全文获取类型
收费全文 | 26071篇 |
免费 | 600篇 |
国内免费 | 156篇 |
专业分类
化学 | 17488篇 |
晶体学 | 206篇 |
力学 | 663篇 |
综合类 | 1篇 |
数学 | 4423篇 |
物理学 | 4046篇 |
出版年
2021年 | 172篇 |
2020年 | 213篇 |
2019年 | 284篇 |
2018年 | 232篇 |
2017年 | 208篇 |
2016年 | 444篇 |
2015年 | 346篇 |
2014年 | 460篇 |
2013年 | 1327篇 |
2012年 | 1193篇 |
2011年 | 1510篇 |
2010年 | 781篇 |
2009年 | 626篇 |
2008年 | 1367篇 |
2007年 | 1418篇 |
2006年 | 1410篇 |
2005年 | 1416篇 |
2004年 | 1241篇 |
2003年 | 1044篇 |
2002年 | 1018篇 |
2001年 | 368篇 |
2000年 | 296篇 |
1999年 | 246篇 |
1998年 | 286篇 |
1997年 | 313篇 |
1996年 | 397篇 |
1995年 | 298篇 |
1994年 | 284篇 |
1993年 | 290篇 |
1992年 | 275篇 |
1991年 | 239篇 |
1990年 | 224篇 |
1989年 | 241篇 |
1988年 | 268篇 |
1987年 | 250篇 |
1986年 | 245篇 |
1985年 | 386篇 |
1984年 | 438篇 |
1983年 | 322篇 |
1982年 | 427篇 |
1981年 | 402篇 |
1980年 | 342篇 |
1979年 | 324篇 |
1978年 | 346篇 |
1977年 | 331篇 |
1976年 | 275篇 |
1975年 | 270篇 |
1974年 | 260篇 |
1973年 | 239篇 |
1972年 | 145篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
181.
182.
Heptakis(2,6-di-O-methyl)-β-cyclodextrin interacts with 5,15-diphenylporphine to produce a 2:1 complex in dimethyl sulfoxide. This complex possesses a hydrophobic groove that circumscribes the metal binding site of the porphyrin moiety. 相似文献
183.
The solvent dependence of the 2-naphthyl(carbomethoxy)carbene (2) singlet-triplet energy gap has been examined by time-resolved infrared (TRIR) and computational methods. The ground state of 2 changes from the triplet state in hexane to the singlet state in acetonitrile. Preferential stabilization of the singlet carbene is the result of its increased dipole moment in polar solvents. Variable-temperature TRIR experiments provide measurements of the enthalpic and entropic differences between (1)2 and (3)2 and suggest that solvent and geometry effects on the entropy of singlet and triplet carbenes can offset differences arising from spin multiplicity. B3LYP calculations using the polarizable continuum solvation model (PCM) reproduce the general trends in enthalpic differences seen experimentally. 相似文献
184.
A comprehensive two-dimensional gas chromatography (GC x GC) instrument has been created by coupling a flow-switching modulator and a standard gas chromatograph. The instrument was used to characterize the aromatic composition of gasoline. The high-resolution separation produced by flow-switching GC x GC allowed gasoline aromatics to be fully resolved from saturated components. The aromatic compounds were further separated into groups having the same carbon number. A standard gasoline sample was analyzed to evaluate the quantitative accuracy and precision of this technique. The data show that flow-switching GC x GC produces results that are comparable to gas chromatography-mass spectrometry (GC-MS) and thermal modulation GC x GC. The simple, low-cost, and robust nature of flow-switching GC x GC makes it an ideal technique for the routine analysis of aromatic compounds in gasoline. 相似文献
185.
186.
187.
The oxymercuration—demercuration (OM—DM) of representative trimethylsilylalkenes in aqueous tetrahydrofuran was investigated. Vinyltrimethylsilane gives only the anti-Markovnikov alcohol, 2-trimethylsilylethanol in 90% yield. Allyltrimethylsilane gives mainly allylmercuric acetate which reacts further to give a 1,3-dimercurated-2-propanol. This species reacts slowly with allyltrimethylsilane to give allylmercuric acetate. 3-Buten-1-yltrimethylsilane gives only the Markovnikov product, 1-trimethylsilyl-3-butanol in 99% yield. cis-1-Propenyl-trimethylsilane gives 1-trimethylsilyl-2-propanol in 44% yield along with cleavage products. The formation of one of these products, 2-propanol, was found by PMR to arise from the OM—DM of cis-1-propenylmercuric acetate which gives the 1,1-dimercurated-2-propanol. While the oxymercuration of cis-1-propenyl-mercuric acetate is regiospecific giving exclusive placement of the mercury α to the silicon, it is completely nonstereospecific giving equal amounts of diastereomeric adducts. The OM—DM of 2-propenyl trimethylsilane (XIX) leads only to desilylation and oxidation products. Possible reasons for the diverse behavior of these silylated alkenes are discussed. 相似文献
188.
HYDROXYSTILBAZOLES AND HYDROXYAZAPHENANTHRENES: PHOTOCYCLIZATION AND FLUORESCENCE STUDIES 总被引:1,自引:0,他引:1
Abstract— As models for novel fluorescent probes, we have synthesized three isomeric hydroxystilbazole systems (4′-hydroxy-substituted 2-, 3-, and 4-stiIbazoles), examined their photocyclization-oxi-dation to four hydroxyazaphenanthrene systems, and made a preliminary study of their absorption and fluorescence spectra. All three stilbazoles can be prepared easily by addition of the isomeric picoline anions to 4-methoxybenzaldehyde, followed by dehydration and deprotection. Photocyclization proceeds efficiently, furnishing a single product isomer from each of the 2- and 4-stilbazole systems, and two isomeric azaphenanthrenes from the 3-stilbazole. The stilbazoles all have intense UV absorbance bands whose maxima depend upon solvent and pH; all three isomers have relatively similar spectra under neutral conditions and all three show a large red shift in base; in acid, however, the 2-and 4-stilbazole isomers show a greater red shift than the 3-stilbazole. The fluorescence of the stilbazoles is also solvent dependent, shifting to the red in more polar medium; red shifts are also observed in acid and base, but in acid, the 3-stilbazole shows a larger shift. The azaphenanthrene photocyclization products show absorbance spectra typical for quinolines and isoquinolines; their absorptivities are less than the stilbazoles, but their fluorescence is more intense. In general, the benzoquinolines have longer wavelength but weaker fluorescence than the benzoisoquinolines. Also, those isomers in which the resonance effects of the hydroxy and nitrogen groups can reinforce one another show longer wavelength emissions of greater intensity. All seven systems show dual fluorescence in water under neutral conditions, suggesting the emission from both non-ionized and ionized species in the excited state. In one case, the benzoisoquinoline system derived from 4′-hydroxy-4-stilbazole, an emission at 640 nm, observed in water over a wide pH range, is ascribed to a zwitterionic phototautomer. These stilbazoles, benzoquinolines and benzoisoquinolines may prove to be useful spectroscopic probes. 相似文献
189.
Ballou S Goodpaster J MacCrehan W Reeder D 《Analytical and bioanalytical chemistry》2003,376(8):1149-1150
190.
Effects of sugar inhibition on cellulases and β-glucosidase during enzymatic hydrolysis of softwood substrates 总被引:1,自引:0,他引:1
Zhizhuang Xiao Xiao Zhang David J. Gregg John N. Saddler 《Applied biochemistry and biotechnology》2004,115(1-3):1115-1126
A quantitative approach was taken to determine the inhibition effects of glucose and other sugar monomers during cellulase
and β-Glucosidase hydrolysis of two types of cellulosic material: Avicel and acetic acid-pretreated softwood. The increased
glucose content in the hydrolysate resulted in a dramatic increase in the degrees of inhibition on both β-Glucosidase and
cellulase activities. Supplementation of mannose, xylose, and galactose during cellobiose hydrolysis did not show any inhibitory
effects on β-Glucosidase activity. However, these sugars were shown to have significant inhibitory effects on cellulase activity
during cellulose hydrolysis. Our study suggests that high-substrate consistency hydrolysis with supplementation of hemicellulose
is likely to be a practical solution to minimizing end-product inhibition effects while producing hydrolysate with high glucose
concentration. 相似文献