全文获取类型
收费全文 | 1243篇 |
免费 | 27篇 |
国内免费 | 8篇 |
专业分类
化学 | 898篇 |
晶体学 | 3篇 |
力学 | 26篇 |
数学 | 178篇 |
物理学 | 173篇 |
出版年
2023年 | 8篇 |
2022年 | 13篇 |
2021年 | 21篇 |
2020年 | 23篇 |
2019年 | 12篇 |
2018年 | 17篇 |
2017年 | 14篇 |
2016年 | 40篇 |
2015年 | 40篇 |
2014年 | 27篇 |
2013年 | 59篇 |
2012年 | 88篇 |
2011年 | 87篇 |
2010年 | 60篇 |
2009年 | 58篇 |
2008年 | 80篇 |
2007年 | 77篇 |
2006年 | 79篇 |
2005年 | 67篇 |
2004年 | 69篇 |
2003年 | 50篇 |
2002年 | 42篇 |
2001年 | 32篇 |
2000年 | 37篇 |
1999年 | 13篇 |
1998年 | 12篇 |
1997年 | 11篇 |
1996年 | 18篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 14篇 |
1992年 | 15篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 11篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 11篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 5篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1959年 | 2篇 |
排序方式: 共有1278条查询结果,搜索用时 0 毫秒
101.
Kinetic resolution of racemic mixtures is a well-established methodology for the preparation of optically active compounds. However, excellent enantioselectivities are required to obtain them in enantiopure form, due to the decrease in ee when conversion values are close to 50%. To overcome this limitation, a parallel (asymmetric) reaction can remove the disfavored enantiomer. In this review, several examples of this strategy showing its wide range of applicability are described, as well as their mathematical treatment and some new applications in combinatorial chemistry. 相似文献
102.
L. M. Gandía M. A. Vicente P. Oelker P. Grange A. Gil 《Reaction Kinetics and Catalysis Letters》1998,64(1):145-151
A Na-montmorillonite was intercalated with manganese- and samarium-manganese-aluminium solutions. The pillared montmorillonites
were characterized by X-ray diffraction and nitrogen adsorption. The presence of manganese and samarium-manganese citrates
in the intercalation process modify the distance between the clay sheets and the microporous structure of the pillared montmorillonites. 相似文献
103.
Zúñiga-Pérez J Muñoz-Sanjosé V Palacios-Lidón E Colchero J 《Physical review letters》2005,95(22):226105
The surface electrical properties of ZnO thin films grown along the nonpolar [1120] direction have been investigated by Kelvin probe microscopy on a nanometer scale. Two different charge domains, with a 75 meV work function difference, coexist within the ZnO surface, which is covered by rhombohedral pyramids whose sidewalls are shown to be {1011}-type planes. The presence and relative orientation of the two kinds of charge domains are explained in terms of the atomic arrangement at the {1011} polar surfaces. 相似文献
104.
Giménez-Romero D Agrisuelas J García-Jareño JJ Gregori J Gabrielli C Perrot H Vicente F 《Journal of the American Chemical Society》2007,129(22):7121-7126
This paper demonstrates the importance of the structural changeover in controlling the physical-chemical properties of hexacyanometalate-like materials (Prussian Blue). A meticulous in situ study of compositional variations using electroacoustic impedance techniques associated to electrogravimetric techniques in hexacyanoferrates containing K+ alkali metals reveals the existence of a nanostructural changeover coupled to a change of the magnetic properties of these electromagnetic materials. In the same way, the electroacoustic impedance techniques can be useful both in the understanding and in the in situ monitoring of the structural changeovers and the magnetic behavior of all kinds of materials. 相似文献
105.
Sanchez-Escribano V Arrighi L Riani P Marazza R Busca G 《Langmuir : the ACS journal of surfaces and colloids》2006,22(22):9214-9219
Cu-Pd/Al2O3 bimetallic catalysts have been characterized by XRD, TEM, and EDX techniques. The surface structure has been investigated by FT-IR spectroscopy of low-temperature adsorbed CO in the reduced and in the oxidized state. Evidence has been provided of the formation of Cu-Pd alloy nanoparticles, both of the alpha-phase (disordered fcc) and of the beta-phase (ordered CsCl-type). IR spectra suggest that Cu likely decorates the edges while Pd mostly stays at the main faces. Part of copper disperses as Cu+ on the support even after reduction. The presence of copper seems to modify strongly the sate of oxidized Pd centers in oxidized high-Pd content materials. The redox chemistry of the system, where Pd is reduced more easily than Cu, appears to be very complex. 相似文献
106.
Lutz Ackermann Prof. Dr. Rubén Vicente Dr. Anant R. Kapdi Dr. 《Angewandte Chemie (International ed. in English)》2009,48(52):9792-9826
The area of transition‐metal‐catalyzed direct arylation through cleavage of C? H bonds has undergone rapid development in recent years, and is becoming an increasingly viable alternative to traditional cross‐coupling reactions with organometallic reagents. In particular, palladium and ruthenium catalysts have been described that enable the direct arylation of (hetero)arenes with challenging coupling partners—including electrophilic aryl chlorides and tosylates as well as simple arenes in cross‐dehydrogenative arylations. Furthermore, less expensive copper, iron, and nickel complexes were recently shown to be effective for economically attractive direct arylations. 相似文献
107.
We report the direct electrochemistry of cytochrome c at screen printed graphite electrodes which exhibits quasi-reversible voltammetric responses without the need for any chemical or electrochemical pre-treatment, use of mediators or nanomaterials. Through voltammetric studies and X-ray photoelectron spectroscopy (XPS) it is shown that carbonyl and carboxylic surface oxygenated species likely residing at edge plane like- sites/defects of the graphite comprising the screen printed electrodes are responsible for the favourable interaction of the cytochrome c with that of the screen printed electrochemical sensing platform. 相似文献
108.
Anhydrides as acylating agents in the enzymatic resolution of an intermediate of (-)-Paroxetine 总被引:2,自引:0,他引:2
de Gonzalo G Brieva R Sánchez VM Bayod M Gotor V 《The Journal of organic chemistry》2003,68(8):3333-3336
A new chemoenzymatic method for the preparation of an intermediate of (-)-Paroxetine is reported. Cyclic anhydrides are used as acylating agents in the lipase-catalyzed esterification of trans-4-(4'-fluorophenyl)-3-hydroxymethyl-N-phenyloxycarbonylpiperidine in organic solvents. The best enantioselectivities are obtained with two different lipases from Candida antarctica. These two lipases show opposite stereochemical preference in these processes, so that both enantiomers can be obtained in their optically pure forms. The (3S,4R) isomer is an intermediate for the synthesis of (-)-Paroxetine. 相似文献
109.
Abu-Youssef MA Drillon M Escuer A Goher MA Mautner FA Vicente R 《Inorganic chemistry》2000,39(22):5022-5027
The reaction of manganese(II) and pyridine derivatives such as 3-methylpyridine (3-Mepy) and 3,4-dimethylpyridine (3,4-Dmepy) led to the new one-dimensional systems trans-[Mn(3-Mepy)2(N3)2]n (1) and trans-[Mn(3,4-Dmepy)2(N3)2]n (2). Compound 1 crystallizes in the monoclinic system, space group P2(1)/n, a = 11.201(3) A, b = 14.499(4) A, c = 14.308(4) A, Z = 6, and compound 2 crystallizes in the triclinic system, space group P1, a = 11.502(4) A, b = 14.246(5) A, c = 16.200(8) A, Z = 6. The two compounds show the same general one-dimensional arrangement of double azido bridges between neighboring manganese atoms with the unprecedented -Mn-(mu(1,3)-N3)2-Mn-(mu(1,3)-N3)2-Mn-(mu(1,1)-N3)2-Mn- sequence. Susceptibility and magnetization measurements reveal a ferrimagnetic-like behavior derived from the topology of the chain. A model of the Heisenberg chain, comprising classical spins coupled through alternating exchange interactions J1J1J2... is proposed to describe the magnetic behavior. 相似文献
110.
Spectrafluorimetric method for the rapid screening of toxic heavy metals in water samples 总被引:1,自引:0,他引:1
B. San Vicente de la RivaJ.M. Costa-Fernández R. PereiroA. Sanz-Medel 《Analytica chimica acta》2002,451(2):203-210
A fast and inexpensive sensitive screening test for recognising potential wastewater contamination by the presence of highly toxic heavy metals is described. The test is based on the reaction of the toxic heavy metals Hg(II), Cd(II), Pb(II) and Ag(I) with 6-mercaptopurine (6-MP) to produce highly fluorescent complexes.Optimum experimental conditions include a buffer of pH 7.2 (0.1 M citric acid/0.2 M Na2HPO4), a chelating reagent concentration of 6×10−4 M and the addition of 10−4 M of o-phenanthroline. The fluorescence emitted by the complexes was measured at 380 and 540 nm for excitation and emission wavelengths, respectively.Detection limits of 4, 3, 6 and 3 μg l−1 were achieved for Hg, Cd, Pb and Ag. Relative standard deviation (R.S.D.) were between ±2 and ±6% of the fluorescence signals for five identical samples. Potential interference effects from other heavy metals (Zn, Mn, Co, Fe, Ni and Cu), which could affect the response of the proposed screening test was investigated. Results showed that none of these metals give rise to noticeable fluorescence signals under the above described experimental conditions.Finally, the capability of the proposed heavy metal screening test for the analysis of contaminated water samples is discussed. 相似文献