全文获取类型
收费全文 | 1546篇 |
免费 | 50篇 |
国内免费 | 10篇 |
专业分类
化学 | 1172篇 |
晶体学 | 9篇 |
力学 | 14篇 |
数学 | 252篇 |
物理学 | 159篇 |
出版年
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 31篇 |
2020年 | 20篇 |
2019年 | 19篇 |
2018年 | 11篇 |
2017年 | 19篇 |
2016年 | 40篇 |
2015年 | 38篇 |
2014年 | 59篇 |
2013年 | 68篇 |
2012年 | 88篇 |
2011年 | 100篇 |
2010年 | 64篇 |
2009年 | 51篇 |
2008年 | 107篇 |
2007年 | 81篇 |
2006年 | 103篇 |
2005年 | 80篇 |
2004年 | 72篇 |
2003年 | 55篇 |
2002年 | 57篇 |
2001年 | 22篇 |
2000年 | 20篇 |
1999年 | 19篇 |
1998年 | 19篇 |
1997年 | 21篇 |
1996年 | 15篇 |
1995年 | 21篇 |
1994年 | 24篇 |
1993年 | 22篇 |
1992年 | 12篇 |
1990年 | 18篇 |
1989年 | 13篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 14篇 |
1983年 | 11篇 |
1982年 | 7篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1975年 | 6篇 |
1974年 | 9篇 |
1973年 | 6篇 |
1972年 | 9篇 |
1971年 | 6篇 |
1969年 | 6篇 |
1968年 | 6篇 |
排序方式: 共有1606条查询结果,搜索用时 301 毫秒
991.
Metal diborides (MB(2)) often have interesting thermal, mechanical, and superconducting properties. MgB(2) was put into focus some years ago for its high transition temperature (39 K) in combination with its simple AlB(2) structure. The boron structure in MB(2) is assumed to be dependent on the electron transfer from the nearby positioned metal atoms. An electronic and structural comparison has been performed here for various initially planar and puckered transition-metal borides, using quantum mechanical density functional theory (DFT) calculations under periodic boundary conditions. In comparison to MgB(2), the experimentally planar transition-metal diborides (ZrB(2), NbB(2), and MoB(2)) and the experimentally puckered ones (TcB(2), RuB(2), RhB(2), and PdB(2)) have been examined. The results indicate that the energetic stability generally follows the experimentally obtained results. The metals that are less electronegative than boron donate electrons to boron, which in turn induce planar boron structures (graphitic-like). The metals that prefer to be planar donate more than one electron, while the trend for metals which favor puckered B structures is that they donate less than one electron per metal atom. Two donated electrons per metal atom (or very close to) will result in the most stable AlB(2) structure. 相似文献
992.
Mattsson A Leideborg M Larsson K Westin G Osterlund L 《The journal of physical chemistry. B》2006,110(3):1210-1220
Adsorption and solar light decomposition of acetone was studied on nanostructured anatase TiO2 and Nb-doped TiO2 films made by sol-gel methods (10 and 20 mol % NbO2.5). A detailed characterization of the film materials show that films contain only nanoparticles with the anatase modification with pentavalent Nb oxide dissolved into the anatase structure, which is interpreted as formation of substituted Nb=O clusters in the anatase lattice. The Nb-doped films displayed a slight yellow color and an enhanced the visible light absorption with a red-shift of the optical absorption edge from 394 nm for the pure TiO2 film to 411 nm for 20 mol % NbO2.5. In-situ Fourier transform infrared (FTIR) transmission spectroscopy shows that acetone adsorbs associatively with eta1-coordination to the surface cations on all films. On Nb-doped TiO2 films, the carbonyl bonding to the surface is stabilized, which is evidenced by a lowering of the nu(C=O) frequency by about 20 cm(-1) to 1672 cm(-1). Upon solar light illumination acetone is readily decomposed on TiO2, and stable surface coordinated intermediates are formed. The decomposition rate is an order of magnitude smaller on the Nb-doped films despite an enhanced visible light absorption in these materials. The quantum yield is determined to be 0.053, 0.004 and 0.002 for the pure, 10% Nb:TiO2, and 20%Nb:TiO2, respectively. Using an interplay between FTIR and DFT calculations we show that the key surface intermediates are bidentate bridged formate and carbonate, and H-bonded bicarbonate, respectively, whose concentration on the surface can be correlated with their heats of formation and bond strength to coordinatively unsaturated surface Ti and Nb atoms at the surface. The oxidation rate of these intermediates is substantially slower than the initial acetone decomposition rate, and limits the total oxidation rate at t>7 min on TiO2, while no decrease of the rate is observed on the Nb-doped films. The rate of degradation of key surface intermediates is different on pure TiO2 and Nb-doped TiO2, but cannot explain the overall lower total oxidation rate for the Nb-doped films. Instead the inferior photocatalytic activity in Nb-doped TiO2 is attributed to an enhanced electron-hole pair recombination rate due to Nb=O cluster and cation vacancy formation. 相似文献
993.
994.
Nienhaus K Deng P Belyea J Franzen S Nienhaus GU 《The journal of physical chemistry. B》2006,110(26):13264-13276
Dehaloperoxidase (DHP) is a globular heme enzyme found in the marine worm Amphitrite ornata that can catalyze the dehalogenation of halophenols to the corresponding quinones by using hydrogen peroxide as a cosubstrate. Its three-dimensional fold is surprisingly similar to that of the oxygen storage protein myoglobin (Mb). A key structural feature common to both DHP and Mb is the existence of multiple conformations of the distal histidine. In DHP, the conformational flexibility may be involved in promotion of substrate and cosubstrate entry and exit. Here we have explored the dynamics of substrate binding in DHP using Fourier transform infrared spectroscopy and flash photolysis. A number of discrete conformations at the active site were identified from the appearance of multiple CO absorbance bands in the infrared region of the spectrum. Upon photolysis at cryogenic temperatures, the CO molecules are trapped at docking sites within the protein matrix, as inferred from the appearance of several photoproduct bands characteristic of each site. Substrate binding stabilizes the protein by approximately 20 kJ/mol. The low yield of substrate-bound DHP at ambient temperature points toward a steric inhibition of substrate binding by carbon monoxide. 相似文献
995.
996.
Dr. Karin Rustler Dr. Galyna Maleeva Alexandre M. J. Gomila Prof. Dr. Pau Gorostiza Prof. Dr. Piotr Bregestovski Prof. Dr. Burkhard König 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(56):12722-12727
Optogenetic and photopharmacological tools to manipulate neuronal inhibition have limited efficacy and reversibility. We report the design, synthesis, and biological evaluation of Fulgazepam, a fulgimide derivative of benzodiazepine that behaves as a pure potentiator of ionotropic γ-aminobutyric acid receptors (GABAARs) and displays full and reversible photoswitching in vitro and in vivo. The compound enables high-resolution studies of GABAergic neurotransmission, and phototherapies based on localized, acute, and reversible neuroinhibition. 相似文献
997.
998.
Organoselenium compounds containing pyrazole or phenylthiazole groups: Synthesis,structure, tin(IV) complexes and antiproliferative activity
下载免费PDF全文
![点击此处可从《应用有机金属化学》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The diorganodiselenides (pzCH2CH2)2Se2 ( 1 ) and (PhtzCH2)2Se2 ( 2 ) were prepared by reacting Na2Se2 with 1‐(2‐bromoethyl)‐1H‐pyrazole and 4‐(chloromethyl)‐2‐phenylthiazole, respectively, while the reactions between 1‐(2‐bromoethyl)‐1H‐pyrazole or 4‐(chloromethyl)‐2‐phenylthiazole and the lithium organoselenolates [2‐(Et2NCH2)C6H4]SeLi and [2‐{O(CH2CH2)2NCH2}C6H4]SeLi in a 1:1 molar ratio resulted in the heteroleptic diorganoselenium(II) compounds [2‐(Et2NCH2)C6H4](R)Se (R = pzCH2CH2 ( 3 ) or PhtzCH2 ( 5 )) and [2‐{O(CH2CH2)2NCH2}C6H4](R)Se (R = pzCH2CH2 ( 4 ) or PhtzCH2 ( 6 )). The diorganotin(IV) bis(organoselenolato) derivatives of type R2Sn(SeCH2CH2pz)2 (R = 2‐(Me2NCH2)C6H4 ( 7 ) or Me ( 8 )) were obtained by reacting (pzCH2CH2)SeNa with the appropriate diorganotin(IV)dichloride in a 2:1 molar ratio. All compounds were investigated using NMR spectroscopy (1H, 13C, 77Se, 119Sn as appropriate) and ESI+ mass spectrometry. The molecular structures of 2 and 6 were determined using single‐crystal X‐ray diffraction. The formation of a 10–Se–3 hypercoordinated species was evidenced for 6 in the solid state, as a consequence of the C,N coordination behaviour of the 2‐{O(CH2CH2)2NCH2}C6H4 group. Compounds 1 , 7 and 8 were investigated for their antiproliferative activity towards the mouse colon carcinoma C26 cell line with the preliminary results showing a better activity than 5‐fluorouracil. 相似文献
999.
1000.