全文获取类型
收费全文 | 3462篇 |
免费 | 184篇 |
国内免费 | 18篇 |
专业分类
化学 | 2420篇 |
晶体学 | 13篇 |
力学 | 76篇 |
数学 | 601篇 |
物理学 | 554篇 |
出版年
2023年 | 31篇 |
2022年 | 40篇 |
2021年 | 61篇 |
2020年 | 79篇 |
2019年 | 76篇 |
2018年 | 70篇 |
2017年 | 76篇 |
2016年 | 177篇 |
2015年 | 138篇 |
2014年 | 122篇 |
2013年 | 223篇 |
2012年 | 266篇 |
2011年 | 280篇 |
2010年 | 163篇 |
2009年 | 117篇 |
2008年 | 229篇 |
2007年 | 193篇 |
2006年 | 163篇 |
2005年 | 169篇 |
2004年 | 149篇 |
2003年 | 137篇 |
2002年 | 123篇 |
2001年 | 38篇 |
2000年 | 29篇 |
1999年 | 37篇 |
1998年 | 37篇 |
1997年 | 25篇 |
1996年 | 38篇 |
1995年 | 12篇 |
1994年 | 20篇 |
1993年 | 28篇 |
1992年 | 18篇 |
1991年 | 15篇 |
1990年 | 15篇 |
1989年 | 11篇 |
1988年 | 15篇 |
1987年 | 13篇 |
1986年 | 14篇 |
1985年 | 31篇 |
1984年 | 23篇 |
1983年 | 11篇 |
1982年 | 13篇 |
1981年 | 9篇 |
1980年 | 10篇 |
1979年 | 18篇 |
1978年 | 12篇 |
1977年 | 12篇 |
1975年 | 9篇 |
1974年 | 13篇 |
1973年 | 12篇 |
排序方式: 共有3664条查询结果,搜索用时 0 毫秒
111.
Synthesis and Conformational Analysis of Cyclic Homooligomers from Pyranoid ε‐Sugar Amino Acids 下载免费PDF全文
Dr. Andrés Feher‐Voelger Jorge Borges‐González Dr. Romen Carrillo Dr. Ezequiel Q. Morales Dr. Javier González‐Platas Dr. Tomás Martín 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(14):4007-4022
New pyranoid ε‐sugar amino acids were designed as building blocks, in which the carboxylic acid and the amine groups were placed in positions C2 and C3 with respect to the tetrahydropyran oxygen atom. By using standard solution‐phase coupling procedures, cyclic homooligomers containing pyranoid ε‐sugar amino acids were synthesized. Conformation analysis was performed by using NMR spectroscopic experiments, FTIR spectroscopic studies, X‐ray analysis, and a theoretical conformation search. These studies reveal that the presence of a methoxy group in the position C4 of the pyran ring produces an important structural change in the cyclodipeptides. When the methoxy groups are present, the structure collapses through interresidue hydrogen bonds between the oxygen atoms of the pyran ring and the amide protons. However, when the cyclodipeptide lacks the methoxy groups, a U‐shape structure is adopted, in which there is a hydrophilic concave face with four oxygen atoms and two amide protons directed toward the center of the cavity. Additionally, we found important evidence of the key role played by weak electrostatic interactions, such as the five‐membered hydrogen‐bonded pseudocycles (C5) between the amide protons and the ether oxygen atoms, in the conformation equilibrium of the macrocycles and in the cyclization step of the cyclic tetrapeptides. 相似文献
112.
Rapid Assembly of Functionalised Spirocyclic Indolines by Palladium‐Catalysed Dearomatising Diallylation of Indoles with Allyl Acetate 下载免费PDF全文
Dr. Persis Dhankher Dr. Laure Benhamou Dr. Tom D. Sheppard 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(41):13375-13381
Herein, we report the application of allyl acetate to the palladium‐catalysed dearomatising diallylation of indoles. The reaction can be carried out by using a readily available palladium catalyst at room temperature, and can be applied to a wide range of substituted indoles to provide access to the corresponding 3,3‐diallylindolinines. These compounds are versatile synthetic intermediates that readily undergo Ugi reactions or proline‐catalysed asymmetric Mannich reactions. Alternatively, acylation of the 3,3‐diallylindolinines with an acid chloride or a chloroformate, followed by treatment with aluminium chloride, enables 2,3‐diallylindoles to be prepared. By using ring‐closing metathesis, functionalised spirocyclic indoline scaffolds can be accessed from the Ugi products, and a dihydrocarbazole can be prepared from the corresponding 2,3‐diallylindole. 相似文献
113.
Víctor Manuel Castor-Villegas José Manuel Guevara-Vela Wilmer E. Vallejo Narváez Ángel Martín Pendás Tomás Rocha-Rinza Alberto Fernández-Alarcón 《Journal of computational chemistry》2020,41(26):2266-2277
Hydrogen bonds (HB) are arguably the most important noncovalent interactions in chemistry. We study herein how differences in connectivity alter the strength of HBs within water clusters of different sizes. We used for this purpose the interacting quantum atoms energy partition, which allows for the quantification of HB formation energies within a molecular cluster. We could expand our previously reported hierarchy of HB strength in these systems (Phys. Chem. Chem. Phys., 2016, 18 , 19557) to include tetracoordinated monomers. Surprisingly, the HBs between tetracoordinated water molecules are not the strongest HBs despite the widespread occurrence of these motifs (e.g., in ice Ih). The strongest HBs within H2O clusters involve tricoordinated monomers. Nonetheless, HB tetracoordination is preferred in large water clusters because (a) it reduces HB anticooperativity associated with double HB donors and acceptors and (b) it results in a larger number of favorable interactions in the system. Finally, we also discuss (a) the importance of exchange-correlation to discriminate among the different examined types of HBs within H2O clusters, (b) the use of the above-mentioned scale to quickly assess the relative stability of different isomers of a given water cluster, and (c) how the findings of this research can be exploited to indagate about the formation of polymorphs in crystallography. Overall, we expect that this investigation will provide valuable insights into the subtle interplay of tri- and tetracoordination in HB donors and acceptors as well as the ensuing interaction energies within H2O clusters. 相似文献
114.
Celina Love Jan Steinkühler David T. Gonzales Naresh Yandrapalli Tom Robinson Rumiana Dimova Dr. T.-Y. Dora Tang 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(15):6006-6013
In situ, reversible coacervate formation within lipid vesicles represents a key step in the development of responsive synthetic cellular models. Herein, we exploit the pH responsiveness of a polycation above and below its pKa, to drive liquid–liquid phase separation, to form single coacervate droplets within lipid vesicles. The process is completely reversible as coacervate droplets can be disassembled by increasing the pH above the pKa. We further show that pH-triggered coacervation in the presence of low concentrations of enzymes activates dormant enzyme reactions by increasing the local concentration within the coacervate droplets and changing the local environment around the enzyme. In conclusion, this work establishes a tunable, pH responsive, enzymatically active multi-compartment synthetic cell. The system is readily transferred into microfluidics, making it a robust model for addressing general questions in biology, such as the role of phase separation and its effect on enzymatic reactions using a bottom-up synthetic biology approach. 相似文献
115.
116.
Guangcan Xu Sbastien Leloux Pinglu Zhang Jorge Meijide Surez Yongmin Zhang Etienne Derat Mickaël Mnand Olivia Bistri‐Aslanoff Sylvain Roland Tom Leyssens Olivier Riant Matthieu Sollogoub 《Angewandte Chemie (International ed. in English)》2020,59(19):7591-7597
The encapsulation of copper inside a cyclodextrin capped with an N‐heterocyclic carbene (ICyD) allowed both to catch the elusive monomeric (L)CuH and a cavity‐controlled chemoselective copper‐catalyzed hydrosilylation of α,β‐unsaturated ketones. Remarkably, (α‐ICyD)CuCl promoted the 1,2‐addition exclusively, while (β‐ICyD)CuCl produced the fully reduced product. The chemoselectivity is controlled by the size of the cavity and weak interactions between the substrate and internal C?H bonds of the cyclodextrin. 相似文献
117.
Al-Shantir Omar Csáki Štefan Ondro Tomáš Keppert Martin Minárik Peter Vrabec Marek Trník Anton 《Journal of Thermal Analysis and Calorimetry》2020,142(5):1759-1766
Journal of Thermal Analysis and Calorimetry - This paper presents mechanical-acoustic study of samples made from electroporcelain mixture (type C 130) under five different compression... 相似文献
118.
Xu Jiaohui Zhang Zhengce Caraballo Tomás 《Journal of Dynamics and Differential Equations》2022,34(1):583-603
Journal of Dynamics and Differential Equations - In this paper, the well-posedness of stochastic time fractional 2D-Stokes equations of order $$\alpha \in (0,1)$$ containig finite or infinite delay... 相似文献
119.
120.
The application of the frozen-core approximation to the calculation of the shielding tensor of nuclear magnetic resonance (NMR) spectroscopy is discussed and an implementation is presented. A complete formulation of the shielding calculation within the frozen-core approximation is given, both in general terms and for the special case of density functional theory (DFT) and “gauge including atomic orbitals” (GIAOs). The practical implementation is validated by a detailed discussion of the consequences of the approximation. The general conclusion is drawn that the frozen-core approximation is a useful tool for shielding calculations—if the valence space is increased to contain at least the ns, np, (n − 1)p, (n − 1)d (fourth period and higher) shells, where n is the number of the given period in the periodic table of elements. The new method is applied to 77Se shieldings and chemical shifts for a small number of compounds. The agreement between theory and experiment is good for relative shifts, whereas calculated absolute shieldings are generally too small by about 300–400 ppm. This difference is attributed to the relativistic contraction of the core density at the selenium atom that had been explicitly incorporated into the experimental absolute shielding scale. © 1996 John Wiley & Sons, Inc. 相似文献