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   538篇
  免费   7篇
  国内免费   25篇
化学   362篇
晶体学   1篇
力学   38篇
综合类   11篇
数学   53篇
物理学   105篇
  2024年   2篇
  2023年   4篇
  2022年   16篇
  2021年   22篇
  2020年   19篇
  2019年   13篇
  2018年   19篇
  2017年   12篇
  2016年   24篇
  2015年   15篇
  2014年   18篇
  2013年   52篇
  2012年   41篇
  2011年   38篇
  2010年   21篇
  2009年   19篇
  2008年   27篇
  2007年   38篇
  2006年   21篇
  2005年   22篇
  2004年   6篇
  2003年   18篇
  2002年   28篇
  2001年   7篇
  1999年   7篇
  1998年   6篇
  1997年   5篇
  1995年   2篇
  1994年   3篇
  1993年   7篇
  1992年   1篇
  1991年   6篇
  1990年   1篇
  1989年   2篇
  1988年   4篇
  1986年   1篇
  1985年   1篇
  1984年   1篇
  1983年   1篇
  1982年   3篇
  1979年   3篇
  1977年   2篇
  1973年   1篇
  1972年   1篇
  1970年   2篇
  1968年   2篇
  1961年   1篇
  1960年   1篇
  1959年   1篇
  1957年   1篇
排序方式: 共有570条查询结果,搜索用时 140 毫秒
11.
Phenols are converted to salicylaldehydes with paraformaldehyde, MgCl2-Et3N in THF, and subsequent treatment with methyl (triphenylphosphoranylidene)acetate gave the corresponding methyl ortho-hydroxycinnamate derivatives. The sequence is conveniently carried out as a one-pot procedure.  相似文献   
12.
We introduce a new numerical technique for the calculation of the pore size distribution in two-dimensional disordered systems. Our method is based on a triangulation technique which allows a closer measurement of pores surface without any morphological hypothesis.In this work, we focus our calculations in simulated gels. Such materials are modeled in two different conditions: by means of the Diffusion-Limited and Reaction-Limited Cluster-cluster Aggregation algorithms, DLCA and RLCA, respectively. In both situations, when the particles concentration decreases, the average pores size increases. The more compact cluster in RLCA, compared with DLCA, is consistent with the pore size distribution we have calculated. The simulated mean pore size is quantitatively in agreement with experimental data from literature.  相似文献   
13.
The complexation of triprolidine hydrochloride (TRP) and β-cyclodextrin (β-CD) in deuterium oxide was investigated by 400 MHz 1^H NMR spectroscopy. The 800 MHz 2D ROESY data revealed that two 1 :1 and one 2 : 1 β-CD-TRP inclusion complexes were formed. Both aromatic moieties (p-tolyl and pyridyl ring) has entered into the β-CD cavity, confirming the existence of two different equilibria for 1 : 1 inclusion complexes in which p-tolyl ring of the guest is more tightly held by the host cavity. The ROE intermolecular interactions provided the plausible structures of these 1 : 1 and 2 : 1 stoichiometric inclusion complexes of β-CD-triprolidine hydrochloride in solu- tion.  相似文献   
14.
The title mol­ecule, C13H13N3O3·H2O, is in the form of a mono­hydrated zwitterion. The tetra­hydro­pyridinium ring adopts an envelope conformation and is nearly coplanar with the plane of the imidazoline ring. The water solvate mol­ecule plays an important role as a bridge between zwitterions, forming molecular chains running along the c direction, which are interconnected by N—H?O hydrogen bonds into molecular ribbons. The crystal packing is further stabilized by another N—H?O and one O—H?N hydrogen bond, which interconnect the molecular ribbons.  相似文献   
15.
Chalcones (α,β‐unsaturated ketones) are effective antitumour agents. It has been proved that having halogen or methoxy groups substituted in various positions of the phenyl ring enhances the activity of chalcones many times. The title compounds, C21H20O5 and C19H15BrO3, respectively, were chosen for crystallographic study in order to determine their structures and conformations. In both compounds, the keto group is in the scis conformation and is almost planar. There are weak intramolecular interactions in both structures.  相似文献   
16.
Summary The synergistic mixture comprising picrolonic acid (HPA) and benzo-15-crown-5 (B15C5) in chloroform has been used for the extraction of Ce(III), Eu(III) and Tm(III) as representatives of lanthanide(III) ions from pH 1-2 solutions having ionic strength of 0.1 mol. dm-3(K+/H+, Cl-). The composition of the extracted species has been determined as M(PA)3. nB15C5 where M is Ce, Eu and Tm and n=1 or 2. The influence of various anions and cations on the extraction of these ions has also been studied and only oxalate, cyanide and tartrate have some deleterious effect. The extraction equilibrium constants have been evaluated and discussed.  相似文献   
17.
A study on the adsorption characteristics of Pb(II) and Cr(III) cations onto C‐4‐methoxyphenylcalix‐[4]resorcinarene (CMPCR) in batch and fixed bed column systems has been conducted. CMPCR was produced by one step synthesis from resorcinol, 4‐methoxybenzaldehyde, and HCl. The synthesis was carried out at 78 °C for 24 hours and afforded the adsorbent in 85.7% as a 3:2 mixture of C:C isomer. Most parameters in batch and fixed bed column systems confirm that CMPCR is a good adsorbent for Pb(II) and Cr(III), though Pb(II) adsorption was more favorable than that of Cr(III). The adsorption kinetic of Pb(II) and Cr(III) adsorptions in batch and fixed bed column systems followed a pseudo 2n order kinetics model. The rate constant of Pb(II) was higher than that of Cr(III) in the batch system, but this result was contrary to the result obtained in a fixed bed column system. Desorption studies to recover the adsorbed Pb(II) and Cr(III) were performed sequentially with distilled water and HCl, and the results showed that the adsorption was dominated by chemisorption.  相似文献   
18.
The formation of tetragonal Zr(0.5)Ce(0.5)O(2) solid solution nanocrystallites of 5 +/- 1 nm size by a hydrothermal method at 120 degrees C for 6 h was confirmed by careful Raman and XRD studies for the first time. It was characterized as the t' '-form with an axial ratio of c/a = 1 but with oxygen ion displacements. The as-prepared sample was hydrous in nature, which is responsible for the lattice expansion. However, most of the water held in the structure can be expelled by heating the sample above 600 degrees C. Above 1050 +/- 50 degrees C the t' '-form of tetragonal Zr(0.5)Ce(0.5)O(2) solid solution dissociates into two phases, cubic phase and the t-form of tetragonal phases.  相似文献   
19.
A new Schiff base ligand (HL) was prepared via a condensation reaction of quinoline‐2‐carboxaldhyde with 2‐aminophenol in a molar ratio of 1:1. Its transition metal mixed ligand complexes with 1,10‐phenanthroline (1,10‐phen) as co‐ligand were also synthesized in a 1:1:1 ratio. HL and its mixed ligand complexes were characterized using elemental analysis, infrared, 1H NMR, mass and UV–visible spectroscopies, molar conductance, magnetic measurements, solid reflectance, thermal analysis, electron spin resonance and X‐ray diffraction. Molar conductance measurements showed that all complexes have an electrolytic nature, except Cd(II) complex. From elemental and spectral data, the formulae [M(L)(1,10‐phen)(H2O)]Clx?nH2O (where M = Cr(III) (x = n = 2), Mn(II) and Ni(II) (x = 1, n = 2), Fe(III) (x = n = 2), Co(II), Cu(II) and Zn(II) (x = 1, n = 2)) and [Cd(L)(1,10‐phen)Cl]?3H2O for the metal complexes have been proposed. The geometric structures of complexes were found to be octahedral. Powder X‐ray diffraction reflected the crystalline nature of the complexes; however, the Schiff base is amorphous. HL and its mixed ligand complexes were screened against Gram‐positive bacteria (Streptococcus pneumoniae and Bacillus subtilis) and Gram‐negative bacteria (Pseudomonas aeruginosa and Escherichia coli). Antifungal activity was determined against Aspergillus fumigatus and Candida albicans, the data showing that most complexes had activity less than that of the Schiff base while Mn(II), Fe(III) and Ni(II) complexes showed no significant antifungal activity. The anticancer activity of HL and its metal complexes was also studied against breast and colon cell lines. The metal complexes showed IC50 higher than that of HL, especially the Cu(II) complex which showed the highest IC50 against breast cell line.  相似文献   
20.
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