全文获取类型
收费全文 | 214篇 |
免费 | 15篇 |
专业分类
化学 | 135篇 |
晶体学 | 1篇 |
力学 | 5篇 |
数学 | 41篇 |
物理学 | 47篇 |
出版年
2023年 | 7篇 |
2022年 | 2篇 |
2021年 | 6篇 |
2020年 | 4篇 |
2019年 | 7篇 |
2018年 | 4篇 |
2017年 | 2篇 |
2016年 | 4篇 |
2015年 | 12篇 |
2014年 | 10篇 |
2013年 | 7篇 |
2012年 | 16篇 |
2011年 | 20篇 |
2010年 | 15篇 |
2009年 | 7篇 |
2008年 | 10篇 |
2007年 | 9篇 |
2006年 | 12篇 |
2005年 | 7篇 |
2004年 | 8篇 |
2003年 | 5篇 |
2002年 | 9篇 |
2001年 | 2篇 |
2000年 | 6篇 |
1999年 | 1篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 3篇 |
1974年 | 1篇 |
1904年 | 1篇 |
1889年 | 1篇 |
1886年 | 2篇 |
1884年 | 2篇 |
1883年 | 1篇 |
排序方式: 共有229条查询结果,搜索用时 62 毫秒
101.
Leydet Y Gavara R Cunha-Silva L Parola AJ Pina F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(13):3663-3671
New trans‐2‐hydroxychalcones bearing a carboxylate group at position 2′ ( Ct ?) were synthesized (compounds 2 and 3 ). These compounds lead to a network of chemical reactions depending on pH value, light, and solvent. In water, when the pH value is lowered, the ionized trans‐chalcone is protonated and the flavylium cation A H+ is formed at very acidic pH values through hemiketal B and cis‐chalcone Cc , with global acidity constants of pK′a ≤?1 and ≈0.1, respectively, for 2 and 3 . The electron‐acceptor character of the carboxylic substituent not only increases the observed acidity of the flavylium cation, but also decreases the rate of the ring‐opening/‐closing from a subsecond timescale to hours relative to model compound 1 (without carboxylate). The photochemistry of the network was studied in detail by means of continuous irradiation, monitored by UV/Vis absorption and 1H and 13C NMR spectroscopic analysis. Although compound 3 is only slightly photoactive, compound 2 ( Ct? ) reacts in aqueous solutions (λirr=313 nm) to form B? and Cc? , with a global quantum yield of 0.15, and fully reverts back to Ct? with a rate constant of k=6.7×10?5 s?1. The flavylium cation is no longer formed in methanol, and irradiation of Ct? leads to the formation of B ? and the new lactone‐trapped chromene species La . The formation of La takes place through a sequence of three photochemical steps: photoisomerization of Ct ?, photo‐ring‐closing reaction of Cc ?, and photolactonization of B ?. Only the cis/trans isomerization and ring‐closing reactions are thermally reversible on a timescale of seconds and hours, respectively. A photochromic system was achieved in rigid matrices of methanol (at 77 K) and 1‐dodecanol (5 °C) by irradiating lactone La to give a red ortho‐quinone allide through a photo‐ring‐opening reaction; the color disappears with a rate constant of k=1.25×10?2 s?1 in 1‐dodecanol at 5 °C. 相似文献
102.
The MIAplex? device is a miniaturized detector, devoted to the high sensitive detection of superparamagnetic nanoprobes for multiparametric immunoassays. It measures a signal corresponding to the second derivative of the magnetization around zero field. Like any new technology, the real success of the MIAplex? detector can only be exploited through a deep understanding of the magnetic signature. In this letter, we study the magnetic behavior around zero-field of diluted lab-made and commercial ferrofluids by comparing together conventional SQUID magnetization and MIAplex? signature. 相似文献
103.
Jean-Pierre Aubin 《NoDEA : Nonlinear Differential Equations and Applications》2013,20(2):187-211
We state and prove a “Lax–Hopf formula” characterizing viable capture basins of targets investigated in viability theory and derive a “Max-Plus” morphism of capture basins with respect to the target. Capture basins are used to define “viability solutions” to Hamilton–Jacobi equations satisfying “trajectory conditions” (initial, boundary or Lagrangian conditions). The Max-Plus morphism property of Lax–Hopf formula implies the fact that the solution associated with inf-convolution of trajectory conditions is the inf-convolution of the solutions for each trajectory condition. For instance, Lipschitz regularization or decreasing envelopes of trajectory condition imply the Lipschitz regulation or decreasing envelopes of the solutions. 相似文献
104.
Direct α-arylation reactions of secondary β-keto amides with arynes, generated by fluoride-induced elimination of ortho-silyl aryltriflates, are described. The transformation proceeds via an interrupted insertion reaction of arynes and leads to densely functionalized aromatic compounds exhibiting a chiral 'all carbon' quaternary center under transition-metal-free conditions. An organocatalytic asymmetric version of the reaction also proved possible, affording the proof of concept that arynes can be involved in enantioselective transformations. 相似文献
105.
S. Archambault F. Aubin M. Auger E. Behnke B. Beltran K. Clark X. Dai A. Davour J. Farine R. Faust M.-H. Genest G. Giroux R. Gornea C. Krauss S. Kumaratunga I. Lawson C. Leroy L. Lessard C. Levy I. Levine R. MacDonald J.-P. Martin P. Nadeau A. Noble M.-C. Piro S. Pospisil T. Shepherd N. Starinski I. Stekl C. Storey U. Wichoski V. Zacek 《Physics letters. [Part B]》2009
106.
107.
108.
109.
110.
Madeleine Aubin Yves Bdard Marie-France Morrissette Robert E. Prud'homme 《Journal of Polymer Science.Polymer Physics》1983,21(2):233-240
Differential scanning calorimetry was used to determine the miscibility behavior of several polyester/Saran blends, the two polymers forming these blends being semicrystalline. It was found that Saran is miscible with polycaprolactone (PCL), polyvalerolactone, poly(butylene adipate), and poly(hexamethylene sebacate) since a single glass transition temperature Tg was observed at each composition. However, immiscibility was found between Saran and poly(ethylene adipate), poly-(ethylene succinate), poly(β-propiolactone), and poly(α-methyl-α-n-propyl-β-propiolactone) since two Tg's were recorded at several compositions. Blends were then obtained containing, over a wide range of composition, a miscible amorphous phase and two different types of crystals. From melting-point depression data on PCL and Saran crystals, thermodynamic interaction parameters χ were calculated and found to be different for PCL-rich blends and for Saran-rich blends. This result suggests a variation of χ with composition. Saran is a polymer which does not contain α-hydrogens and its miscibility with polyesters may result from a β-hydrogen bonding interaction or a C?O/C? Cl dipole-dipole interaction. 相似文献