全文获取类型
收费全文 | 4636篇 |
免费 | 188篇 |
国内免费 | 25篇 |
专业分类
化学 | 2972篇 |
晶体学 | 25篇 |
力学 | 121篇 |
数学 | 864篇 |
物理学 | 867篇 |
出版年
2023年 | 35篇 |
2022年 | 65篇 |
2021年 | 94篇 |
2020年 | 71篇 |
2019年 | 118篇 |
2018年 | 81篇 |
2017年 | 84篇 |
2016年 | 164篇 |
2015年 | 140篇 |
2014年 | 150篇 |
2013年 | 262篇 |
2012年 | 319篇 |
2011年 | 361篇 |
2010年 | 202篇 |
2009年 | 211篇 |
2008年 | 299篇 |
2007年 | 287篇 |
2006年 | 271篇 |
2005年 | 203篇 |
2004年 | 210篇 |
2003年 | 152篇 |
2002年 | 146篇 |
2001年 | 72篇 |
2000年 | 75篇 |
1999年 | 48篇 |
1998年 | 47篇 |
1997年 | 42篇 |
1996年 | 50篇 |
1995年 | 28篇 |
1994年 | 30篇 |
1993年 | 29篇 |
1992年 | 45篇 |
1991年 | 43篇 |
1990年 | 43篇 |
1989年 | 24篇 |
1988年 | 26篇 |
1987年 | 20篇 |
1986年 | 27篇 |
1985年 | 38篇 |
1984年 | 39篇 |
1983年 | 29篇 |
1982年 | 18篇 |
1981年 | 23篇 |
1980年 | 18篇 |
1979年 | 18篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 8篇 |
排序方式: 共有4849条查询结果,搜索用时 31 毫秒
981.
Ernest M. Asani Victor N. Khrustalev Rachel M. Williamson Rodolfo A. Martinez Clifford J. Unkefer Tatiana V. Timofeeva 《Journal of chemical crystallography》2007,37(10):663-667
[1,2,3-13C3]-1-(Phenylsulfinyl)-3-benzyloxyacetone, C16H16O3S, (3) has been synthesized and its crystal structure has been determined by a single-crystal X-ray diffraction analysis. The X-ray diffraction study revealed that compound 3 crystallizes in the monoclinic crystal system in the acentric space group Pc, with cell constants at T = 100 K: a = 16.073(5), b = 5.5079(16), c = 7.949(2) Å, β = 100.221(4)°, V = 692.6(3) Å3, Z = 2, d calc = 1.383 g/cm3. Compound 3 contains the chiral tetravalent three-coordinated sulfur atom, which has a distorted tetrahedral configuration with a lone electron pair occupying one of the tetrahedron vertices. In the crystal, the molecules are packed in stacks along the b axis; the stacks consist of the molecules of the same chirality. Furthermore, the stacks of the molecules of the opposite chirality alternate along the c axis. The molecules in neighboring stacks are arranged by head-to-tail orientations. There are no short intermolecular contacts in the crystal of 3. 相似文献
982.
Summary : We demonstrate that it is possible to link multi-chain molecular dynamics simulations with the tube model using a single chain slip-links model as a bridge. This hierarchical approach allows significant speed up of simulations, permitting us to span the time scales relevant for a comparison with the tube theory. Fitting the mean-square displacement of individual monomers in molecular dynamics simulations with the slip-spring model, we show that it is possible to predict the stress relaxation. Then, we analyze the stress relaxation from slip-spring simulations in the framework of the tube theory. In the absence of constraint release, we establish that the relaxation modulus can be decomposed as the sum of contributions from fast and longitudinal Rouse modes, and tube survival. Finally, we discuss some open questions regarding possible future directions that could be profitable in rendering the tube model quantitative, even for mildly entangled polymers. 相似文献
983.
Rubn A. Toscano Emma Maldonado Alfredo Ortega Jorge Crdenas 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(7):846-847
The absolute configuration of the neoclerodane glycoside amarisolide, presented here as the monohydrate, C26H36O9·H2O, has been determined by association with the known configuration of the glucose moiety. Its structure was established as 2β‐(O‐β‐d ‐glucopyranosyl)neocleroda‐3,13(16),14‐trien‐15,16‐epoxy‐18,19‐olide. Extensive hydrogen bonding among the hydroxyl groups of the sugar moiety forms layers which are interconnected by water molecules. 相似文献
984.
Manuel Gadella Sebastin Fortín Juan Pablo Jorge Marcelo Losada 《Entropy (Basel, Switzerland)》2022,24(6)
We review some results in the theory of non-relativistic quantum unstable systems. We account for the most important definitions of quantum resonances that we identify with unstable quantum systems. Then, we recall the properties and construction of Gamow states as vectors in some extensions of Hilbert spaces, called Rigged Hilbert Spaces. Gamow states account for the purely exponential decaying part of a resonance; the experimental exponential decay for long periods of time physically characterizes a resonance. We briefly discuss one of the most usual models for resonances: the Friedrichs model. Using an algebraic formalism for states and observables, we show that Gamow states cannot be pure states or mixtures from a standard view point. We discuss some additional properties of Gamow states, such as the possibility of obtaining mean values of certain observables on Gamow states. A modification of the time evolution law for the linear space spanned by Gamow shows that some non-commuting observables on this space become commuting for large values of time. We apply Gamow states for a possible explanation of the Loschmidt echo. 相似文献
985.
Jos S. Cmara Rosa Perestrelo Cristina V. Berenguer Carolina F. P. Andrade Telma M. Gomes Basit Olayanju Abuzar Kabir Cristina M. R. Rocha Jos Antnio Teixeira Jorge A. M. Pereira 《Molecules (Basel, Switzerland)》2022,27(9)
Green extraction techniques (GreETs) emerged in the last decade as greener and sustainable alternatives to classical sample preparation procedures aiming to improve the selectivity and sensitivity of analytical methods, simultaneously reducing the deleterious side effects of classical extraction techniques (CETs) for both the operator and the environment. The implementation of improved processes that overcome the main constraints of classical methods in terms of efficiency and ability to minimize or eliminate the use and generation of harmful substances will promote more efficient use of energy and resources in close association with the principles supporting the concept of green chemistry. The current review aims to update the state of the art of some cutting-edge GreETs developed and implemented in recent years focusing on the improvement of the main analytical features, practical aspects, and relevant applications in the biological, food, and environmental fields. Approaches to improve and accelerate the extraction efficiency and to lower solvent consumption, including sorbent-based techniques, such as solid-phase microextraction (SPME) and fabric-phase sorbent extraction (FPSE), and solvent-based techniques (μQuEChERS; micro quick, easy, cheap, effective, rugged, and safe), ultrasound-assisted extraction (UAE), and microwave-assisted extraction (MAE), in addition to supercritical fluid extraction (SFE) and pressurized solvent extraction (PSE), are highlighted. 相似文献
986.
David E. Mery Amanda J. Compadre Paola E. Ordez Edward J. Selvik Vladimir Morocho Jorge Contreras Omar Malagn Darin E. Jones Philip J. Breen Michael J. Balick Flavio G. Gaudio Monica L. Guzman Cesar M. Compadre 《Molecules (Basel, Switzerland)》2022,27(9)
A method to identify anticancer compounds in plants was proposed based on the hypothesis that these compounds are primarily present in plants to provide them with an ecological advantage over neighboring plants and other competitors. According to this view, identifying plants that contain compounds that inhibit or interfere with the development of other plant species may facilitate the discovery of novel anticancer agents. The method was developed and tested using Magnolia grandiflora, Gynoxys verrucosa, Picradeniopsis oppositifolia, and Hedyosmum racemosum, which are plant species known to possess compounds with cytotoxic activities. Plant extracts were screened for growth inhibitory activity, and then a thin-layer chromatography bioautography assay was conducted. This located the major antileukemic compounds 1, 2, 4, and 5 in the extracts. Once the active compounds were located, they were extracted and purified, and their structures were determined. The growth inhibitory activity of the purified compounds showed a significant correlation with their antileukemic activity. The proposed approach is rapid, inexpensive, and can easily be implemented in areas of the world with high biodiversity but with less access to advanced facilities and biological assays. 相似文献
987.
Lucas Wickemeyer Niklas Aders Andreas Mix Beate Neumann Hans-Georg Stammler Jorge J. Cabrera-Trujillo Israel Fernndez Norbert W. Mitzel 《Chemical science》2022,13(27):8088
The reaction of tBu2P(O)H with Bis2AlH (Bis = CH(SiMe3)2) afforded the adduct tBu2P(H)–O–Al(H)Bis2 (3). It slowly releases H2 to form the first oxygen-bridged geminal Al/P frustrated Lewis pair tBu2P–O–AlBis2. It is capable of reversibly binding molecular hydrogen to afford 3, shown by NMR and H/D scrambling experiments, and forms a 1,2-adduct with CO2. Importantly, the H2 adduct 3 reduces CO2 in a stoichiometric reaction leading to the formic acid adduct tBu2P(H)–O–Al(CO2H)Bis2. The formation of the different species was explored by density functional theory calculations which provide support for the experimental results. All products were characterized by NMR spectroscopy as well as X-ray diffraction experiments and elemental analyses.Addition vs. reduction: the geminal FLP Bis2Al–O–PtBu2 can reversibly bind molecular hydrogen, it reacts with CO2 to give an adduct, and its hydrogen adduct reduces CO2 to an adduct of formic acid. 相似文献
988.
Rafael Rojas-Hernndez Juan Luis Díaz-de-Len-Santiago Grettel Barcel-Alonso Jorge Bautista-Lpez Valentin Trujillo-Mora Julio Csar Salgado-Ramírez 《Entropy (Basel, Switzerland)》2022,24(7)
This paper introduces a new method of compressing digital images by using the Difference Transform applied in medical imaging. The Difference Transform algorithm performs the decorrelation process of image data, and in this way improves the encoding process, achieving a file with a smaller size than the original. The proposed method proves to be competitive and in many cases better than the standards used for medical images such as TIFF or PNG. In addition, the Difference Transform can replace other transforms like Cosine or Wavelet. 相似文献
989.
Espinoza-Vergara Javier Pavez Jorge Gonzalez Evelyn Ramirez Cristián Páez Maritza Gulppi Miguel 《Journal of Solid State Electrochemistry》2019,23(12):3225-3229
Journal of Solid State Electrochemistry - Here we demonstrate that the metal ion excess after precipitating a sparingly soluble metal salt can be conveniently determined by a secondary... 相似文献
990.
Jorge Gonzlez‐Camus Valentin Keyantuo Carlos Lizama Mahamadi Warma 《Mathematical Methods in the Applied Sciences》2019,42(14):4688-4711
We prove representation results for solutions of a time‐fractional differential equation involving the discrete fractional Laplace operator in terms of generalized Wright functions. Such equations arise in the modeling of many physical systems, for example, chain processes in chemistry and radioactivity. Our focus is in the problem , where 0<β ≤ 2, 0<α ≤ 1, , (?Δd)α is the discrete fractional Laplacian, and is the Caputo fractional derivative of order β. We discuss important special cases as consequences of the representations obtained. 相似文献