation on the molecular and electronic structures of these ring systems. These reactions have been monitored by UV-visible and 31P NMR spectroscopies and the structures of crystalline adducts have been determined by X-ray crystallography. For example, the interaction of 1,3-(Ph2PN)2(SN)2 with MeSO3CF3 gives a product in which the methyl group is attached to a nitrogen atom between phosphorus and sulfur. The resulting structural distortion in this and related adducts will be analyzed in the context of the electronic structures of mixed phosphazene-thiazene systems and their adducts with Lewis acids.  相似文献   
242.
Abstract

Structural studies of a series of laterally attached side chain liquid crystalline polysiloxanes have been made by X-ray diffraction. All but one exhibit a nematic phase with SC-like short range ordering. The diffraction has been interpreted in terms of a model in which the mesogenic units form a ‘jacket’ around the polymer backbone. The backbone conformation has been studied by small angle X-ray scattering from oriented samples of mesomorphic solutions and by infrared dichroism measurements on oriented samples of the polymers. In both cases the results were consistent with the mesogens being oriented parallel to the backbone which is consistent with the ‘mesogen jacket’ model.  相似文献   
243.
The X-ray crystal structure and magnetic properties of a molecular crystal consisting of 1,5-dimethyl-3-(2-pyridyl)-6-oxoverdazyl radical and hydroquinone (pyvd:hq) are presented. The structure contains a two-dimensional network of hydrogen bonds involving the hydroquinones and the pyridine ring of the pyvd radical. The radicals adopt an unusual head-over-tail (antiparallel) pi-stacked array perpendicular to the hydrogen-bonded planes. The variable-temperature magnetic susceptibility data can be modeled using a one-dimensional antiferromagnetic chain model, with J = -58 cm(-1). The strength of the magnetic coupling is very unusual because there are no close intermolecular radical-radical contacts to provide conventional pathways for magnetic interactions. A pathway for coupling is proposed involving the mediation of magnetic exchange interactions between radical centers by the pyridine rings. Density functional calculations on the pyvd radical, as well as aggregates thereof based on the X-ray structure, have been employed in attempts to understand the possible mechanisms by which the strong magnetic interactions are achieved.  相似文献   
244.
Global societal challenges emphasize the importance of collaboration between scientists and policy-makers, while the participation of a diverse group of professionals, including early-career scientists, is critical towards a sustainable future. The European Young Chemists’ Network (EYCN) has been actively working with the European Chemical Society (EuChemS) to create a platform for early-career chemists in policy advice. This article comments on the possible roles of scientists in policy-making and provides an overview of relevant initiatives and platforms at the European level that could facilitate involvement. Opportunities for participation in policy advice from the perspective of early-career chemists are discussed and examples of impact are provided, hoping to stimulate further discussions and engagement in policy-making.  相似文献   
245.
In this study, a series of twelve ring-substituted salicylanilides and carbamoylphenylcarbamates were prepared and characterized. The compounds were analyzed using RP-HPLC to determine lipophilicity. They were tested for their activity related to the inhibition of photosynthetic electron transport (PET) in spinach (Spinacia oleracea L.) chloroplasts. Moreover, their site of action in the photosynthetic apparatus was determined. Primary in vitro screening of the synthesized compounds was also performed against mycobacterial, bacterial and fungal strains. Several compounds showed biological activity comparable with or higher than the standards 3-(3,4-dichlorophenyl)-1,1-dimethylurea, isoniazid, penicillin G, ciprofloxacin or fluconazole. The most active compounds showed minimal anti-proliferative activity against human cells in culture, indicating they would have low cytotoxicity. For all compounds, the relationships between lipophilicity and the chemical structure are discussed.  相似文献   
246.
We report a high-energy optical parametric oscillator (OPO) synchronously pumped by a 7.19 MHz, Yb:fiber-amplified, picosecond, gain-switched laser diode. The 42-m-long ring cavity maintains a compact design through the use of an intracavity optical fiber. The periodically poled MgO-doped LiNbO(3) OPO provides output pulse energies as high as 0.49 μJ at 1.5 μm (signal) and 0.19 μJ at 3.6 μm (idler). Tunability from 1.5 to 1.7 μm and from 2.9 to 3.6 μm is demonstrated, and typical M(2) values of 1.5 × 1.3 and 2.8 × 1.9 are measured for the signal and idler, respectively, at high power.  相似文献   
247.
A category of lattice-valued fuzzy interior operator spaces is defined and studied. Axioms are given in order for this category to be isomorphic to the category whose objects consist of all the stratified, lattice-valued, pretopological convergence spaces.  相似文献   
248.
The palladium-catalyzed oxidation of allylic esters and carbonates using a novel potassium nitronate has been developed. This method is highly chemoselective, leaving other esters, alcohols, thioethers, and amines undisturbed. The oxidation can be operated in two modes: an achiral mode, using either PPh3 or rac-2 as ligand, or a chiral and highly enantioselective mode, using 2 as ligand. The oxidative enantioselective desymmetrization of meso bis-esters provides a significantly shorter method to arrive at commonly used synthetic intermediates compared to other strategies. Highly efficient kinetic resolution is also possible using this methodology.  相似文献   
249.
Cytosine was adsorbed onto a Cu(110) surface under UHV conditions. Annealing to 370 K resulted in the formation of a (6 x 6)gg low energy electron diffraction (LEED) pattern, even at submonolayer coverages. Examination of this structure with scanning tunneling microscopy (STM) revealed islands of zigzag chains at low coverages and large ordered domains at monolayer saturation. Further annealing to 480 K initiated a phase transition to a (6 x 2)gg structure observed both by LEED and STM. High resolution electron energy loss spectroscopy spectra for both overlayer structures exhibited mainly in-plane modes suggesting upright/tilted species on the surface. Based on the experimental data and supported by density functional theory calculations, a model is proposed for the (6 x 2)gg structure, which involves the formation of deprotonated hydrogen bridge-bonded cytosine dimers, adsorbed through the oxygen atoms.  相似文献   
250.
[reaction: see text] Lysophospholipase D (lysoPLD), also known as autotaxin (ATX), is an important source of the potent mitogen lysophosphatidic acid (LPA). Two fluorogenic substrate analogues for lysoPLD were synthesized in nine steps from (S)-PMB-glycerol. The substrates (FS-2 and FS-3) show significant increases in fluorescence when treated with recombinant ATX and have potential applications in screening for this emerging drug target.  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