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   197篇
  免费   8篇
化学   184篇
晶体学   1篇
力学   2篇
数学   3篇
物理学   15篇
  2023年   2篇
  2022年   3篇
  2021年   7篇
  2020年   2篇
  2019年   5篇
  2018年   4篇
  2017年   1篇
  2016年   11篇
  2015年   7篇
  2014年   6篇
  2013年   11篇
  2012年   19篇
  2011年   15篇
  2010年   6篇
  2009年   7篇
  2008年   6篇
  2007年   14篇
  2006年   9篇
  2005年   12篇
  2004年   16篇
  2003年   9篇
  2002年   7篇
  2001年   3篇
  2000年   2篇
  1998年   2篇
  1996年   1篇
  1994年   1篇
  1993年   2篇
  1992年   1篇
  1991年   2篇
  1990年   2篇
  1989年   1篇
  1988年   2篇
  1985年   1篇
  1982年   2篇
  1981年   1篇
  1977年   1篇
  1975年   1篇
  1969年   1篇
排序方式: 共有205条查询结果,搜索用时 9 毫秒
131.
A novel indomethacin ester prodrug, 2-[N-[3-(3-(piperidinomethyl)phenoxy)propyl]carbamoylmethylthio ]ethyl 1-(p-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetate (1) was prepared from a new histamine H2-receptor antagonist, N-[3-(3-(piperidinomethyl)phenoxy)propyl]-2-(2-hydroxyethylthio )acetamide (2) and indomethacin (3). The compound 1 was shown to be essentially similar to 3 in its antiinflammatory action and to almost completely inhibit carrageenin-induced hind-paw edema in the rat at a very high dose of 230 mg/kg (280 mumol/kg), which is comparable to that of 100 mg/kg (280 mumol/kg) of 3, without producing gastric lesions. On a molar basis, the acute gastric lesioning properties of 1 were near one-hundred times less than those of 3, resulting in over a twenty-fold improvement in the ratio of antiedema activity to ulcerogenicity. The effect of the co-administration of histamine H2-receptor antagonists on antiedema activity and ulcerogenicity caused by 3 is also discussed.  相似文献   
132.
The isolation of the bartolosides, unprecedented cyanobacterial glycolipids featuring aliphatic chains with chlorine substituents and C‐glycosyl moieties, is reported. Their chlorinated dialkylresorcinol (DAR) core presented a major structural‐elucidation challenge. To overcome this, we discovered the bartoloside (brt) biosynthetic gene cluster and linked it to the natural products through in vitro characterization of the DAR‐forming ketosynthase and aromatase. Bioinformatic analysis also revealed a novel potential halogenase. Knowledge of the bartoloside biosynthesis constrained the DAR core structure by defining key pathway intermediates, ultimately allowing us to determine the full structures of the bartolosides. This work illustrates the power of genomics to enable the use of biosynthetic information for structure elucidation.  相似文献   
133.
The ratio of the easily interconverting rotational isomers of biphenyl-2,2'-diylbis[bis(4-dimethylaminophenyl)methylium] (R)/(S)-1a(2+) can be biased to prefer an R configuration upon 1:1 complexation with γ-cyclodextrin in water. Through the reaction with Na(2)S, the preference of 1a(2+)@γ-CyD for an axial chirality of R can be fixed as the M-helicity of dihydrothiepin 2.  相似文献   
134.
[meso-Triaryl-21,23-didehydro-23H-5-oxaporphyrinato](trifluoroacetato)zinc(II) was prepared by the reaction of meso-triarylbilindione with acetic anhydride and zinc acetate, and it was isolated as a trifluoroacetate salt. The X-ray crystallographic study demonstrated that the trifluoroacetate anion was coordinated to the zinc ion. [21,23-Didehydro-10,15,20-tris(4-methoxycarbonylphenyl)-23H-5-oxaporphyrinato](trifluoroacetato)zinc(II) 3a was dissolved in various organic solvents such as toluene, chloroform, diethyl ether, ethyl acetate, acetone, acetonitrile, methanol, DMSO, and DMF, although it readily reacted with alcohols and DMF to yield linear tetrapyrroles. The solubility of 3a in toluene was 4.2 ± 0.1 g dm(-3) at room temperature. 3a showed characteristic UV-vis absorption at 649 nm and fluorescence emission at 657 nm in chloroform. The fluorescence quantum yields of 3a, [21,23-didehydro-10,15,20-triphenyl-23H-5-oxaporphyrinato](trifluoroacetato)zinc(II) (3c), and [21,23-didehydro-10,15,20-tris(4-methoxyphenyl)-23H-5-oxaporphyrinato](trifluoroacetato)zinc(II) (3b) were 0.071, 0.071, and 0.050, respectively. Reaction of 3a with EtOH afforded the zinc complex of 19-ethoxybilinone, and it proceeded 2 orders of magnitude faster than that of [β-octaalkyl-21,23-didehydro-23H-5-oxaporphyrinato]zinc(II). The reaction with alcohols was sensitive to steric bulk of the alcohols; the rate of reaction with i-PrOH was 2700 times faster than that of t-BuOH at 303 K. The reaction of [meso-triaryl-21,23-didehydro-23H-5-oxaporphyrinato]zinc(II) with water proceeded 3 orders of magnitude slower than that with EtOH.  相似文献   
135.
In order to avoid denaturation of biomolecules due to strong adsorption on solid surfaces, a soft substrate has to be used for atomic force microscopy (AFM) observation. We propose a hydrophilic agarose gel surface as a soft substrate for AFM to observe liposomes and lipid membranes. Although our simple method does not require any delicate control at the molecular level, an agarose gel surface can be simply flattened to 0.3 nm in roughness using an atomically flat solid surface during gelation. The AFM images revealed that liposomes were unruptured on the gel surface at low liposome density, whereas an unruptured state was difficult to obtain on a solid surface like mica. This indicates that the weak interaction between the liposome and the soft surface inhibits the liposome from rupturing, and also that the surface rougher than the solid surface prevents lateral diffusion of the liposomes along the surface to be fused. Increasing the liposome density resulted in a lipid membrane at various thicknesses forming on the hydrogel surface by the fusion and rupture of liposomes. Using the soft substrate, it can be expected to promote investigations of structures and functions of biomolecules at the nanometer scale under physiological conditions with AFM.  相似文献   
136.
A grating interferometer composed of two transmission gratings in tandem irradiated incoherently was studied for an atom interferometer. Determination of its interference effect was made with consideration of the optical transfer function. An atom beam had a velocity distribution, which made the influence on the interference phenomenon. The calculated results show that the fringe (grating image) with relatively high contrast is obtained under a certain grating arrangement even if the interferometer is irradiated by a spatially incoherent polychromatic atom beam.  相似文献   
137.
A macroporous silica-based supramolecular recognition absorbent (Calix[4]?+?Dodecanol)/SiO2?CP, was prepared by successive impregnation and fixing the 1,3-[(2,4-diethylheptylethoxy)oxy]-2,4-crown-6-Calix[4]arene (Calix[4]arene-R14) and its molecule modifier 1-Dodecanol onto SiO2 silica-based polymer support. The characterization of (Calix[4]?+?Dodecanol)/SiO2?CP was examined by thermal gravimetry and differential thermal analysis and electron probe microanalysis. Relatively large separation factors of Cs and other metal ions (?? Cs/M n+ ) above 60 were obtained in the presence of 3?M HNO3. The adsorption data of Cs(I) fitted well with Langmuir isotherm and the maximum adsorption capacity was estimated to be 0.19?mmol?g?1. The Cs(I) in 3?M HNO3 were also effectively adsorption on (Calix[4]?+?Dodecanol)/SiO2?CP in the column operation, and the loaded Cs(I) was successfully eluted with an eluent of H2O. The column packed with (Calix[4]?+?Dodecanol)/SiO2?CP had excellent reusability after three cycles.  相似文献   
138.
The purpose of this study was to investigate the usefulness and feasibility of magnetic resonance imaging (MRI) with ultrasmall superparamagnetic iron oxide (USPIO) (USPIO-enhanced MRI) for imaging inflammatory tissues. First, we investigated the relationship between the apparent transverse relaxation rate (R2*) and the concentration of USPIO by phantom studies and measured the apparent transverse relaxivity (r2*) of USPIO. Second, we performed animal experiments using a total of 30 mice. The mice were divided into five groups [A (n=6), B (n=6), C (n=6), sham control (n=6), and control (n=6)]. The mice in Groups A, B, C and control were subcutaneously injected with 0.1 ml of turpentine oil on Day 0, while those in the sham control group were subcutaneously injected with 0.1 ml of saline. The mice in Groups A, B, C and sham control were intraperitoneally injected with 200 μmol Fe per kilogram body weight of USPIO (28 nm in diameter) immediately after the first MRI study on Days 3, 5, 7 and 7, respectively, and those in the control group were not injected with USPIO. The second and third MRI studies were performed at 24 and 48 h after USPIO administration, respectively. The maps of R2* were generated from the apparent transverse relaxation time (T2*)-weighted images with six different echo times. The phantom studies showed that there was a linear relationship between R2* and the concentration of USPIO (r=0.99) and the r2* value of USPIO was 105.7 mM−1 s−1. There was a significant increase of R2* in inflammatory tissues in Group C at 24 h after USPIO administration compared with the precontrast R2* value. Our results suggest that USPIO-enhanced MRI combined with R2* measurement is useful for detecting inflammatory tissues.  相似文献   
139.
The directional properties of bottlenose dolphin clicks, burst-pulse, and whistle signals were measured using a five element array, at horizontal angles of 0°, 45°, 90°, 135°, and 180° relative to a dolphin stationed on an underwater biteplate. Clicks and burst-pulse signals were highly directional with directivity indices of ~11 dB for both signal types. Higher frequencies and higher amplitudes dominated the forward, on-axis sound field. A similar result was found with whistles, where higher frequency harmonics had greater directivity indices than lower frequency harmonics. The results suggest the directional properties of these signals not only provide enhanced information to the sound producer (as in echolocation) but can provide valuable information to conspecific listeners during group coordination and socialization.  相似文献   
140.
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