全文获取类型
收费全文 | 2506篇 |
免费 | 86篇 |
国内免费 | 24篇 |
专业分类
化学 | 1694篇 |
晶体学 | 9篇 |
力学 | 67篇 |
数学 | 475篇 |
物理学 | 371篇 |
出版年
2023年 | 20篇 |
2022年 | 67篇 |
2021年 | 61篇 |
2020年 | 57篇 |
2019年 | 54篇 |
2018年 | 59篇 |
2017年 | 62篇 |
2016年 | 99篇 |
2015年 | 61篇 |
2014年 | 91篇 |
2013年 | 182篇 |
2012年 | 168篇 |
2011年 | 199篇 |
2010年 | 141篇 |
2009年 | 105篇 |
2008年 | 149篇 |
2007年 | 154篇 |
2006年 | 143篇 |
2005年 | 121篇 |
2004年 | 87篇 |
2003年 | 73篇 |
2002年 | 65篇 |
2001年 | 27篇 |
2000年 | 28篇 |
1999年 | 14篇 |
1998年 | 20篇 |
1997年 | 26篇 |
1996年 | 27篇 |
1995年 | 11篇 |
1994年 | 11篇 |
1993年 | 16篇 |
1992年 | 15篇 |
1991年 | 21篇 |
1990年 | 13篇 |
1989年 | 10篇 |
1988年 | 17篇 |
1987年 | 7篇 |
1986年 | 18篇 |
1985年 | 17篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 22篇 |
1981年 | 18篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1971年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有2616条查询结果,搜索用时 15 毫秒
91.
92.
93.
Vladimir I. Galkin Gulchira M. Saakyan Irina V. Galkina Rafael A. Cherkasov 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4)
Abstract Elaboration of effective methods of selective phosphorylation of α,β-unsaturated carbonyl compounds containing simultaneously C[dbnd]C and C[dbnd]O bonds, which may compete with each other or react consistently one after another, is one of the most complex problems in the modern organophosphorus synthesis. On the basis of our previous systematic investigations of kinetics and mechanism of the Pudovik and Abramov reactions in the series of α,β-unsaturated carbonyl compounds we have suggested three possible ways to solve this problem: a) soft reaction conditions, regulated by solvent, catalyst and temperature; b) interface catalysis; c) metallocomlex catalysis (reaction in the coordination sphere of transition metal). 相似文献
94.
Klaus Bieger Miguel Tomás José Barluenga Rafael Santiago Santiago García-Granda 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4)
Abstract The title compound is the main product of the 2:1 reaction of DMAD with diazaphosphinines. The supposed mechanisms with intermediates and related products will be presented. 相似文献
95.
96.
Amanda B. Lima Eric O. Faria Rodrigo H. O. Montes Rafael R. Cunha Eduardo M. Richter Rodrigo A. A. Munoz Wallans T. P. dos Santos 《Electroanalysis》2013,25(7):1585-1588
The electrochemical oxidation of ibuprofen at a boron‐doped diamond electrode (BDDE) and its voltammetric determination is reported for the first time. A well‐defined oxidation peak was observed at around 1.6 V in 0.1 mol L?1 H2SO4 solution with 10 % (v/v) ethanol at the BDDE surface activated by either cathodic or anodic pretreatments. A differential‐pulse voltammetric method for the determination of ibuprofen in pharmaceutical formulations was optimized with a detection limit of 5 µmol L?1 and compared with the British Pharmacopeia method. 相似文献
97.
Guillermo Lasarte-Aragonés Rafael Lucena Soledad Cárdenas Miguel Valcárcel 《Analytical and bioanalytical chemistry》2013,405(10):3269-3277
Extraction techniques are surface-dependent processes since their kinetic directly depends on the contact area between the sample and the extractant phase. The dispersion of the extractant (liquid or solid) increases this area improving the extraction efficiency. In this article, the dispersion of a nanostructured sorbent at the very low milligram level is achieved by effervescence thanks to the in situ generation of carbon dioxide. For this purpose, a special tablet containing the effervescence precursors (sodium carbonate as carbon dioxide source and sodium dihydrogen phosphate as proton donor) and the sorbent [multiwalled carbon nanotubes (MWCNTs)] is prepared. All the microextraction steps take place in a glass beaker containing 100 mL of the sample. After the extraction, the MWCNTs, enriched with the extracted analytes, are recovered by vacuum filtration. Methanol was selected to elute the retained analytes. The extraction mode is optimized and characterized using the determination of nine herbicides in water samples as model analytical problem. The absolute recoveries of the analytes were in the range 48–76 %, while relative recoveries were close to 100 % in all cases. These values permit the determination of these analytes at the low microgram per liter range with good precision (relative standard deviations lower than 9.3 %) using ultra performance liquid chromatography (UPLC) combined with ultraviolet detection (UV). 相似文献
98.
Gustavo Santiso‐Quinones Rafael E. Rodriguez‐Lugo 《Acta Crystallographica. Section C, Structural Chemistry》2013,69(8):859-861
The compound [2‐(aminomethyl)pyridine‐κ2N,N′][chlorido/trifluoromethanesulfonato(0.91/0.09)][(10,11‐η)‐5H‐dibenzo[a,d]cyclohepten‐5‐amine‐κN](triphenylphosphane‐κP)ruthenium(II) trifluoromethanesulfonate dichloromethane 0.91‐solvate, [Ru(CF3SO3)0.09Cl0.91(C6H8N2)(C15H13N)(C18H15P)]CF3SO3·0.91CH2Cl2, belongs to a series of RuII complexes that had been tested for transfer hydrogenation, hydrogenation of polar bonds and catalytic transfer hydrogenation. The crystal structure determination of this complex revealed disorder in the form of two different anionic ligands sharing the same coordination site, which other spectroscopic methods failed to characterize. The reduced catalytic activity of the title compound was not fully understood until the crystallographic data provided evidence for the mixed ligand species. The crystal structure clearly shows that the majority of the synthesized material has a chloride ligand present. Only a small portion of the material is the expected complex [RuII(OTf)(ampy)(η2‐tropNH2)(PPh3)]OTf, where OTf is triflate or trifluoromethanesulfonate, ampy is 2‐(aminomethyl)pyridine and tropNH2 is 5H‐dibenzo[a,d]cyclohepten‐5‐amine. 相似文献
99.
Noelia Negreira Isaac Rodríguez Rosario Rodil Elisa Rubí Rafael Cela 《International journal of environmental analytical chemistry》2013,93(11):1174-1188
A low solvent consumption method for the determination of eight ultraviolet (UV) filters, displaying low to medium polarities, in freeze-dried samples of marine bivalves and fish is proposed. Matrix solid-phase dispersion (MSPD) and gas chromatography with mass spectrometry (GC-MS) were used as sample preparation and determination techniques, respectively. This work describes the influence of several parameters (type and amount of dispersant and clean-up sorbents, as well as elution solvent) on the yield and the selectivity of the MSPD extraction. Under optimized conditions, samples (0.5?g) were ground with 2?g of Florisil in a mortar with a pestle and transferred into a polypropylene syringe, which contained 1?g of C18 as clean-up sorbent. Analytes were eluted with 5?mL of acetonitrile. This extract was concentrated to dryness, re-constituted with 1?mL of ethyl acetate and injected in the GC-MS system without any further clean-up. The global average recoveries, measured for three different biota samples, spiked at three different levels (between 50 and 1000?ng?g?1), ranged from 80% to 101% with associated standard deviations below 10%. The inter-day precision of the method varied from 4% to 15% and the achieved LOQs (defined for a signal to noise ratio of 10) ranged from 4 to 28?ng?g?1, referred to the freeze-dried matrix. Octocrylene (OCR) was found in some samples of fish and mussels at concentrations between 15 and 20?ng?g?1, referred to dry mass. 相似文献
100.
José L. Arias Pankaj Sharma Armando Cabrera Fernando Beristain Rafael Sampere Cesar Arizmendi 《Transition Metal Chemistry》2013,38(7):787-792
In the cobalt-catalysed hydroformylation of 3,4-dihydro[2H]pyran, the influence of different reaction parameters such as time, pressure, triphenylphosphine addition, catalyst and substrate concentration has been investigated. 2-formyl-tetrahydropyran, tetrahydropyran and a hydroalkylcarbonylation product were the main reaction products. The selectivity towards 2-formyl-tetrahydropyran formation is favoured at constant catalyst and substrate concentration. The coordination of the pyran’s oxygen to the cobalt atom seems to be an important intermediate for the formation of 2-formyl-tetrahydropyran. Different substrate or catalyst concentrations promote the formation of other reduced products. The addition of triphenylphosphine to the catalyst leads to a less active species, which decreases the yield and promotes the hydroalkylcarbonylation reaction. 相似文献