全文获取类型
收费全文 | 5057篇 |
免费 | 187篇 |
国内免费 | 16篇 |
专业分类
化学 | 3322篇 |
晶体学 | 54篇 |
力学 | 109篇 |
数学 | 555篇 |
物理学 | 1220篇 |
出版年
2023年 | 47篇 |
2022年 | 103篇 |
2021年 | 154篇 |
2020年 | 123篇 |
2019年 | 147篇 |
2018年 | 123篇 |
2017年 | 116篇 |
2016年 | 195篇 |
2015年 | 162篇 |
2014年 | 158篇 |
2013年 | 329篇 |
2012年 | 353篇 |
2011年 | 413篇 |
2010年 | 261篇 |
2009年 | 260篇 |
2008年 | 349篇 |
2007年 | 313篇 |
2006年 | 251篇 |
2005年 | 223篇 |
2004年 | 212篇 |
2003年 | 143篇 |
2002年 | 115篇 |
2001年 | 88篇 |
2000年 | 83篇 |
1999年 | 61篇 |
1998年 | 34篇 |
1997年 | 39篇 |
1996年 | 44篇 |
1995年 | 42篇 |
1994年 | 34篇 |
1993年 | 37篇 |
1992年 | 46篇 |
1991年 | 25篇 |
1990年 | 16篇 |
1989年 | 16篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 10篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 9篇 |
1978年 | 8篇 |
1977年 | 10篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1968年 | 4篇 |
排序方式: 共有5260条查询结果,搜索用时 31 毫秒
911.
Willian R. Rocha Josefredo R. Pliego Stella M. Resende Hlio F. Dos Santos Marcos A. De Oliveira Wagner B. De Almeida 《Journal of computational chemistry》1998,19(5):524-534
The potential energy surface (PES) for the cyclooctane molecule was comprehensively investigated at the Hartree–Fock (HF) level of theory employing the 3–21G, 6–31G, and 6–31G* basis sets. Six distinct true minimum energy structures (named B, BB, BC, CROWN, TBC, and TCC1), characterized through harmonic frequency analysis, were located on the multidimensional PES. Two transition state structures were also located on the PES for the cyclooctane molecule. Electron correlation effects were accounted for using the Møller–Plesset second-order perturbation theory (MP2) approach. The predicted global minimum energy structure on the ab initio PES for the cyclooctane molecule is the BC conformer. A gas phase electron diffraction study at 300 K suggested a conformational mixture while an NMR study in solution at 161.5 K predicted the BC conformer as the predominant form. The equilibrium constants reported in the present study, which were evaluated from the ab initio calculated total Gibbs free energy change values, were in good agreement with both experimental investigations. The ab initio results showed that the low temperature condition significantly favored the BC conformer while above room temperature both BC and CROWN structures can coexist. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 524–534, 1998 相似文献
912.
913.
914.
The reactions of the title substrate (1) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0°C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, pseudo‐first‐order rate coefficients (kobs) are obtained. Plots of kobs against [NH], where NH is the free amine, are nonlinear upwards, except the reactions of piperidine, which show linear plots. According to the kinetic results and the analysis of products, a reaction scheme is proposed with two tetrahedral intermediates, one zwitterionic (T±) and another anionic (T−), with a kinetically significant proton transfer from T± to an amine to yield T− (k3 step). By nonlinear least‐squares fitting of an equation derived from the scheme to the experimental points, the rate microcoefficients involved in the reactions are determined. Comparison of the kinetics of the title reactions with the linear kobs vs. [NH] plots found in the same aminolysis of O‐ethyl 4‐nitrophenyl dithiocarbonate (2) in the same solvent shows that the rate coefficient for leaving group expulsion from T± (k2) is larger for 2 due to a stronger push by EtO than PhO. The k3 value is the same for both reactions since both proton transfers are diffusion controlled. Comparison of the title reactions with the same aminolysis of phenyl 4‐nitrophenyl thionocarbonate (3) in water indicates that (i) the k2 value is larger for the aminolysis of 1 due to the less basic nucleofuge involved and the small solvent effect on k2, (ii) the k3 value is smaller for the reactions of 1 due to the more viscous solvent, (iii) the rate coefficient for amine expulsion from T± (k−1) is larger for the aminolysis of 1 than that of 3 due to a solvent effect, and (iv) the value of the rate coefficient for amine attack (k1) is smaller for the aminolysis of 1 in aqueous ethanol, which can be explained by a predominant solvent effect relative to the electron‐withdrawing effect from the nucleofuge. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 839–845, 1999 相似文献
915.
The Wiener–Hopf factorization of a class of 2×2 symbols including matrices of Daniele–Khrapkov type is studied. The partial indices and the factors are determined, both in the canonical and non‐canonical cases. A non‐linear method is used which reduces the solution of a homogeneous Riemann–Hilbert problem to a non‐linear scalar equation. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
916.
Joo Henrique Zimnoch Dos Santos Marcelo Barbosa Da Rosa Cristiano Krug Fernanda Chiarello Stedile Monica Carcuchinski Haag Jairton Dupont Madalena De Camargo Forte 《Journal of polymer science. Part A, Polymer chemistry》1999,37(13):1987-1996
The effects of polymerization conditions were evaluated on the production of polyethylene by silica-supported (n-BuCp)2ZrCl2 grafted under optimized conditions and cocatalyzed by methylaluminoxane (MAO). The Al : Zr molar ratio, reaction temperature, monomer pressure, and the age and concentration of the catalyst were systematically varied. Most reactions were performed in toluene. Hexane, with the addition of triisobutilaluminum (TIBA) to MAO, was also tested as a polymerization solvent for both homogeneous and heterogeneous catalyst systems. Polymerization reactions in hexane showed their highest activities with MAO : TIBA ratios of 3 : 1 and 1 : 1 for the homogeneous and supported systems, respectively. Catalyst activity increased continuously as Al : Zr molar ratios increased from 0 to 2000, and remained constant up to 5000. The highest activity was observed at 333 K. High monomer pressures (≈ 4 atm) appeared to stabilize active species during polymerization, producing polyethylenes with high molecular weight (≈ 3 × 105 g mol−1). Catalyst concentration had no significant effect on polymerization activity or polymer properties. Catalyst aging under inert atmosphere was evaluated over 6 months; a pronounced reduction in catalyst activity [from 20 to 13 × 105 g PE (mol Zr h)−1] was observed only after the first two days following preparation. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1987–1996, 1999 相似文献
917.
918.
919.
Anna Irto Paola Cardiano Karam Chand Rosalia Maria Cigala Francesco Crea Concetta De Stefano Maria Amlia Santos 《Molecules (Basel, Switzerland)》2021,26(23)
The binding ability of five bifunctional 3-hydroxy-4-pyridinones towards Cu2+ and Fe3+ was studied by means of potentiometric and UV–Vis spectrophotometric measurements carried out at I = 0.15 mol L−1 in NaCl(aq), T = 298.15 K and 310.15 K. The data treatments allowed us to determine speciation schemes featured by metal-ligand species with different stoichiometry and stability, owing to the various functional groups present in the 3-hydroxy-4-pyridinones structures, which could potentially participate in the metal complexation, and in the Cu2+ and Fe3+ behaviour in aqueous solution. Furthermore, the sequestering ability and metal chelating affinity of the ligands were investigated by the determination of pL0.5 and pM parameters at different pH conditions. Finally, a comparison between the Cu2+ and Fe3+/3-hydroxy-4-pyridinones data herein presented with those already reported in the literature on the interaction of Zn2+ and Al3+ with the same ligands showed that, from the thermodynamic point of view, the 3-hydroxy-4-pyridinones are particularly selective towards Fe3+ and could therefore be considered promising iron-chelating agents, also avoiding the possibility of competition, and eventually the depletion, of essential metal cations of biological and environmental relevance, such as Cu2+ and Zn2+. 相似文献
920.
An eco‐friendly protocol for the synthesis of azobenzene dyes by oxidative coupling of primary aromatic amines is reported. As efficient biocatalytic systems, CotA laccase and CotA laccase/ABTS (2,2′‐azinobis(3‐ethylbenzothiazoline‐6‐sulfonic acid)) enable the oxidation of various substituted anilines, in aqueous medium, ambient atmosphere and under mild reaction conditions of pH and temperature. A series of azobenzene dyes were prepared in good to excellent yields in an one‐pot reaction. A mechanistic proposal for the formation of the azo derivatives is presented. Our strategy offers an alternative approach for the direct synthesis of azobenzene dyes, avoiding the harsh conditions generally required for most of the traditional synthetic methods. 相似文献