全文获取类型
收费全文 | 1144篇 |
免费 | 76篇 |
国内免费 | 23篇 |
专业分类
化学 | 833篇 |
晶体学 | 2篇 |
力学 | 73篇 |
数学 | 122篇 |
物理学 | 213篇 |
出版年
2024年 | 2篇 |
2023年 | 9篇 |
2022年 | 42篇 |
2021年 | 40篇 |
2020年 | 63篇 |
2019年 | 68篇 |
2018年 | 107篇 |
2017年 | 68篇 |
2016年 | 103篇 |
2015年 | 64篇 |
2014年 | 84篇 |
2013年 | 141篇 |
2012年 | 104篇 |
2011年 | 81篇 |
2010年 | 54篇 |
2009年 | 36篇 |
2008年 | 39篇 |
2007年 | 37篇 |
2006年 | 30篇 |
2005年 | 27篇 |
2004年 | 18篇 |
2003年 | 8篇 |
2002年 | 7篇 |
2001年 | 2篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1989年 | 1篇 |
1985年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有1243条查询结果,搜索用时 15 毫秒
91.
Tetrafluorobenzyne thermochemistry: experiment and theory 总被引:1,自引:0,他引:1
Pratt LM Fattahi A Kass SR 《European journal of mass spectrometry (Chichester, England)》2004,10(6):813-818
Gas-phase thermodynamic properties of 1,2,3,4-tetrafluorobenzyne (1 H-(2)) were determined by Fourier transform mass spectrometry and ab initio and density functional theory methods. 1,2,3,4-Tetrafluorobenzyne radical anion was generated by abstraction of a proton and a hydrogen atom upon reaction of 1,2,3,4-tetrafluorobenzene (1) with O(-.). The resulting structure was confirmed by converting it to a species which could be independently prepared. Bracketing results provided the proton affinity of 1,2,3,4-tetrafluorobenzyne radical anion and the electron affinities of 1,2,3,4-tetrafluorobenzyne and 1,2,3,4-tetrafluorophenyl radical. These measured values were combined in a thermodynamic cycle to provide the heat of hydrogenation of 1 H(2) (DeltaH degree (hyd) = 367 +/- 18 kJ mol(-1)) and the first and second CH bond dissociation energies of 1 (481 +/- 11 and 321 +/- 13 kJ mol(-1)). The same approach failed for the meta and para isomers, but their energetics were examined using B3LYP and CCSD(T) computations. 相似文献
92.
A new, convenient, and high yielding procedure for the synthesis of triazolo[1,2-a]indazole-triones by the condensation reaction between dimedone, aryl aldehydes, and ueazoles in the presence of a catalytic amount of sulfonated polyethylene glycol (PEG-SO3H) as a highly stable and reusable eco-friendly degradable polymeric catalyst is described under solvent-free conditions. This procedure has also been applied successfully for the synthesis of novel spiro triazolo[1,2-a]indazole-tetraones. 相似文献
93.
A simple method is introduced providing a highly clean microextraction for the determination of some anti‐inflammatory drugs as the model analytes in human urine and environmental matrices. This method is based upon the implementation of two consecutive emulsification liquid‐phase microextractions, which are separated by a syringe filtration step. In this method, the organic extraction solvent (dihexyl ether) is dispersed into the aqueous sample solution (20 mL), and the resulting cloudy mixture is passed through a hydrophilic polytetrafluoroethylene syringe filter. By this action, the extraction phase containing the analytes and many interfering species that could be transferred into the organic phase is retained behind the hydrophilic membrane. The filter is then detached from the syringe and attached to another syringe containing an aqueous solution (pH 12.0, 150 μL), and by the in‐syringe dispersion of the organic phase into the aqueous phase, the analytes are selectively back‐extracted into the aqueous phase. The developed method is centrifuge‐free and very simple, and provides a high sample clean‐up in a few minutes. Under the optimized experimental conditions, the developed method provided a linearity in the range of 2.0–2000 ng/mL, a low limit of detection (0.5 ng/mL), and enrichment factors of 47–53. 相似文献
94.
Jamal Aalaie Ghader Khanbabaie Alireza R. Khoshniyat Ali Rahmatpour 《Journal of Macromolecular Science: Physics》2013,52(2):305-316
Nanocomposites of two different grades of polyamide 6 (PA6) with organically modified nanoclay were prepared via melt compounding in a twin‐screw extruder. The rheological behavior, morphology and mechanical properties of the nanocomposites were studied using a capillary rheometer, x‐ray diffraction (XRD), tapping‐mode atomic force microscopy (AFM), and tensile and flexural tests. XRD patterns indicate that the organically modified layered silicate was well dispersed in the PA6 matrix. From the AFM images the surface roughness of PA6 slightly increases with addition of organoclay. The rheological studies showed that the prepared nanocomposites have shear thinning behavior, obeying the power law equation. Addition of organoclay increases the shear stress and shear viscosity. At high rate of shear deformation the viscosity of nanocomposites are comparable to those of the pure polyamides. The activation energy of flow decreases with increasing nanoclay content. For most of the prepared nanocomposites the activation energy values increase with increasing shear rate. The tensile strength and flexural modulus and strength of the nanocomposites increase with increase of nanoclay content, but the extension at yield decreases with increasing clay loading. 相似文献
95.
Maryam Sadat Hosseini-ZareMohammad Mahdavi Mina SaeediMehdi Asadi Shahrzad JavanshirAbbas Shafiee Alireza Foroumadi 《Tetrahedron letters》2012,53(27):3448-3451
We describe a new one-pot synthesis of 2,3-diaryl-5H-imidazo[2,1-a]isoindol-5-ones via the reaction of 1,2-diketones, 2-formylbenzoic acids, and ammonium acetate in acetic acid under reflux conditions and in the absence of a catalyst. 相似文献
96.
Mehdi Bakavoli Alireza Motavalizadeh Kakhky Ali Shiri Mahdieh Ghabdian Abolghasem Davoodnia Hossein Eshghi Mola Khatami 《中国化学快报》2010,21(6):651-655
<正>DBUH-Br_3 catalyzed selective conversion of sulfides to sulfoxides in the presence of H_2O_2 as oxidizing agent is described.The reaction was performed selectively at room temperature and relatively short reaction times. 相似文献
97.
98.
Shahla Masoudian Hassan Hosseini Monfared Alireza Aghaei 《Transition Metal Chemistry》2011,36(5):521-530
Various aerogels of silica gel doped with Fe2O3 were prepared by sol–gel method. They were calcined to produce nanoparticle solids. The nanosized mixed oxides were active
in the oxidation of alcohols and produced carbonyl compounds in very good to excellent yields using hydrogen peroxide. 相似文献
99.
100.
Alireza Banaei Afshin Saadat Mina Mohammad Goli Patrick McArdle Eslam Pourbasheer Parinaz Pargolghasemi 《Heteroatom Chemistry》2016,27(6):353-360
Two novel dithiophosphonate ligands, HS2P(p‐C6H4OMe)(OCH2CH2CH(CH3)2) ( 1 ) and HS2P(p‐C6H4OMe)(OCH(CH3)2) ( 2 ), were synthesized and characterized by multinuclear (1H, 31P, and 13C) NMR, infrared spectroscopy as well as elemental analysis. The reactions of 1 and 2 with NiCl2·6H2O and Cd(NO3)2·4H2O in methanol led to novel complexes 3 and 4 . The single crystal X‐ray structures of 3 and 4 showed tetracoordinated structure with square planar geometry for the nickel complex, while it showed pentacoordinated structure with distorted square‐pyramid environment for the cadmium complex. 相似文献