全文获取类型
收费全文 | 556篇 |
免费 | 44篇 |
专业分类
化学 | 355篇 |
晶体学 | 1篇 |
力学 | 5篇 |
数学 | 92篇 |
物理学 | 147篇 |
出版年
2022年 | 3篇 |
2021年 | 7篇 |
2020年 | 15篇 |
2019年 | 10篇 |
2017年 | 3篇 |
2016年 | 35篇 |
2015年 | 25篇 |
2014年 | 26篇 |
2013年 | 33篇 |
2012年 | 30篇 |
2011年 | 40篇 |
2010年 | 29篇 |
2009年 | 17篇 |
2008年 | 34篇 |
2007年 | 22篇 |
2006年 | 22篇 |
2005年 | 22篇 |
2004年 | 24篇 |
2003年 | 15篇 |
2002年 | 10篇 |
2001年 | 7篇 |
2000年 | 11篇 |
1999年 | 8篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1994年 | 10篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1991年 | 3篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 9篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1972年 | 4篇 |
1971年 | 4篇 |
1968年 | 7篇 |
1926年 | 2篇 |
1893年 | 2篇 |
1891年 | 2篇 |
1890年 | 2篇 |
排序方式: 共有600条查询结果,搜索用时 328 毫秒
581.
Fustero S Cuñat AC Flores S Báez C Oliver J Cynamon M Gütschow M Mertens MD Delgado O Tresadern G Trabanco AA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(52):14772-14784
The preparation of novel fluorinated allylamines and their use as key fragments for the stereoselective synthesis of hydroxyethyl secondary amine (HEA)-type peptidomimetics is described. Our strategy employs chiral sulfinyl imines as synthesis intermediates, by treatment of hemiaminal precursors with two equivalents of vinylmagnesium bromide. The subsequent oxidation of the allylic amines to the corresponding epoxides was achieved by treatment with methyl(trifluoromethyl)dioxirane. Finally, epoxide ring opening with a range of nitrogen nucleophiles provided a library of HEA-derived peptidomimetics with a phenyldifluoromethylene moiety. The biological evaluation of these derivatives revealed compounds with remarkable BACE1 inhibitory activity. Docking studies revealed the influence of the fluorine atoms in the binding mode of the synthesized ligands. Furthermore, the biological evaluation of our final products and synthesis intermediates led to the discovery of compounds with antimicrobial activity against Mycobacterium and Nocardia species. 相似文献
582.
Biocatalytic organic synthesis of optically pure (S)-scoulerine and berbine and benzylisoquinoline alkaloids 总被引:1,自引:0,他引:1
Schrittwieser JH Resch V Wallner S Lienhart WD Sattler JH Resch J Macheroux P Kroutil W 《The Journal of organic chemistry》2011,76(16):6703-6714
A chemoenzymatic approach for the asymmetric total synthesis of the title compounds is described that employs an enantioselective oxidative C-C bond formation catalyzed by berberine bridge enzyme (BBE) in the asymmetric key step. This unique reaction yielded enantiomerically pure (R)-benzylisoquinoline derivatives and (S)-berbines such as the natural product (S)-scoulerine, a sedative and muscle relaxing agent. The racemic substrates rac-1 required for the biotransformation were prepared in 4-8 linear steps using either a Bischler-Napieralski cyclization or a C1-Cα alkylation approach. The chemoenzymatic synthesis was applied to the preparation of fourteen enantiomerically pure alkaloids, including the natural products (S)-scoulerine and (R)-reticuline, and gave overall yields of up to 20% over 5-9 linear steps. 相似文献
583.
Bart De Ketelaere Kristof Mertens Frank Mathijs Daniel Sabin Diaz Josse De Baerdemaeker 《商业与工业应用随机模型》2011,27(4):367-376
Statistical process control (SPC) is a powerful framework that is used in many industries to decrease process variability and to pinpoint special cause variation. Although a broad range of techniques have been developed to do so, often the real‐life situation does not fully comply with the basic assumptions that are made in SPC resulting in poor results. One of the main violations against the assumptions is the fact that industrial processes rarely behave in a stationary manner — this is evidently the case for biological processes but is also an important issue when monitoring industrial processes. Besides, the ever increasing amount of data, with a clear shift towards multivariate and even multiway quality control, makes the classical univariate approach not feasible anymore. These two observations pose important challenges to statisticians to develop novel SPC algorithms that are broadly applicable in modern industries. In this contribution we discuss both issues and use two very different case studies to show the reader recent directions and developments in the SPC landscape. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
584.
Deepali Waghray Arvid Cloet Prof. Dr. Kristof Van Hecke Dr. Stijn F. L. Mertens Prof. Dr. Steven De Feyter Prof. Dr. Luc Van Meervelt Prof. Dr. Mark Van der Auweraer Prof. Dr. Wim Dehaen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(36):12077-12085
Diazadithia[7]helicenes were synthesized from the readily available building block ethyl 7‐chloro‐8‐formylthieno[3,2‐f]quinoline‐2‐carboxylate by a Wittig reaction–photocyclization strategy. The helicene core was functionalized by nucleophilic aromatic substitution with a variety of nucleophiles (e.g., O‐, N‐, and C‐centered) and palladium‐catalyzed reactions such as Suzuki coupling and Buchwald–Hartwig amination. Racemization studies confirmed that the enantiopure forms of these [7]helicenes are conformationally stable compared to their lower analogues. The solid‐state structures of the novel diazadithia[7]helicenes were determined by single‐crystal X‐ray diffraction. The crystal structures of these azathia[7]helicenes show columnar stacking in antiparallel fashion. The HOMO–LUMO gaps of the new compounds were determined on the basis of electrochemical and optical measurements. 相似文献
585.
The effect of a wave with a varying traveling component on the bubble activity as well as the physical force generated by microbubbles on a surface has been studied. The acoustic emission from a collection of bubbles is measured in a 928 kHz sound field. Particle removal tests on a surface, which actually measures the applied physical force by the bubbles on that surface, indicate a very strong dependence on the angle of incidence. In other words, when the traveling wave component is maximized, the average physical force applied by microbubbles reaches a maximum. Almost complete particle removal for 78 nm silica particles was obtained for a traveling wave, while particle removal efficiency was reduced to only a few percent when a standing wave was applied. This increase in particle removal for a traveling wave is probably caused by a decrease in bubble trapping at nodes and antinodes in a standing wave field. 相似文献
586.
587.
Dr. Eike T. Spielberg Eugen Edengeiser Dr. Bert Mallick Prof. Dr. Martina Havenith Prof. Dr. Anja‐Verena Mudring 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(18):5338-5345
The compound (C4C1py)[Cu(SCN)2], (C4C1py=1‐Butyl‐4‐methyl‐pyridinium), which can be obtained from CuSCN and the ionic liquid (C4C1py)(SCN), turns out to be a new organic–inorganic hybrid material as it qualifies both, as a coordination polymer and an ionic liquid. It features linked [Cu(SCN)2]? units, in which the thiocyanates bridge the copper ions in a μ1,3‐fashion. The resulting one‐dimensional chains run along the a axis, separated by the C4C1py counterions. Powder X‐ray diffraction not only confirms the single‐crystal X‐ray structure solution but proves the reformation of the coordination polymer from an isotropic melt. However, the materials shows a complex thermal behavior often encountered for ionic liquids such as a strong tendency to form a supercooled melt. At a relatively high cooling rate, glass formation is observed. When heating this melt in differential scanning calorimetry (DSC) and temperature‐dependent polarizing optical microscopy (POM), investigations reveal the existence of a less thermodynamically stable crystalline polymorph. Raman measurements conducted at 10 and 100 °C point towards the formation of polyanionic chain fragments in the melt. Solid‐state UV/Vis spectroscopy shows a broad absorption band around 18 870 cm?1 (530 nm) and another strong one below 20 000 cm?1 (<500 nm). The latter is attributed to the d(CuI)→π*(SCN)‐MLCT (metal‐to‐ligand charge transfer) transition within the coordination polymer yielding an energy gap of 2.4 eV. At room temperature and upon irradiation with UV light, the material shows a weak fluorescence band at 15 870 cm?1 (630 nm) with a quantum efficiency of 0.90(2) % and a lifetime of 131(2) ns. Upon lowering the temperature, the luminescence intensity strongly increases. Simultaneously, the band around 450 nm in the excitation spectrum decreases. 相似文献
588.
Verena Schnitzler 《Journal of organometallic chemistry》2008,693(15):2610-2614
Reaction of 3,4-dimethylphospholylthallium (Tl-1) with [Cp∗MCl2]2 (M = Rh, Ir) leads to the formation of the dimeric species [(Cp∗M)2(Me2C4H2P)3]+2 and 3 with bridging μ-η1:η1-phospholyl ligands. The phosphametallocenium sandwich complexes [Cp∗M(Me2C4(SiMe3)2P)]+7 (M = Rh) and 8 (M = Ir) could be obtained from the reaction of [Cp∗MCl2]2 and the 2,5-bis(trimethylsilyl)-1-trimethylstannylphosphole 6, with the bulky trimethylsilyl groups preventing the phosphole from η1- and enforcing a η5-coordination. The structures of phospharhodocenium cation 7 and a byproduct 9 containing a phosphairidocenium moiety could be determined by X-ray diffraction. 相似文献
589.
Indium(I) tetraiodoaluminate, InAlI4, was obtained from the respective molar amounts of In, InI3 and AlI3 at 553 K. The crystal structure [monoclinic, P21/m (no. 11), a = 713.5(4), b = 1182.2(3), c = 1108.3(6) pm, β = 96.32(6)°, V = 1013.2(8)·106 pm3, Z = 4, R1 for 981 reflections with I0>2σ(I0): 0.0404] contains discrete indium(I) cations and tetraiodoaluminate anions. No evidence was found for a stereochemical activity of the In‐5s2 electron pair. InAlI4 (Person code mP24) represents a new structure type within the ABX4 structure family. 相似文献
590.
Photonic nanostructures for advanced light trapping in silicon solar cells: the impact of etching on the material electronic quality 下载免费PDF全文
Christos Trompoukis Andre Stesmans Eddy Simoen Valérie Depauw Ounsi El Daif Kidong Lee Ivan Gordon Robert Mertens Jef Poortmans 《固体物理学:研究快报》2016,10(2):158-163
Dry plasma etching, commonly used by the Photonics community as the etching technique for the fabrication of photonic nanostructures, could be a source of device performance limitations when used in the frame of silicon photovoltaics. So far, the lack of silicon solar cells with state‐of‐the‐art efficiencies utilizing nanophotonic concepts shows how challenging their integration is, owing to the trade‐off between optical and electrical properties. In this study we show that dry plasma etching results in the degradation of the silicon material quality due to (i) a high density of dangling bonds and (ii) the presence of sub‐surface defects, resulting in high surface recombination velocities and low minority carrier lifetimes. On the contrary, wet chemical anisotropic etching used as an alternative, leads to the formation of inverted nanopyramids that result in low surface recombination velocity and low density of dangling bonds. The proposed inverted nanopyramids could enable high efficiency photonic assisted solar cells by offering the potential to achieve higher short‐circuit current without degrading the open circuit voltage. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) 相似文献