全文获取类型
收费全文 | 2588篇 |
免费 | 83篇 |
国内免费 | 26篇 |
专业分类
化学 | 1766篇 |
晶体学 | 10篇 |
力学 | 53篇 |
数学 | 493篇 |
物理学 | 375篇 |
出版年
2023年 | 20篇 |
2022年 | 69篇 |
2021年 | 61篇 |
2020年 | 57篇 |
2019年 | 52篇 |
2018年 | 60篇 |
2017年 | 53篇 |
2016年 | 96篇 |
2015年 | 60篇 |
2014年 | 96篇 |
2013年 | 187篇 |
2012年 | 166篇 |
2011年 | 195篇 |
2010年 | 140篇 |
2009年 | 107篇 |
2008年 | 154篇 |
2007年 | 158篇 |
2006年 | 147篇 |
2005年 | 128篇 |
2004年 | 93篇 |
2003年 | 77篇 |
2002年 | 71篇 |
2001年 | 43篇 |
2000年 | 33篇 |
1999年 | 23篇 |
1998年 | 20篇 |
1997年 | 26篇 |
1996年 | 29篇 |
1995年 | 13篇 |
1994年 | 13篇 |
1993年 | 16篇 |
1992年 | 11篇 |
1991年 | 25篇 |
1990年 | 13篇 |
1989年 | 6篇 |
1988年 | 14篇 |
1987年 | 9篇 |
1986年 | 16篇 |
1985年 | 22篇 |
1984年 | 11篇 |
1983年 | 11篇 |
1982年 | 22篇 |
1981年 | 22篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1973年 | 3篇 |
1957年 | 2篇 |
排序方式: 共有2697条查询结果,搜索用时 15 毫秒
51.
52.
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
53.
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
54.
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
55.
56.
Rafael Omar Saavedra Díaz Longzhu Q. Shen Alexander D. Ryabov 《Journal of Coordination Chemistry》2014,67(23-24):3909-3919
The synthesis of 1,2-C6H4(NHCOCMe2NHCO)2-3,3′-(2,2′-bpy) (3), a TAML (tetraamido macrocyclic ligand) incorporating the peripheral 2,2′-bipyridine unit, is described. Its geometry after optimization by density functional theory (DFT) indicated a rather unfavorable conformation of four N–H amide units for forming macrocyclic transition metal complexes. This explains why the iron(III) derivative of 3 could not be obtained even after deprotonation of the N–H bonds by n-BuLi. Nevertheless, the macrocyclic complex of CoIII was synthesized in moderate yield, characterized, and explored by DFT. Our data suggest a strongly distorted square-planar geometry of the macrocyclic complex between CoIII and 3. The dihedral angle between the pyridine rings equals 80° ruling out the possibility of metal chelation by the bipyridine unit. 相似文献
57.
Belén Abarca Rafael Ballesteros Rosa Adam Rafael Ballesteros-Garrido Frédéric R. Leroux Françoise Colobert Ibon Alkorta José Elguero 《Tetrahedron》2014
A dynamic 1H NMR study, together with DFT calculations, of bis-([1,2,3]triazolo)[1,5-a:5′,1′-k][1,10]phenanthroline 2 has allowed to identify the ring and open forms of a new example of ring/chain tautomerism, as well as their interconversion barriers (ring/ring and ring/open). The barrier of the exchange process between the chain forms and the ring form was found higher than the ‘racemization’ process in the closed form, so the ring opening does not contribute to the ‘racemization’. The di-1,10-methyl and di-1,10-iodo derivatives have been prepared and their properties calculated. 相似文献
58.
Michele Tomasini Dr. Lucia Caporaso Jonathan Trouvé Dr. Jordi Poater Dr. Rafael Gramage-Doria Dr. Albert Poater 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(57):e202201970
Non-biological catalysts following the governing principles of enzymes are attractive systems to disclose unprecedented reactivities. Most of those existing catalysts feature an adaptable molecular recognition site for substrate binding that are prone to undergo conformational selection pathways. Herein, we present a non-biological catalyst that is able to bind substrates via the induced fit model according to in-depth computational calculations. The system, which is constituted by an inflexible substrate-recognition site derived from a zinc-porphyrin in the second coordination sphere, features destabilization of ground states as well as stabilization of transition states for the relevant iridium-catalyzed C−H bond borylation of pyridine. In addition, this catalyst appears to be most suited to tightly bind the transition state rather than the substrate. Besides these features, which are reminiscent of the action modes of enzymes, new elementary catalytic steps (i. e. C−B bond formation and catalyst regeneration) have been disclosed owing to the unique distortions encountered in the different intermediates and transition states. 相似文献
59.
de Oliveira Thomas Rafael Tannous Katia de Lima Elisangela Cristina Trevisan 《Journal of Thermal Analysis and Calorimetry》2022,147(13):7431-7448
Journal of Thermal Analysis and Calorimetry - This work aims to investigate the thermal decomposition and reaction kinetics of the hybrid energy cane, as well as its thermochemical properties.... 相似文献
60.
Jeferson Rodrigo Souza Pina Joo Victor Silva-Silva Josiwander Miranda Carvalho Heriberto Rodrigues Bitencourt Luciano Almeida Watanabe Juan Matheus Pereira Fernandes Guilherme Eduardo de Souza Anna Caroline Campos Aguiar Rafael Victorio Carvalho Guido Fernando Almeida-Souza Ktia da Silva Calabrese Patrícia Santana Barbosa Marinho Andrey Moacir do Rosario Marinho 《Molecules (Basel, Switzerland)》2021,26(11)
The natural compound ravenelin was isolated from the biomass extracts of Exserohilum rostratum fungus, and its antimicrobial, antiplasmodial, and trypanocidal activities were evaluated. Ravenelin was isolated by column chromatography and HPLC and identified by NMR and MS. The susceptibility of Gram-positive and Gram-negative bacteria strains to ravenelin was determined by microbroth dilution assay. Cytotoxicity was evaluated in hepatocarcinoma cells (HepG2) and BALB/c peritoneal macrophages by using MTT. SYBR Green I-based assay was used in the asexual stages of Plasmodium falciparum. Trypanocidal activity was tested against the epimastigote and intracellular amastigote forms of Trypanosoma cruzi. Ravenelin was active against Gram-positive bacteria strains, with emphasis on Bacillus subtilis (MIC value of 7.5 µM). Ravenelin’s antiparasitic activities were assessed against both the epimastigote (IC50 value of 5 ± 1 µM) and the intracellular amastigote forms of T. cruzi (IC50 value of 9 ± 2 µM), as well as against P. falciparum (IC50 value of 3.4 ± 0.4 µM). Ravenelin showed low cytotoxic effects on both HepG2 (CC50 > 50 µM) and peritoneal macrophage (CC50 = 185 ± 1 µM) cells with attractive selectivity for the parasites (SI values > 15). These findings indicate that ravenelin is a natural compound with both antibacterial and antiparasitic activities, and considerable selectivity indexes. Therefore, ravenelin is an attractive candidate for hit-to-lead development. 相似文献