全文获取类型
收费全文 | 5786篇 |
免费 | 310篇 |
国内免费 | 26篇 |
专业分类
化学 | 4310篇 |
晶体学 | 14篇 |
力学 | 119篇 |
数学 | 807篇 |
物理学 | 872篇 |
出版年
2023年 | 56篇 |
2022年 | 30篇 |
2021年 | 90篇 |
2020年 | 116篇 |
2019年 | 123篇 |
2018年 | 72篇 |
2017年 | 66篇 |
2016年 | 244篇 |
2015年 | 204篇 |
2014年 | 197篇 |
2013年 | 311篇 |
2012年 | 395篇 |
2011年 | 417篇 |
2010年 | 265篇 |
2009年 | 238篇 |
2008年 | 344篇 |
2007年 | 300篇 |
2006年 | 314篇 |
2005年 | 315篇 |
2004年 | 270篇 |
2003年 | 188篇 |
2002年 | 208篇 |
2001年 | 112篇 |
2000年 | 90篇 |
1999年 | 89篇 |
1998年 | 78篇 |
1997年 | 93篇 |
1996年 | 70篇 |
1995年 | 77篇 |
1994年 | 57篇 |
1993年 | 61篇 |
1992年 | 47篇 |
1991年 | 26篇 |
1990年 | 29篇 |
1989年 | 35篇 |
1988年 | 18篇 |
1987年 | 23篇 |
1986年 | 20篇 |
1985年 | 21篇 |
1984年 | 25篇 |
1982年 | 23篇 |
1981年 | 18篇 |
1980年 | 17篇 |
1979年 | 15篇 |
1978年 | 24篇 |
1977年 | 20篇 |
1975年 | 18篇 |
1974年 | 19篇 |
1973年 | 22篇 |
1967年 | 14篇 |
排序方式: 共有6122条查询结果,搜索用时 15 毫秒
991.
In this Letter we study networks that have been optimized to realize a trade-off between communication efficiency and dynamical resilience. While the first is related to the average shortest pathlength, we argue that the second can be measured by the largest eigenvalue of the adjacency matrix of the network. Best efficiency is realized in star-like configurations, while enhanced resilience is related to the avoidance of short loops and degree homogeneity. Thus crucially, very efficient networks are not resilient while very resilient networks lack in efficiency. Networks that realize a trade-off between both limiting cases exhibit core-periphery structures, where the average degree of core nodes decreases but core size increases as the weight is gradually shifted from a strong requirement for efficiency and limited resilience towards a smaller requirement for efficiency and a strong demand for resilience. We argue that both, efficiency and resilience are important requirements for network design and highlight how networks can be constructed that allow for both. 相似文献
992.
In this paper, we construct an explicit quasi-isomorphism to study the cyclic cohomology of a deformation quantization over a Riemannian étale groupoid. Such a quasi-isomorphism allows us to propose a general algebraic index problem for Riemannian étale groupoids. We discuss solutions to that index problem when the groupoid is proper or defined by a constant Dirac structure on a 3-dimensional torus. 相似文献
993.
Davide Bonifazi Gianluca Accorsi Nicola Armaroli Fayi Song Amit Palkar Luis Echegoyen Markus Scholl Paul Seiler Bernhard Jaun Franois Diederich 《Helvetica chimica acta》2005,88(7):1839-1884
We report the synthesis and physical properties of novel fullerene–oligoporphyrin dyads. In these systems, the C‐spheres are singly linked to the terminal tetrapyrrolic macrocycles of rod‐like meso,meso‐linked or triply‐linked oligoporphyrin arrays. Monofullerene–mono(ZnII porphyrin) conjugate 3 was synthesized to establish a general protocol for the preparation of the target molecules (Scheme 1). The synthesis of the meso,meso‐linked oligopophyrin–bisfullerene conjugates 4 – 6 , extending in size up to 4.1 nm ( 6 ), was accomplished by functionalization (iodination followed by Suzuki cross‐coupling) of the two free meso‐positions in oligomers 21 – 23 (Schemes 2 and 3). The attractive interactions between a fullerene and a ZnII porphyrin chromophore in these dyads was quantified as ΔG=−3.3 kcal mol−1 by variable‐temperature (VT) 1H‐NMR spectroscopy (Table 1). As a result of this interaction, the C‐spheres adopt a close tangential orientation relative to the plane of the adjacent porphyrin nucleus, as was unambiguously established by 1H‐ and 13C‐NMR (Figs. 9 and 10), and UV/VIS spectroscopy (Figs. 13–15). The synthesis of triply‐linked diporphyrin–bis[60]fullerene conjugate 8 was accomplished by Bingel cyclopropanation of bis‐malonate 45 with two C60 molecules (Scheme 5). Contrary to the meso,meso‐linked systems 4 – 6 , only a weak chromophoric interaction was observed for 8 by UV/VIS spectroscopy (Fig. 16 and Table 2), and the 1H‐NMR spectra did not provide any evidence for distinct orientational preferences of the C‐spheres. Comprehensive steady‐state and time‐resolved UV/VIS absorption and emission studies demonstrated that the photophysical properties of 8 differ completely from those of 4 – 6 and the many other known porphyrin–fullerene dyads: photoexcitation of the methano[60]fullerene moieties results in quantitative sensitization of the lowest singlet level of the porphyrin tape, which is low‐lying and very short lived. The meso,meso‐linked oligoporphyrins exhibit 1O2 sensitization capability, whereas the triply‐fused systems are unable to sensitize the formation of 1O2 because of the low energy content of their lowest excited states (Fig. 18). Electrochemical investigations (Table 3, and Figs. 19 and 20) revealed that all oligoporphyrin arrays, with or without appended methano[60]fullerene moieties, have an exceptional multicharge storage capacity due to the large number of electrons that can be reversibly exchanged. Some of the ZnII porphyrins prepared in this study form infinite, one‐dimensional supramolecular networks in the solid state, in which the macrocycles interact with each other either through H‐bonding or metal ion coordination (Figs. 6 and 7). 相似文献
994.
995.
Polygonal finite elements provide great flexibility meshing complex structures and for the refinement of meshes. The actual task in the development of these elements is to offer adequate and secure numerical results compared with regular finite elements at low computational costs. Particulary, finding an efficient and appropriate interpolation of the arbitrary element domain exhibits strong difficulties. Based on the general interpolant equation three element formulations are shown that use different interpolation strategies. The elements' performances are shown with a numerical example considering 2d linear elasticity. The results of the different element formulations are compared among each other, with analytical as well as with regular elements' results. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
996.
997.
Anthony Linden Markus Furegati Andreas J. Rippert 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(4):o223-o225
The title compounds, C20H24O7 and C40H32O, respectively, are racemic oxepines, the molecules of which contain a chiral axis. Both molecules possess crystallographic C2 symmetry and the seven‐membered ring adopts a twisted‐boat conformation. 相似文献
998.
Markus Ganzera Ivo Nischang Christian Siegl Birgit Senzenberger Frantisek Svec Hermann Stuppner 《Electrophoresis》2009,30(21):3757-3763
Two microscale separation techniques for the analysis of bioactive naphthoquinones in Eleutherine americana were developed and validated. By MEKC four compounds (eleuthoside B, isoeleutherin, eleutherol and eleutherinoside A) could be determined in plant extracts using an aqueous electrolyte solution composed of 25 mM sodium tetraborate, 50 mM sodium cholate and 20% THF. CEC on a polymeric methacrylate‐based monolith with strong cationic properties showed promising results, as it additionally enabled the separation of two enantiomers, eleutherin and isoeleutherin. The mobile phase for CEC experiments comprised 3 mM ammonium formate in a mixture of ACN and water. At an applied voltage of ?25 kV, all five markers were baseline separated in less than 12 min. Both methods were successfully validated for linearity (MEKC: R2≥0.999; CEC: R2≥0.997), sensitivity (MEKC: LOD=4–5 μg/mL; CEC: LOD=2–8 μg/mL), accuracy (MEKC: 96.5–102.7% recovery; CEC: 97.1–103.5% recovery) and precision (MEKC: σrel≤2.43%; CEC: σrel≤2.21%). The quantitative analysis of naphthoquinone derivatives in several E. americana samples showed that both methods are suitable for practical applications, because the results were well comparable to those obtained by established techniques such as HPLC. 相似文献
999.
Markus H. J. Seifert 《Journal of computer-aided molecular design》2009,23(9):633-644
Target-specific optimization of scoring functions for protein–ligand docking is an effective method for significantly improving
the discrimination of active and inactive molecules in virtual screening applications. Its applicability, however, is limited
due to the narrow focus on, e.g., single protein structures. Using an ensemble of protein kinase structures, the publically
available directory of useful decoys ligand dataset, and a novel multi-factorial optimization procedure, it is shown here
that scoring functions can be tuned to multiple targets of a target class simultaneously. This leads to an improved robustness
of the resulting scoring function parameters. Extensive validation experiments clearly demonstrate that (1) virtual screening
performance for kinases improves significantly; (2) variations in database content affect this kind of machine-learning strategy
to a lesser extent than binary QSAR models, and (3) the reweighting of interaction types is of particular importance for improved
screening performance.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
1000.
Markus Döbbelin Cristina Pozo‐Gonzalo Rebeca Marcilla Raúl Blanco José L. Segura Jose A. Pomposo David Mecerreyes 《Journal of polymer science. Part A, Polymer chemistry》2009,47(12):3010-3021
Novel poly(3,4‐ethylenedioxythiophene) (PEDOT) polymers bearing imidazolium‐ionic liquid moieties were synthesized by electrochemical polymerizations. For this purpose, new functional monomers were synthesized having an 3,4‐ethylenedioxythiophene (EDOT) unit and an imidazolium‐ionic liquid with different anions such as tetrafluoroborate (BF), bis(trifluoromethane)sulfonimide ((CF3SO2)2N?), and hexafluorophosphate (PF). Next, polymer films were obtained by electrochemical synthesis in dicholoromethane solutions. Obtained polymers were characterized, revealing the characteristics of PEDOT in terms of electrochemical and spectroelectrochemical properties, FTIR, 1H NMR, and AFM microscopy. Interestingly, the hydrophobic character of electropolymerized films could be modified depending on the anion type. The hydrophobicity followed the trend PF > (CF3SO2)2N? > BF > pure PEDOT as determined by water contact angle measurements. Furthermore, the polymers could be dissolved in a range of polar organic solvents such as dimethylformamide, propylene carbonate, and dimethyl sulfoxide making these polymers interesting candidates for wet processing methods. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3010–3021, 2009 相似文献