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   255篇
  免费   12篇
化学   207篇
力学   12篇
数学   17篇
物理学   31篇
  2022年   1篇
  2021年   3篇
  2020年   4篇
  2019年   3篇
  2018年   12篇
  2017年   6篇
  2016年   4篇
  2015年   9篇
  2014年   11篇
  2013年   21篇
  2012年   18篇
  2011年   20篇
  2010年   18篇
  2009年   14篇
  2008年   19篇
  2007年   8篇
  2006年   6篇
  2005年   6篇
  2004年   5篇
  2003年   11篇
  2002年   6篇
  2001年   3篇
  2000年   2篇
  1998年   3篇
  1997年   1篇
  1996年   2篇
  1995年   3篇
  1994年   2篇
  1993年   2篇
  1992年   2篇
  1991年   4篇
  1990年   6篇
  1989年   2篇
  1988年   1篇
  1987年   2篇
  1986年   1篇
  1985年   6篇
  1984年   2篇
  1983年   2篇
  1982年   1篇
  1981年   1篇
  1980年   5篇
  1979年   3篇
  1971年   1篇
  1970年   1篇
  1968年   2篇
  1966年   2篇
排序方式: 共有267条查询结果,搜索用时 31 毫秒
1.
4(3H)-Quinazolinones have been synthesized in high to excellent yields through the one-pot condensation of anthranilic acid, trimethyl orthoformate and primary amines in the presence of 5 mol % of Bi(TFA)3 immobilized on [nbp]FeCl4 as a room temperature ionic liquid.  相似文献   
2.
The passive control of bluff body flows using porous media is investigated by means of the penalization method. This method is used to create intermediate porous media between solid obstacles and the fluid in order to modify the boundary layer behaviour. The study covers a wide range of two‐dimensional flows from low transitional flow to fully established turbulence by direct numerical simulation of incompressible Navier–Stokes equations. A parametric study is performed to illustrate the effect of the porous layer permeability and thickness on the passive control. The numerical results reveal the ability of porous media to both regularize the flow and to reduce the drag forces up to 30%. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
3.
Summary.  Benzyltriphenylphosphonium peroxymonosulfate in the presence of catalytic amounts of bismuth chloride was found to be an efficient and mild reagent for the oxidative cleavage of oximes and semicarbazones to the corresponding carbonyl compounds under microwave irradiation. Corresponding author. E-mail: haji@cc.iut.ac.ir Received March 4, 2002; accepted (revised) April 8, 2002  相似文献   
4.
Primary and secondary trimethylsilyl ethers were converted to their corresponding ethers in the presence Nafion-H® with good to excellent yields under mild and heterogeneous conditions.  相似文献   
5.
6.
Pt(II) and Pd(II) dichloride complexes with 5,7-ditertbutyl-1,2,4-triazolo[1,5-a]pyrimidine (dbtp) have been synthesized and characterized by infrared and 1H, 13C NMR, 13C CPMAS spectroscopy. The structures of the cis-PtCl2(dbtp)2 · EtOH (1) and cis-PdCl2(dbtp)(dmso) (2) has been determined by signal-crystal X-ray diffraction. In both complexes the X-ray crystal structures shows that heterocycle ligand (dbtp) binds the central atom monodentate via nitrogen atom N(3). In addition, compound (2) is interesting for its structural features, because it is the first report of mixed dichloride Pd(II) complexes with N-donor (triazolopyrimidine) and S-donor (dimethylsulfoxide) ligands. In this structure the Pd–Cl distances are: 2.302(1) and 2.281(1) Å, Pd–N 2.041(3) Å and Pd–S 2.245(1) Å. The 1H, 13C NMR studies show clearly that these structures are retained in solution.  相似文献   
7.
8.
9.
Solid-state structures have been determined for cis- and trans-1-cyclohexyl-2-phenyl-3-(p-toluyl)aziridines using single-crystal X-ray diffraction techniques. The cis isomer crystallizes in the centrosymmetric monoclinic space group P21/c (No. 14), with a = 18.669(3)Å, b = 5.709(1)Å, c = 17.412(2)Å, β = 96.29(1)° and Z = 4; the trans isomer crystallizes in the noncentrosymmetric orthorhombic space group Pna21 (No. 33), with a = 17.089(2)Å, b = 18.729(3)Å, c = 5.749(1)Å and Z = 4. Full-matrix least-squares refinement of the structural parameters led to the following final agreement factors: R1 (unweighted, based on F) = 0.040 and R2 (weighted, based on F) = 0.054 for the 2592 independent reflections of the cis isomer having 2θMoK¯α <55° and I>3σ1, and R1 = 0.033 and R2 = 0.031 for the 1504 independent reflections of the trans isomer having 2θMoK¯α <55° and I>3σ1. The statistically significant differences that exist between the two isomers for two bond lengths and ten bond angles (p < 0.05) appear to be the direct result of the p-toluyl group orientation with respect to the cyclohexyl and phenyl substituents. In the cis isomer it is anti with respect to the N-cyclohexyl group and cis with respect to the phenyl group, whereas in the trans isomer it is syn with respect to the N-cyclohexyl and trans with respect to the phenyl group. Three-ring to carbonyl hyperconjugation is correlated with stereoelectronic interactions in the trans isomer. Bonding, determined by X-ray and nmr studies, is discussed for the three-membered aziridine ring proper; while bonding, determined by X-ray studies, is discussed for substituents of the aziridine ring. These aziridinyl ketone compounds are of importance as potential mammalian DNA alkylating anti-tumor agents in solid-state solid-state systems. To date only a trans isomer has demonstrated this biological activity in tumor-bearing rats.  相似文献   
10.
The magnetization fluctuations are considered in thin films treated as a set of monoatomic layers parallel to the film surfaces. The correlation functions have been determined and their forms are discussed. The influence of inhomogeneity of magnetization on the equation of state for a magnetic system described by Valenta's model is studied. The behaviour of the spontaneous magnetization as well as the magnetic susceptibility near the Curie point is described.This work is partially supported by the Institute of Physics of the P.A.S.  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