全文获取类型
收费全文 | 7878篇 |
免费 | 203篇 |
国内免费 | 49篇 |
专业分类
化学 | 4642篇 |
晶体学 | 62篇 |
力学 | 226篇 |
数学 | 823篇 |
物理学 | 1508篇 |
无线电 | 869篇 |
出版年
2021年 | 52篇 |
2020年 | 96篇 |
2019年 | 79篇 |
2018年 | 70篇 |
2017年 | 58篇 |
2016年 | 151篇 |
2015年 | 132篇 |
2014年 | 146篇 |
2013年 | 338篇 |
2012年 | 327篇 |
2011年 | 371篇 |
2010年 | 234篇 |
2009年 | 215篇 |
2008年 | 387篇 |
2007年 | 418篇 |
2006年 | 411篇 |
2005年 | 357篇 |
2004年 | 314篇 |
2003年 | 316篇 |
2002年 | 279篇 |
2001年 | 200篇 |
2000年 | 214篇 |
1999年 | 123篇 |
1998年 | 111篇 |
1997年 | 109篇 |
1996年 | 153篇 |
1995年 | 101篇 |
1994年 | 119篇 |
1993年 | 118篇 |
1992年 | 123篇 |
1991年 | 108篇 |
1990年 | 101篇 |
1989年 | 81篇 |
1988年 | 111篇 |
1987年 | 84篇 |
1986年 | 83篇 |
1985年 | 105篇 |
1984年 | 101篇 |
1983年 | 51篇 |
1982年 | 77篇 |
1981年 | 81篇 |
1980年 | 68篇 |
1979年 | 85篇 |
1978年 | 65篇 |
1977年 | 98篇 |
1976年 | 77篇 |
1975年 | 69篇 |
1974年 | 78篇 |
1973年 | 75篇 |
1971年 | 42篇 |
排序方式: 共有8130条查询结果,搜索用时 359 毫秒
101.
Miss Itawati Pratanata L. R. Williams R. N. Williams 《Journal of mass spectrometry : JMS》1974,9(4):418-421
When subjected to electron-impact, chloromethyl aryl sulphones (II, X = Cl) fragment predominantly by the loss of CH2Cl from the molecular ion followed by the loss of SO2 and in most cases the appropriate metastable peaks are present to confirm the transitions. In addition, alkyl–oxygen bond formation in the molecular ion was revealed by the presence of prominent peaks corresponding to the [R? ?-SO]+ ions. In most spectra no evidence for aryl–oxygen bond formation could be found and thus the presence of the chlorine atom was able to effectively reverse the direction of skeletal rearrangement reported for aryl methyl sulphones. 相似文献
102.
Pyrolytic process has a promising potential for the environmentally friendly upgrading of lignocellulosic and plastic waste.
Thermogravimetry and pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) were used to get information about the reactive
decomposition of PCL in binary mixtures with microcrystalline cellulose (MC) or sisal fibres (SF). Preliminary thermogravimetric
investigation showed that biomass is thermally degraded at lower temperatures than PCL and this process has a predominant
influence on the thermal behaviour of the mixtures. Discrepancies between the experimental and predicted TG/DTG profiles were
considered as a measurement of the extent of interactions occurring on co-pyrolysis. It was found that reactivity of PCL was
slightly increased in PCL-SF binary mixtures. Evolution of acidic products from cellulose and hemicelluloses decomposition
may promote PCL degradation in binary mixtures with SF. It seems that the co-pyrolysis process could have potential for the
environmentally friendly transformation of biocomposites. 相似文献
103.
Peri F Jiménez-Barbero J García-Aparicio V Tvaroska I Nicotra F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(6):1433-1444
N(OMe)-linked disaccharide analogues, isosteric to the corresponding natural disaccharides, have been synthesized by chemoselective assembly of unprotected natural monosaccharides with methyl 6-deoxy-6-methoxyamino-alpha-D-glucopyranoside in an aqueous environment. The coupling reactions were found to be chemo- and stereoselective affording beta-(1-->6) disaccharide mimics when using Glc and GlcNAc; in the case of Gal, the beta-anomer was prevalent (beta:alpha=7:1). An iterative method for the synthesis of linear N(OMe) oligosaccharide analogues was demonstrated, based on the use of an unprotected monosaccharide building block in which an oxime functionality at C-6 is converted during the synthesis into the corresponding methoxyamino group. The conformational analysis of these compounds was carried out by using NMR spectroscopy, ab initio, molecular mechanics, and molecular dynamics methods. Optimized geometries and energies of fourteen conformers for each compound have been calculated at the B3LYP/6-31G* level. Predicted conformational equilibria were compared with the results based on NMR experiments and good agreement was found. It appears that N(OMe)-linked disaccharide analogues exhibit a slightly different conformational behavior to their parent natural disaccharides. 相似文献
104.
[formula: see text] The diastereoselective synthesis of cis-2,5-disubstituted-3-methylenetetrahydrofurans via Pd(0)-catalyzed cyclization of 2-methylene-1,4-diols is described. Investigations into the scope of the reaction and its application toward the synthesis of amphidinolide K is reported. 相似文献
105.
The ADE fragment of nakadomarin A has been synthesized in nine linear steps from commercial material. The key transformation is an asymmetric azomethine ylide [1,3]-dipolar cycloaddition to establish the AD-spirocyclic system containing three of the four stereocenters of the natural product. [reaction: see text] 相似文献
106.
Bernardi A Potenza D Capelli AM García-Herrero A Cañada FJ Jiménez-Barbero J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(20):4598-4612
As a step to delineate a strategy of ligand design for cholera toxin (CT), NMR studies were performed on several mimics of the GM1 ganglioside oligosaccharide. The conformation of these analogues was investigated first in solution and then upon binding to cholera toxin by transferred nuclear Overhauser effect (TR-NOE) measurements. It was demonstrated that CT selects a conformation similar to the global minima of the free saccharides from the ensemble of presented conformations. No evidence of major conformational distortions was obtained, but one or two of the available conformers of the hydroxyacid side chain appear to be selected in the bound state. The NMR data were interpreted with the aid of computer models, generated and analyzed by using a combination of different approaches (MacroModels' MC/EM and MC/SD, Autodock, and GRID). Analysis of the NMR data supported by computational studies allowed us to interpret the experimental observations and to derive workable models of the ligand:toxin complexes. These models suggest that the higher affinity of the (R)-lactic acid derivative 3 may stem from lipophilic interactions with a hydrophobic area in the toxin binding site located in the vicinity of the sialic acid side chain binding region of the CT:GM1 complex, and formed by the side chain of Ile-58 and Lys-34. Thus, the models obtained have allowed us to make useful design suggestions for the improvement of ligand affinity. 相似文献
107.
Rapid screening method for the determination of diethylstilbestrol in edible animal tissue by column liquid chromatography with electrochemical detection 总被引:10,自引:0,他引:10
A rapid and sensitive screening method for the determination of residues of diethylstilbestrol in edible animal tissue is described. The analyte was extracted from the tissue with tert.-butyl methyl ether, reextracted with 1 M sodium hydroxide and further cleaned up by solid-phase extraction with C18 cartridges. Analysis was performed by isocratic elution with a phosphate-buffered mobile phase, methanol-0.05 M phosphate buffer pH 3.5 (67:33), on a Nucleosil 5-microns C18 column with electrochemical detection at +0.90 V. The average recovery of trans-diethylstilbestrol in spiked samples is 66%, with a standard deviation of 14% (n = 22) in the range 0.5-2.0 microgram/kg. The detection limit is 0.1-0.2 microgram/kg, although at this level other compounds may interfere and give rise to false positive results. 相似文献
108.
The arylation of fluorobenzene with pentafluorophenyl radicals obtained by the reaction of the corresponding aniline with pentyl nitrite is described. The reaction involves attact at the 2t?-,3t?-and 4t?-positions. The composition of the mixture and the nature of its components were determined by 19F NMR spectroscopy and were confirmed by gas-liquid chromatography. 相似文献
109.
Dudley Williams and his colleagues discuss how ligands can gain binding energy to their receptors, and substrate transition states to their enzymes, by tightening the protein structures, with a decrease in their dynamic behaviour. 相似文献
110.
Eva Eichler Clarence Stanley Rooney Haydn Windsor Richard Williams 《Journal of heterocyclic chemistry》1976,13(4):841-844
The syntheses of some 1,8-naphthyridines substituted in the 6-position with heterocyclic groups are described. A synthetic route to 6-amino-5,7-dimethyl-1,8-naphthyridin-2(1H)one is also presented. 相似文献