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   306篇
  免费   16篇
  国内免费   1篇
化学   249篇
晶体学   1篇
力学   6篇
数学   21篇
物理学   46篇
  2023年   3篇
  2022年   6篇
  2021年   8篇
  2020年   9篇
  2019年   5篇
  2018年   5篇
  2017年   4篇
  2016年   9篇
  2015年   8篇
  2014年   3篇
  2013年   14篇
  2012年   22篇
  2011年   28篇
  2010年   11篇
  2009年   22篇
  2008年   15篇
  2007年   17篇
  2006年   22篇
  2005年   18篇
  2004年   12篇
  2003年   14篇
  2002年   15篇
  2001年   7篇
  2000年   4篇
  1999年   3篇
  1998年   1篇
  1997年   2篇
  1995年   2篇
  1994年   1篇
  1993年   1篇
  1992年   2篇
  1990年   1篇
  1988年   2篇
  1987年   2篇
  1986年   2篇
  1985年   2篇
  1984年   4篇
  1983年   1篇
  1982年   2篇
  1981年   2篇
  1979年   2篇
  1978年   1篇
  1977年   1篇
  1976年   1篇
  1975年   1篇
  1974年   4篇
  1970年   2篇
排序方式: 共有323条查询结果,搜索用时 15 毫秒
251.
The ability of different cyclodextrins (CDs): CD, 2-hydroxypropyl CD to complex drugs like 3--hydroxy-11-oxoolean- 12-en-30-oic acid, 2-ethylhexyl-3-(4-methoxyphenyl)-2-propanoate and menthol was compared to that of water-soluble polymers: CD-co-epichlorhydrin polymer (pCD/EP) and CD-co-epichlorhydrin polymer partially modified with trimethylammonium groups (pCD/EPN+). 3--Hydroxy-11-oxoolean-12-en-30-oic acid was poorly solubilized by CD compared with other CD derivatives, however the determination of the complexation constants was possible for pCD/EP, K11 = 740, K12 = 4, for pCD/EPN+, K11 = 681, for CD, K11 = 16 and for hydroxypropyl CD, K11 = 114, K12 = 3.4. A significant increase of the solubility was observed for 2-ethylhexyl-3-(4-methoxyphenyl)-2-propanoate with all host molecules, it was 916 times its solubility in pure water with pCD/EPN+, 1116 and 1300 times with 2-hydroxypropyl CD and pCD/EP respectively. The association constants are K11 = 7970, K11 = 4700, K11 = 1470, K11 = 230 and K12 = 200 with pCD/EP, pCD/EPN+, CD, 2-hydroxypropyl CD respectively. An increase of the solubility of menthol was observed with all CD derivatives, up to 36–37 times, except for CD. The complexation constants are similar equal to about 200.  相似文献   
252.
The crystal structure of a new symmetrical pentadentate N2O3 Schiff base: di[4-(phenylimino)pentan-2-one] ether (H2L) is described. In the solid state, the ligand appears as a keto-imine tautomer, while in DMSO solution, the eneamine form is observed. This ligand coordinates cobalt(II), nickel(II), copper(II) and cadmium(II). The structures of these new complexes are described using infrared and electronic spectroscopy, 1H-n.m.r. and d.s.c. The cyclic voltammograms of the ligand and the complexes in DMF are discussed.  相似文献   
253.
The catalytic selective reduction of NO over metal‐exchanged (ZnII, AgI) natural zeolites (mordenite and clinoptilolite) from Cuba using NH3 as a reducing agent in the presence of excess oxygen was studied. Both transition metals slightly improve the catalytic performance for the NO reduction. ZnII‐exchanged zeolites exhibit a moderate catalytic activity, with conversions of NO of ≈58 % and high selectivity to N2 at high temperatures.  相似文献   
254.
The synthesis of P,P'-dimesityl-P,P'-diphenyldiphosphine and benzylmesitylphenylphosphine is described as well as the resolution of the latter ligand by means of homochiral organometallic complexes. The absolute configuration of the phosphine is assigned by NMR spectra, using the homochiral palladacycle as a reference point. The configuration has been confirmed by single crystal X-ray diffraction. Molecular mechanics calculations were performed in [PdCl-(R)-(+)-C10H6CH(Me)NH2(PBnMesPh)], and showed that the rotation around the Pd-P bond is restricted in this complex. [Pd(eta3-2-MeC3H4)Cl(PBnMesPh)] was obtained and used as a precursor in the catalytic hydrovinylation of styrene. Benzylmesitylphenylphosphine has a strong tendency to form phosphapalladacycles by activation of one of the ortho-methyl groups. The formation of this metallacycle from cyclopalladated N-donor derivatives by a ligand-exchange reaction is also described.  相似文献   
255.
(+)- or (-)-p-Menthane-3-carboxaldehyde is made in two easy steps from (+)- or (-)-menthone, respectively. This auxiliary allows for the synthesis of carbonyl compounds bearing a alpha-chiral quaternary carbon. The flexibility, efficiency, and ease of use of the method are demonstrated in a series of examples, which include the total synthesis of (+)-cuparenone as well as a partial synthesis of (-)-cassiol.  相似文献   
256.
257.
We prove , bounds for

and

where , are rational functions. Our bounds depend only on the degrees of the polynomials and, in particular, they do not depend on the coefficients of these polynomials.

  相似文献   

258.
A series of acetyl pyrenes and pyrenyl ynones with and without tert-butyl groups showed distinct mechanofluorochromism (MFC). Four pairs of polymorphic solids were found out of six compounds and interestingly, each of them showed hypsochromic, bathochromic or off-to-on MFC. The MFC properties were rationalized by categorizing the packing schemes into herringbone, sandwich, beta and gamma motifs depending on the relative contributions of C⋅⋅⋅C (or π-π) against C⋅⋅⋅H contacts. The bulky tert-butyl and trimethylsilyl groups served not only to reduce the number of aggregation patterns but also to prohibit the complete back reactions in solid state. Our results suggest that the simple pyrene derivatives may be promising candidates for a novel group of mechanically-sensitive materials.  相似文献   
259.
It has been reported that cellulose is better dissolved in NaOH-water when a certain amount of urea is added. In order to understand the mechanisms of this dissolution and the interactions between the components, the binary phase diagram of urea/water, the ternary urea/NaOH/water phase diagram and the influence of the addition of microcrystalline cellulose in urea/NaOH/water solutions were studied by DSC. Urea/water solutions have a simple eutectic behaviour with a eutectic compound formed by pure urea and ice (one urea per eight water moles), melting at −12.5 °C. In the urea/NaOH/water solutions, urea and NaOH do not interact, each forming their own eutectic mixtures, (NaOH + 5H2O, 4H2O) and (urea, 8H2O), as found in their binary mixtures. When the amount of water is too low to form the two eutectic mixtures, NaOH is attracting water at the expense of urea. In the presence of microcrystalline cellulose, the interactions between cellulose and NaOH/water are exactly the same as without urea, and urea is not interacting with cellulose. A tentative explanation of the role of urea is to bind water, making cellulose-NaOH links more stable. Member of the European Polysaccharide Network of Excellence (EPNOE),  相似文献   
260.
The stabilization of olive recombinant hydroperoxide lyases (rHPLs) was investigated using selected chemical additives. Two rHPLs were studied: HPL full-length and HPL with its chloroplast transit peptide deleted (matured HPL). Both olive rHPLs are relatively stable at 4 °C, and enzyme activity can be preserved (about 100% of the rHPL activities are maintained) during 5 weeks of storage at ?20 or at ?80 °C in the presence of glycerol (10%, v/v). Among the additives used in this study, glycine (2.5% w/v), NaCl (0.5 M), and Na2SO4 (0.25 M) provided the highest activation of HPL full-length activity, while the best matured HPL activity was obtained with Na2SO4 (0.25 M) and NaCl (1 M). Although the inactivation rate constants (k) showed that these additives inactivate both rHPLs, their use is still relevant as they strongly increase HPL activity. Results of C6-aldehyde production assays also showed that glycine, NaCl, and Na2SO4 are appropriate additives and that NaCl appears to be the best additive, at least for hexanal production.  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