全文获取类型
收费全文 | 1155篇 |
免费 | 54篇 |
国内免费 | 9篇 |
专业分类
化学 | 772篇 |
晶体学 | 4篇 |
力学 | 14篇 |
数学 | 108篇 |
物理学 | 320篇 |
出版年
2023年 | 5篇 |
2022年 | 9篇 |
2021年 | 19篇 |
2020年 | 22篇 |
2019年 | 22篇 |
2018年 | 20篇 |
2017年 | 14篇 |
2016年 | 35篇 |
2015年 | 35篇 |
2014年 | 36篇 |
2013年 | 68篇 |
2012年 | 65篇 |
2011年 | 77篇 |
2010年 | 34篇 |
2009年 | 40篇 |
2008年 | 56篇 |
2007年 | 53篇 |
2006年 | 55篇 |
2005年 | 62篇 |
2004年 | 30篇 |
2003年 | 34篇 |
2002年 | 53篇 |
2001年 | 27篇 |
2000年 | 30篇 |
1999年 | 18篇 |
1998年 | 14篇 |
1997年 | 11篇 |
1996年 | 19篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 15篇 |
1992年 | 15篇 |
1991年 | 10篇 |
1990年 | 14篇 |
1989年 | 16篇 |
1988年 | 17篇 |
1987年 | 10篇 |
1986年 | 15篇 |
1985年 | 11篇 |
1984年 | 12篇 |
1983年 | 10篇 |
1982年 | 20篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1974年 | 4篇 |
排序方式: 共有1218条查询结果,搜索用时 531 毫秒
71.
AbstractWe report here an improved, highly efficient, and general method for the ligand-free Suzuki cross-coupling reaction to the synthesis of biaryls, bipyridyls, thienylpyridine, and allylphenols. Microwave irradiation of (het)aryl halides and (hetaryl, allyl)arylboronic acid N-methyl-iminodiacetic acid (MIDA) ester, using polyurea microencapsulated palladium catalyst (Pd EnCat 30), gave the coupling adducts 1a–x in excellent yields in just 10–18 min. 相似文献
72.
73.
Victoria A. Roberts Elaine E. Thompson Michael E. Pique Martin S. Perez L. F. Ten Eyck 《Journal of computational chemistry》2013,34(20):1743-1758
Computational docking is a useful tool for predicting macromolecular complexes, which are often difficult to determine experimentally. Here, we present the DOT2 software suite, an updated version of the DOT intermolecular docking program. DOT2 provides straightforward, automated construction of improved biophysical models based on molecular coordinates, offering checkpoints that guide the user to include critical features. DOT has been updated to run more quickly, allow flexibility in grid size and spacing, and generate an infinitive complete list of favorable candidate configurations. Output can be filtered by experimental data and rescored by the sum of electrostatic and atomic desolvation energies. We show that this rescoring method improves the ranking of correct complexes for a wide range of macromolecular interactions and demonstrate that biologically relevant models are essential for biologically relevant results. The flexibility and versatility of DOT2 accommodate realistic models of complex biological systems, improving the likelihood of a successful docking outcome. © 2013 Wiley Periodicals, Inc. 相似文献
74.
Liliana C. Tomé Susana C. M. Fernandes Denilson Silva Perez Patrizia Sadocco Armando J. D. Silvestre Carlos Pascoal Neto Isabel M. Marrucho Carmen S. R. Freire 《Cellulose (London, England)》2013,20(4):1807-1818
Thin nanocomposite films of thermoplastic starch, chitosan and cellulose nanofibers (bacterial cellulose or nanofibrillated cellulose) were prepared for the first time by solvent casting of water based suspensions of the three polysaccharides. The role of the different bioploymers on the final properties (thermal stability, transparency, mechanical performance and antimicrobial activity) of the films was related with their intrinsic features, contents and synergic effects resulting from the establishment of interactions between them. Thermoplastic starch displays an important role on the thermal stability of the films because it is the most stable polysaccharide; however it has a negative impact on the mechanical performance and transparency of the films. The addition of chitosan improves considerably the transparency (up to 50 % transmittance for 50 % of chitosan, in respect to the amount of starch), mechanical performance and antimicrobial properties (at least 25 % of chitosan and no more than 10 % of cellulose nanofibers are required to observe bacteriostatic or bactericidal activity) but decrease their thermal stability. The incorporation of cellulose nanofibers had the strongest positive impact on the mechanical properties of the materials (increments of up to 15 and 30 MPa on the Young′s modulus and Tensile strength, respectively, for films with 20 % of BC or NFC). Nonetheless, the impact in thermal stability and mechanical performance of the films, promoted by the addition of chitosan and cellulose nanofibres, respectively, was higher than the expected considering their percentage contents certainly because of the establishment of strong and complex interactions between the three polysaccharides. 相似文献
75.
Danny Mortimer Matthew Whiting Joseph P.A. Harrity Simon Jones Iain Coldham 《Tetrahedron letters》2014
Lewis acid mediated intramolecular Mannich reaction between an azocinone and a 3-formylindole was investigated as part of a study towards the synthesis of actinophyllic acid. The intramolecular Mannich reaction resulted in a single diastereomer of the 1-azabicyclo[4.2.1]nonan-5-one core framework, although single crystal X-ray structure analysis revealed that this had the undesired stereochemistry in comparison with the natural product. 相似文献
76.
In a spin‐polarized electron gas, Coulomb interaction couples the spin and motion degrees of freedom to build propagating spin waves. The spin wave stiffness Ssw quantifies the energy cost to trigger such excitation by perturbing the kinetic energy of the electron gas (i.e. putting it in motion). Here we introduce the concept of spin–orbit stiffness, Sso, as the energy necessary to excite a spin wave with a spin polarization induced by spin–orbit coupling. This quantity governs the Coulombic enhancement of the spin–orbit field acting of the spin wave. First‐principles calculations and electronic Raman scattering experiments carried out on a model spin‐polarized electron gas, embedded in a CdMnTe quantum well, demonstrate that Sso = Ssw. Through optical gating of the structure, we demonstrate the reproducible tuning of Sso by a factor of 3, highlighting the great potential of spin–orbit control of spin waves in view of spintronics applications. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) 相似文献
77.
Substitution pattern elucidation of hydroxypropyl Pinus pinaster (Ait.) bark polyflavonoid derivatives by ESI(−)‐MS/MS
下载免费PDF全文
![点击此处可从《Journal of mass spectrometry : JMS》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Danny E. García Marrero Wolfgang G. Glasser Antonio Pizzi Sebastian Paczkowski Marie‐Pierre G. Laborie 《Journal of mass spectrometry : JMS》2014,49(10):1050-1058
The structure of condensed tannins (CTs) from Pinus pinaster bark extract and their hydroxypropylated derivatives with four degrees of substitution (DS 1, 2, 3 and 4) has been characterized for the first time using negative‐ion mode electrospray ionization tandem mass spectrometry (ESI(?)‐MS/MS). The results showed that P. pinaster bark CTs possess structural homogeneity in terms of monomeric units (C15, catechin). The oligomer sizes were detected to be dimers to heptamers. The derivatives showed typical phenyl‐propyl ether mass fragmentation by substituent elimination (58 amu) and inherent C15 flavonoid fissions. The relative abundance of the product ions revealed a preferential triple, tetra‐/penta‐ and octa‐ hydroxypropylation substitution pattern in the monomer, dimer and trimer derivatives, respectively. A defined order of –OH reactivity towards propylene oxide was established by means of multistage experiments (A‐ring ≥ B‐ring > C‐ring). A high structural heterogeneity of the modified oligomers was detected. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
78.
Houssine Sehaqui Uxua Perez de Larraya Peng Liu Numa Pfenninger Aji P. Mathew Tanja Zimmermann Philippe Tingaut 《Cellulose (London, England)》2014,21(4):2831-2844
Biobased nanofibers are increasingly considered in purification technologies due to their high mechanical properties, high specific surface area, versatile surface chemistry and natural abundance. In this work, cellulose and chitin nanofibers functionalized with carboxylate entities have been prepared from pulp residue (i.e., a waste product from the pulp and paper production) and crab shells, respectively, by chemically modifying the initial raw materials with the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) mediated oxidation reaction followed by mechanical disintegration. A thorough investigation has first been carried out in order to evaluate the copper(II) adsorption capacity of the oxidized nanofibers. UV spectrophotometry, X-ray photoelectron spectroscopy and wavelength dispersive X-rays analysis have been employed as characterization tools for this purpose. Pristine nanofibers presented a relatively low content of negative charges on their surface thus adsorbing a low amount of copper(II). The copper adsorption capacity of the nanofibers was enhanced due to the oxidation treatment since the carboxylate groups introduced on the nanofibers surface constituted negative sites for electrostatic attraction of copper ions (Cu2+). The increase in copper adsorption on the nanofibers correlated both with the pH and carboxylate content and reached maximum values of 135 and 55 mg g?1 for highly oxidized cellulose and chitin nanofibers, respectively. Furthermore, the metal ions could be easily removed from the contaminated nanofibers through a washing procedure in acidic water. Finally, the adsorption capacity of oxidized cellulose nanofibers for other metal ions, such as nickel(II), chromium(III) and zinc(II), was also demonstrated. We conclude that TEMPO oxidized biobased nanofibers from waste resources represent an inexpensive and efficient alternative to classical sorbents for heavy metal ions removal from contaminated water. 相似文献
79.
Kaitlyn A. Perez Cameron R. Rogers Emily A. Weiss 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(33):14195-14199
This Communication describes the use of CuInS2/ZnS quantum dots (QDs) as photocatalysts for the reductive deprotection of aryl sulfonyl-protected phenols. For a series of aryl sulfonates with electron-withdrawing substituents, the rate of deprotection for the corresponding phenyl aryl sulfonates increases with decreasing electrochemical potential for the two electron transfers within the catalytic cycle. The rate of deprotection for a substrate that contains a carboxylic acid, a known QD-binding group, is accelerated by more than a factor of ten from that expected from the electrochemical potential for the transformation, a result that suggests that formation of metastable electron donor–acceptor complexes provides a significant kinetic advantage. This deprotection method does not perturb the common NHBoc or toluenesulfonyl protecting groups and, as demonstrated with an estrone substrate, does not perturb proximate ketones, which are generally vulnerable to many chemical reduction methods used for this class of reactions. 相似文献
80.
Danny Schuhknecht Dr. Valeri Leich Dr. Thomas P. Spaniol Iskander Douair Prof. Dr. Laurent Maron Prof. Dr. Jun Okuda 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(13):2821-2825
Potassium silanide [KSiH3]∞ contains 4.2 wt % of hydrogen and has been intensely studied as hydrogen storage material. The macrocyclic ligand Me4TACD (1,4,7,10-tetramethyl-1,4,7,10-tetraaminocyclododecane, L ) stabilizes the full range of triphenylsilyl complexes [( L )MSiPh3]n (M=Li–Cs), which react with H2 or PhSiH3 to form molecular [( L )MSiH3]n that can be isolated in soluble form and fully characterized. 相似文献