全文获取类型
收费全文 | 1608篇 |
免费 | 43篇 |
国内免费 | 4篇 |
专业分类
化学 | 1153篇 |
晶体学 | 16篇 |
力学 | 28篇 |
数学 | 198篇 |
物理学 | 260篇 |
出版年
2021年 | 15篇 |
2020年 | 25篇 |
2019年 | 26篇 |
2018年 | 12篇 |
2017年 | 17篇 |
2016年 | 28篇 |
2015年 | 34篇 |
2014年 | 42篇 |
2013年 | 70篇 |
2012年 | 77篇 |
2011年 | 90篇 |
2010年 | 57篇 |
2009年 | 49篇 |
2008年 | 83篇 |
2007年 | 77篇 |
2006年 | 69篇 |
2005年 | 61篇 |
2004年 | 55篇 |
2003年 | 33篇 |
2002年 | 44篇 |
2001年 | 41篇 |
2000年 | 37篇 |
1999年 | 24篇 |
1998年 | 26篇 |
1997年 | 22篇 |
1996年 | 19篇 |
1995年 | 32篇 |
1994年 | 17篇 |
1993年 | 20篇 |
1992年 | 40篇 |
1991年 | 35篇 |
1990年 | 13篇 |
1989年 | 13篇 |
1988年 | 17篇 |
1986年 | 10篇 |
1985年 | 19篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 21篇 |
1979年 | 10篇 |
1978年 | 20篇 |
1977年 | 16篇 |
1975年 | 12篇 |
1974年 | 10篇 |
1973年 | 15篇 |
1972年 | 10篇 |
1968年 | 9篇 |
1960年 | 8篇 |
排序方式: 共有1655条查询结果,搜索用时 93 毫秒
21.
The possibility of lasing without inversion on the 1079.8 nm line of a HeNe laser is investigated. It is shown that a modified double- scheme can be realized by use of the 877.9 nm line for introducing Zeeman coherence. It is shown experimentally that Zeeman coherence can enforce laser action, even if the inversion is below threshold. A different choice of the polarization of the driving beam can result in suppression of laser action above threshold. The underlying mechanisms are examined. On the 611.8 nm line, laser action below threshold inversion is obtained with the driving beam tuned to 824.9 nm (up-conversion). 相似文献
22.
23.
Schmidt R Blaich T Elze TW Emling H Freiesleben H Grimm K Henning W Holzmann R Keller JG Klingler H Kulessa R Kratz JV Lambrecht D Lange JS Leifels Y Lubkiewicz E Moore EF Wajda E Prokopowicz W Schütter C Spies H Stelzer K Stroth J Walus W Wollersheim HJ Zinser M Zude E 《Physical review letters》1993,70(12):1767-1770
24.
Molecular dynamics simulations have been performed to gain insights into the catalytic mechanism of the hydrolysis of epoxides to vicinal diols by soluble epoxide hydrolase (sEH). The binding of a substrate, 1S,2S-trans-methylstyrene oxide, was studied in two conformations in the active site of the enzyme. It was found that only one is likely to be found in the active enzyme. In the preferred conformation the phenyl group of the substrate is pi-sandwiched between two aromatic residues, Tyr381 and His523, whereas the other conformation is pi-stacked with only one aromatic residue, Trp334. Two simulations were carried out to 1 ns for each conformation to evaluate the protonation state of active site residue His523. It was found that a protonated histidine is essential for keeping the active site from being disrupted. Long time scale, 4 ns, molecular dynamics simulation was done for the structure with the most likely combination of binding conformation and protonation state of His523. Near Attack Conformers (NACs) are present 5.3% of the time and nucleophilic attack on either epoxide carbon atom, approximately 75% on C(1) and approximately 25% on C(2), is found. A maximum of one hydrogen bond between the epoxide oxygen and either of the active site tyrosines, Tyr465 and Tyr381, is present, in agreement with experimental mutagenesis results that reveal a slight loss in activity if one tyrosine is mutated and essential loss of all activity upon double mutation of the two tyrosines in question. It was found that a hydrogen bond from Tyr465 to the substrate oxygen is essential for controlling the regioselectivity of the reaction. Furthermore, a relationship between the presence of this hydrogen bond and the separation of reactants was found. Two groups of amino acid segments were identified each as moving collectively. Furthermore, an overall anti-correlation was found between the movements of these two individually collectively moving groups, made up by parts of the cap-region, including the two tyrosines, and the site of the catalytic triad, respectively. This overall anti-correlated collective domain motion is, perhaps, involved in the conversion of E.NAC to E.TS. 相似文献
25.
S. F. Bureiko N. S. Golubev G. S. Denisov I. Ya. Lange 《Reaction Kinetics and Catalysis Letters》1977,7(2):139-144
The kinetic isotope effect in proton exchange between methanol and water in CCl4 solution was studied by1H and2H NMR and kinetic IR spectroscopy. The exchange rate increases in the series CD3OH+H2O3OD+H2O3OD+D2O. It has been concluded that the limiting step of the reaction involves the formation of an H-bonded cyclic intermediate.
1H 2H - CCl4. , CD3OH+H2O相似文献3OD+H2O 3OD+D2O. , - -.
26.
Asymmetric induction was achieved in the photoaddition of 3-methyl-2-cyclohexenone to optically active enoates 1a–c and the absolute configuration of the major adduct was determined. 相似文献
27.
A novel solid-state NMR concept is introduced that allows for the detection of through-space proton-proton contacts in high spectral resolution. The proposed method not only can be utilized to infer sequential assignments and backbone or side chain conformation in solid-phase polypeptides, but it also provides information about the three-dimensional arrangement of the molecule of interest. As a result, the molecular structure can be studied without additional restrictions regarding sample labeling or magic angle spinning rates. 相似文献
28.
The difference between liquid-crystal and gas-phase values for the nuclear quadrupole coupling constant in D2 and HD is used to obtain the mean electric field gradient in various liquid crystals. Order parameters for small molecules dissolved in liquid crystals are calculated assuming that the orientational order arises from the interaction of the molecular quadrupole moment with the average field gradient. The results obtained are in good agreement with experimental values for hydrogen and several other solutes. 相似文献
29.
Halland N Alstrup Lie M Kjaersgaard A Marigo M Schiøtt B Jørgensen KA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(23):7083-7090
The mechanism for the 2,5-diphenylpyrrolidine-catalyzed enantioselective alpha-chlorination of aldehydes with electrophilic halogenation reagents has been investigated by using experimental and computational methods. These studies have led us to propose a mechanism for the reaction that proceeds through an initial N-chlorination of the chiral catalyst-substrate complex, followed by a 1,3-sigmatropic shift of the chlorine atom to the enamine carbon atom. The suggested reaction course is different from previously proposed mechanisms for organocatalytic enamine reactions, in which the carbon-electrophile bond is formed directly. Furthermore, the rate-determining step in the overall reaction was determined and the presence of nonlinear effects was probed. 相似文献
30.