全文获取类型
收费全文 | 2819篇 |
免费 | 130篇 |
国内免费 | 15篇 |
专业分类
化学 | 2222篇 |
晶体学 | 19篇 |
力学 | 49篇 |
数学 | 263篇 |
物理学 | 411篇 |
出版年
2023年 | 42篇 |
2022年 | 113篇 |
2021年 | 106篇 |
2020年 | 79篇 |
2019年 | 91篇 |
2018年 | 66篇 |
2017年 | 52篇 |
2016年 | 138篇 |
2015年 | 117篇 |
2014年 | 110篇 |
2013年 | 192篇 |
2012年 | 198篇 |
2011年 | 251篇 |
2010年 | 140篇 |
2009年 | 117篇 |
2008年 | 143篇 |
2007年 | 152篇 |
2006年 | 120篇 |
2005年 | 112篇 |
2004年 | 95篇 |
2003年 | 98篇 |
2002年 | 72篇 |
2001年 | 30篇 |
2000年 | 58篇 |
1999年 | 18篇 |
1998年 | 14篇 |
1997年 | 5篇 |
1996年 | 16篇 |
1995年 | 12篇 |
1994年 | 16篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 7篇 |
1985年 | 13篇 |
1984年 | 17篇 |
1983年 | 11篇 |
1982年 | 7篇 |
1981年 | 12篇 |
1980年 | 5篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1973年 | 4篇 |
排序方式: 共有2964条查询结果,搜索用时 23 毫秒
91.
Checkers R. Marshall Emma E. Timmel Sara A. Staudhammer Carl K. Brozek 《Chemical science》2020,11(42):11539
Nanoparticles of metal–organic frameworks (nanoMOFs) boast superior properties compared to their bulk analogs, yet little is known about how common synthetic parameters dictate particle sizes. Here, we provide experimental evidence for the “seesaw” model of nanoMOF growth. Solution acidity, ligand excess, and reactant concentrations are decoupled and shown to form the key independent determinants of nanoMOF sizes, thereby validating the proposal that nanoMOFs arise from coupled equilibria involving ligand deprotonation and metal–ligand complexation. By achieving the first demonstration of a seesaw relationship between nanoMOF sizes and ligand excess, these results provide further proof of the model, as they required deliberate manipulation of relationships outlined by the model. Exploring the relative impacts of these parameters reveals that ligand excess has the greatest ability to decrease sizes, although low acidity and high concentrations can exhibit similar effects. As a complement to existing models of polymer formation and crystal growth, the seesaw model therefore offers a powerful tool for reliable control over nanoMOF sizes.Nanoparticles of metal–organic frameworks (nanoMOFs) boast superior properties compared to their bulk analogs, yet little is known about how common synthetic parameters dictate particle sizes. 相似文献
92.
Alajarin M Bonillo B Ortin MM Orenes RA Vidal A 《Organic & biomolecular chemistry》2011,9(19):6741-6749
N-(2-azidomethyl)phenyl ketenimines and N-(2-azidomethyl)phenyl-N'-alkyl(aryl) carbodiimides undergo, under mild thermal conditions, intramolecular [3 + 2] cycloaddition reactions between the azido group and either the C=C or the distal C=N double bonds of the ketenimine and carbodiimide functions respectively. The reaction products are indolo[1,2-a]quinazolines and/or indolo[2,1-b]quinazolines in the case of azido-ketenimines, and tetrazolo[5,1-b]quinazolines in the case of azido-carbodiimides. The formation of the two classes of indoloquinazolines implies the ulterior dinitrogen extrusion from the non-isolated, putative [3 + 2] cycloadducts between the azide and ketenimine functions, whereas in the case of azido-carbodiimides the initial cycloadducts, tetrazoloquinazolines, were cleanly isolated and further converted into 2-aminoquinazolines by thermally induced dinitrogen extrusion. 相似文献
93.
Kyle Clagg Sara Hold Archana Kumar Stefan G. Koenig Remy Angelaud 《Tetrahedron letters》2019,60(1):5-7
The direct application of Knochel-Hauser derivative arylmagnesium halides in Kumada-Corriu couplings is described. By utilizing the in situ-generated Grignard reagents, the transmetalation with stoichiometric zinc salts to known Negishi coupling conditions is avoided, thereby streamlining the transformation. Several aromatic hetereocycles participate in the deprotonation and couple with a variety of phenyl iodides and bromides in isolated yields up to 81%. The parent system is demonstrated on 6?g scale with 79% yield, and monitored via ReactIR to show the stability of the Grignard and progression of the deprotonation/CC coupling reaction. 相似文献
94.
Estimation and correction of matrix effects in gas chromatographic pesticide multiresidue analytical methods with a nitrogen-phosphorus detector 总被引:1,自引:0,他引:1
Egea González FJ HernándezTorres ME Cuadros Rodríguez L Almansa López E Martínez Vidal JL 《The Analyst》2002,127(8):1038-1044
The assessment of matrix effects in the quantification of organophosphorus pesticides in fruit and vegetables by GC-NPD, were studied applying ANCOVA. Calibration curves prepared in solvent were compared with calibration curves prepared in a blank matrix extract for eight different commodities, establishing whether the matrix induces systematic or proportional errors in the quantification of the pesticides. In such cases correction functions were obtained and validated by quantifying spiked samples using solvent calibrations and applying the correction functions to the data obtained. The results were compared with those obtained by quantification using matrix-matching calibrations and with those from 100% recovery experiments. It was found that the matrix effects can be avoided using the correction functions. Finally the contribution of the correction functions to the uncertainty of the results was estimated as well as their stability during a four month period. 相似文献
95.
Reaction of N-tert-butyloxycarbonylasparagine (Boc-Asn) with 2 equiv of benzyl bromide in presence of cesium carbonate led to N-benzyl-3-Boc-amino-pyrrolidin-2,5-dione 1a (N-benzyl-3-Boc-aminosuccinimide). Borane dimethylsulfide reduced 3-Boc-aminopyrrolidine-2,5-dione 1a into 3-Boc-aminopyrrolidin-2-one 2a. The same procedure could also be used to prepare derivatives 1 and 2 substituted on the aromatic ring. 相似文献
96.
Michele Menotta Sara Biagiotti Laura Streppa Luigia Rossi Mauro Magnani 《Analytica chimica acta》2015
In the present paper we describe an atomic force microscopy (AFM)-based method for the quantitative analysis of FK506 (Tacrolimus) in whole blood (WB) samples. Current reference methods used to quantify this immunosuppressive drug are based on mass spectrometry. In addition, an immunoenzymatic assay (ELISA) has been developed and is widely used in clinic, even though it shows a small but consistent overestimation of the actual drug concentration when compared with the mass spectrometry method. The AFM biosensor presented herein utilises the endogen drug receptor, FKBP12, to quantify Tacrolimus levels. The biosensor was first assayed to detect the free drug in solution, and subsequently used for the detection of Tacrolimus in blood samples. The sensor was suitable to generate a dose–response curve in the full range of clinical drug monitoring. A comparison with the clinically tested ELISA assay is also reported. 相似文献
97.
Znabet A Blanken S Janssen E de Kanter FJ Helliwell M Turner NJ Ruijter E Orru RV 《Organic & biomolecular chemistry》2012,10(5):941-944
An efficient combination of MAO-N-catalyzed desymmetrization of cyclic meso-amines with Ugi-Smiles multicomponent chemistry produced optically pure N-aryl proline amides. This method represents the first report of a fully asymmetric Ugi-Smiles process. 相似文献
98.
Electrostatic interactions in proteins can be probed experimentally through determination of residue-specific acidity constants. We describe here triple-resonance NMR techniques for direct determination of lysine and arginine side-chain protonation states in proteins. The experiments are based on detection of nonexchangeable protons over the full range of pH and temperature and therefore are well suited for pKa determination of individual amino acid side chains. The experiments follow the side-chain 15Nzeta (lysine) and 15Nepsilon or 13Czeta (arginine) chemical shift, which changes due to sizable changes in the heteronuclear electron distribution upon (de)protonation. Since heteronuclear chemical shifts are overwhelmed by the charge state of the amino acid side chain itself, these methods supersede 1H-based NMR in terms of accuracy, sensitivity, and selectivity. Moreover, the 15Nzeta and 15Nepsilon nuclei may be used to probe changes in the local electrostatic environment. Applications to three proteins are described: apo calmodulin, calbindin D9k, and FKBP12. For apo calmodulin, residue-specific pKa values of lysine side chains were determined to fall between 10.7 and 11.2 as a result of the high net negative charge on the protein surface. Ideal two-state titration behavior observed for all lysines indicates the absence of significant direct charge interactions between the basic residues. These results are compared with earlier studies based on chemical modification. 相似文献
99.
Sara Mattsson 《Tetrahedron letters》2007,48(14):2497-2499
When treated with amine bases such as triethylamine and various lithium salts in wet solvents, esters are efficiently hydrolyzed to the corresponding acids in good yields. Esters incorporating an α- or β-heteroatom with respect to the ester carbonyl group are hydrolyzed rapidly even at room temperature. To further demonstrate the usefulness of this method, one example is provided where hydrolysis of acetylated camphorsultam is mediated by LiBr. 相似文献
100.