全文获取类型
收费全文 | 5528篇 |
免费 | 249篇 |
国内免费 | 52篇 |
专业分类
化学 | 3911篇 |
晶体学 | 70篇 |
力学 | 251篇 |
综合类 | 1篇 |
数学 | 606篇 |
物理学 | 990篇 |
出版年
2024年 | 12篇 |
2023年 | 53篇 |
2022年 | 434篇 |
2021年 | 324篇 |
2020年 | 239篇 |
2019年 | 240篇 |
2018年 | 206篇 |
2017年 | 168篇 |
2016年 | 274篇 |
2015年 | 152篇 |
2014年 | 209篇 |
2013年 | 454篇 |
2012年 | 324篇 |
2011年 | 323篇 |
2010年 | 202篇 |
2009年 | 173篇 |
2008年 | 217篇 |
2007年 | 226篇 |
2006年 | 184篇 |
2005年 | 138篇 |
2004年 | 143篇 |
2003年 | 123篇 |
2002年 | 139篇 |
2001年 | 63篇 |
2000年 | 45篇 |
1999年 | 39篇 |
1998年 | 35篇 |
1997年 | 25篇 |
1996年 | 36篇 |
1995年 | 34篇 |
1994年 | 37篇 |
1993年 | 29篇 |
1992年 | 35篇 |
1991年 | 36篇 |
1990年 | 28篇 |
1989年 | 47篇 |
1988年 | 40篇 |
1987年 | 28篇 |
1986年 | 31篇 |
1985年 | 28篇 |
1984年 | 37篇 |
1983年 | 27篇 |
1982年 | 18篇 |
1981年 | 20篇 |
1980年 | 22篇 |
1979年 | 18篇 |
1978年 | 23篇 |
1977年 | 18篇 |
1976年 | 14篇 |
1973年 | 11篇 |
排序方式: 共有5829条查询结果,搜索用时 15 毫秒
81.
A new analytical method is described for the determination of organochlorine pesticides (OCPs) in sewage sludges using GC-ion trap-MS–MS. In this work, 16 organo-chlorine pesticides (OCPs) listed by the US Environmental Protection Agency (US EPA) as priority pollutants were separated and quantified. Sludge samples from three of Kuwaits wastewater treatment plants (WWTPs) were analyzed for organochlorine pesticides (OCPs). Spiked sludge samples were extracted with a mixture of (1:1 v/v) dichloromethane (DCM)/hexane. The extracts were cleaned on a silica/aluminum oxide column, then transferred to a gel permeation chromatography (GPC) column, before undergoing further silica/aluminum oxide clean-up; the presence of OCPs was then confirmed by GC-ion trap-MS–MS. Three extraction techniques, soxtec, soxhlet, and pressurized liquid extractions were utilized, compared and validated using the spiked sludge samples. The methods were validated in term of accuracy (recovery) and precision (RSD). The method recovery values varyied from 76.1 to 92.9% for the three extraction techniques. 相似文献
82.
cis,cis,cis-1,2,3,4-Tetrakis(diphenylphosphinomethyl)cyclopentane/[PdCl(C3H5)]2 efficiently catalyses the Heck reaction of β-substituted enol ethers with aryl bromides. Employing β-methoxystyrene, 3-ethoxyacrylonitrile or methyl 3-methoxyacrylate, the regioselective α-arylation of these enol ethers was observed in all cases, and mixtures of Z and E isomers were generally obtained, which in many cases yielded a single ketone product after acid treatment. The stereoselectivity of this reaction depends on steric and electronic factors, and better stereoselectivities in favour of Z isomers were observed with electron-rich or sterically congested aryl bromides. Better yields were obtained for this reaction with electron-rich or sterically congested aryl bromides than with electron-poor aryl bromides. This observation suggests that with these β-substituted enol ethers the rate-limiting step of the catalytic cycle is not the oxidative addition of the aryl bromide to the palladium complex. 相似文献
83.
The syntheses and the X-ray structures of the tetranuclear gold(I) benzamidinate, Au4[PhNC(Ph)NPh]4, and the tetranuclear gold(I) acetamidinate, Au4[PhNC(CH3)NPh]4, clusters are reported. The clusters are produced by the reaction of the sodium salt of an amidine ligand with the gold precursor
Au(THT)Cl in a (1:1) stoichiometry. The average Au...Au distance between adjacent Au(I) atoms is ∼2.9 ?, typical of compounds
having an aurophilic interaction. The four gold atoms are arranged in a square (Au...Au...Au... = 88–91°) in the acetamidinate
and in a distorted square (Au...Au...Au... = 82–97°) in the benzamidinate derivative. Electrochemical oxidation of the tetranuclear
complex Au4[PhNC(Ph)NPh]4 show three reversible waves at 0.87, 1.19, 1.42 V vs. Ag/AgCl at a scan rate of 100 mV/s in CH2Cl2 similar to the three reversible waves seen before from the tetranuclear complexes Au4[ArNC(H)NAr]4, Ar = C6H4-4-OMe, Ar = C6H4-4-Me, and Ar = C6H3-3,5-Cl. A summary of the chemistry of the tetranuclear Au(I) amidinate complexes Au4[ArNC(H)NAr]4, Ar = C6H4-4-OMe, C6H3-3,5-Cl, C6H4-4-Me, C6H4-3-CF3, C6F5, C10H7 also is presented. The tetranuclear clusters Au4[ArNC(H)NAr]4, Ar = C6H4-4-OMe, Ar = C6H4-3-CF3, Ar = C6H4-4-Me and Ar = C6H4-3,5-Cl are the first tetranuclear gold(I) cluster species from group 11 elements to show fluorescence at room temperature.
The lifetimes of the naphthyl and trifluoromethylphenyl complexes are in the millisecond range indicating phosphorescent processes.
Recently it has been shown that Au4[ArNC(H)NAr]4 are very effective catalysts upon calcination for room temperature CO oxidation.
Congratulations to Dieter Fenske, a superb synthetic chemist with exceptional talents in cluster chemistry, on the occasion
of his 65th birthday. 相似文献
84.
Ahmed Akelah 《European Polymer Journal》1982,18(7):559-561
Insoluble polystyryldiphenyl-methoxymethyl- and methylthiomethyl-phosphonium chlorides have been prepared by treating polystyryldiphenylphosphine with chloromethylmethyl-ether and thioether respectively. These reagents were used for the conversion under mild conditions of carbonyl compounds into vinyl-ethers and thioethers, otherwise difficult to obtain; yields are high and work-up is easy. Moreover, methyl and ethyl β-styryl ethers were also prepared from the polymeric phosphorane by treatment with formate esters. 相似文献
85.
3-[1′(1′H)-Substituent-pyrazol-5′-yl]benzo[5,6]coumarins and 3-(1′,2′-oxazol-5′-yl)benzo[5,6]coumarin were prepared via condensation of 3-(2′-formyl-1′-chlorovinyl)benzo[5,6] coumarin with hydrazine derivatives or hydroxylamine.Reaction of 3-[1′(1′H)-pyrazol-5′-yl]benzo[5,6]coumarin with alkyl halides,olefinic compunds or acid chlorides are described. 相似文献
86.
El-Hendawy Ahmed M. Alqaradawi Siham Y. Al-Madfa Homaid A. 《Transition Metal Chemistry》2000,25(5):572-578
The new complexes [RuIIIbpyL2](PF6) and [RuIIIbpyLCl2] [bpy = 2,2-bipyridine; HL = acetylacetone, trifluoroacetylacetone, benzoylacetone (Hbac), tropolone or maltol; HL = Hbac or dibenzoylmethane) have been prepared and characterized by spectroscopy. Their redox behaviour was studied by cyclic voltammetry. They effectively catalyze the oxidation of alcohols, alkanes and primary aromatic amines. 相似文献
87.
An europium-sensitized time-resolved luminescence (TRL) method was developed to determine oxytetracycline (OTC) in cultivated catfish muscle. Extraction of OTC from fish muscle was performed with pH 4.0 ethylenediaminetetraacetic acid (EDTA)-McIlvaine buffer and clean up with hydrophilic-lipophilic balanced copolymer solid phase extraction (SPE) cartridges. The eluate was used without further concentration for TRL measurement in pH 9.0 micellar tris(hydroxylmethyl)aminomethane (TRIS) buffer. Cetyltrimethylammonium chloride (CTACl) was used as surfactant and EDTA as a co-ligand. The excitation and emission wavelengths were set at 388 and 615 nm, respectively. The linear dynamic range was 0-1000 ng g−1 (R2=0.9995). The recovery was 92-112% in the fortification range of 50-200 ng g−1 and the limits of detection (LOD) ranged from 3 to 7 ng g−1. Incurred catfish samples were used to demonstrate the performance of the method around 100 ng g−1, the European Union maximum residue level. 相似文献
88.
Ahmed H. Osman Amna S. A. Zidan Asma I. El-Said Aref A. M. Aly 《Transition Metal Chemistry》1993,18(1):34-36
Summary Mixed ligand complexes of the type K[Cu(x-HQA)Rxan] [x-HQA = 5-arylazo-8-hydroxyquinoline derivatives; Rxan = methyl or ethylxanthate] have been prepared and characterised by elemental analysis, spectral measurements and conductivity. Upon irradiation, solutions of these complexes photoeliminate dinitrogen and a mechanism for this reaction is proposed. The photosensitivity of the oxine ligand was found to be responsible for the observed photolysis of the complexes. 相似文献
89.
Wack H France S Hafez AM Drury WJ Weatherwax A Lectka T 《The Journal of organic chemistry》2004,69(13):4531-4533
We detail the synthesis of a new C(2)-symmetric bis(cyclophane) ligand system that can be thought of as electronically analogous to binol, but which possesses the added "third dimension" of cyclophane chirality. The ligand synthesis involves a spontaneous (but unexpected) atropisomerization to the desired product. We have employed this ligand to form a metal complex that is an effective cocatalyst for the highly enantio- and diastereoselective catalytic asymmetric synthesis of a beta-lactam. 相似文献
90.
Mohamed R. Mahmoud Hussein S. Elkashef Maher M. Ahmed 《Monatshefte für Chemie / Chemical Monthly》1980,111(5):1213-1224
The electronic absorption spectra of some 2-styryl-4-phenyl-thiazole ethiodides are studied in organic solvents of different polarities. The shorter wavelength band appearing in the visible region is assigned to an intramolecular charge transfer (CT)-transition originating from the phenyl moiety to the positively charged hetero ring, while the longer wavelength one is due to an intermolecular CT-transition from the iodide ion to the 2-styryl-4-phenyl-thiazolinium cation. These assignments are based on the nature of the aldehydic residue and effects of solvent, concentration, and temperature on both the position and absorptivity of the CT complex-band. It is concluded that the CT complex formed will be highly solvated inDMF, DMSO, ethanol and methanol relative to in CHCl3, dioxane and acetone. The formation constant of the CT complex in solutions of different polarities is determined at different temperatures. Furthermore, the thermodynamic parameters H
o, G
o and S
o for complex formation are calculated and discussed.
Absorptionsspektren von 2-Styryl-4-phenyl-thiazol-ethiodiden in verschiedenen Lösungsmitteln und Bestimmung der Bildungskonstanten der Charge-Transfer-Komplexe
Zusammenfassung Die Elektronenanregungsspektren einiger substituierter 2-Styryl-4-phenyl-thiazol-ethiodide wurden in einigen Lösungsmitteln unterschiedlicher Polarität untersucht. Die Absorption bei kürzerer Wellenlänge wird einem intramolekularen Charge-Transfer (CT)-Übergang zugeordnet, die langwellige Bande einem intermolekularen CT-Übergang (Jodid—organ. Kation). Die Diskussion erfolgt basierend auf Substitutions-, Lösungsmittel-, Konzentrations-, und Temperatur-Effekten. Die Komplexbildungskonstanten und die thermodynamischen Parameter H o, G o und S o werden angegeben.相似文献