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141.
Preparation of Enantiomerically Pure 4,4,4-Trifluoro-3-hydroxy-butanoic Acid Derivatives, Branched in the 2- or 3-Position, from 6-Trifluoromethyl-1,3-dioxan- and -dioxin-4-ones Enantiomerically pure 3-hydroxy-3-trifluoromethyl-propionic acid and esters, substituted in the 2- or 3-position, are prepared (13 examples) from (R)- or (S)-4,4,4-trifluoro-3-hydroxy-butanoic acid. Key intermediates are the 2-t-butyl-6-trifluoromethyl-1,3-dioxan- and -dioxin-4-ones. The Li enolate of the cis-dioxanone is generated with t-BuLi and reacts with electrophiles (alkyl halides, aldehydes, imines, nitroolefins, Br2, I2) with predominant formation of trans,trans-2,5,6-trisubstituted dioxanones (9 examples). Elimination of HBr from the 5-Br-substituted dioxanone gives the (R)- or (S)-dioxinone, a chiral derivative of 4,4,4-trifluoro-3-oxo-butanoic acid (trifluoro-acetoacetate). Michael additions of cuprates or of CuCl-doped Grignard reagents to the dioxinone produce 6,6-disubstituted dioxanones (10 examples) bearing a CF3 group in the 6-position. In most cases this addition is highly diastereoselective, with the new substituent winding up in the trans position. There are, however, surprising exceptions, such as the product formed with benzylmagnesium chloride which is an abnormal adduct with a p-quinoid structure ( 26 ) and with the newly introduced group in the cis position with respect to the t-Bu group. The structures of four trisubstituted dioxanones bearing CF3 groups are determined by X-ray crystal structure analysis (Figure 1, Table 1), one of them including the absolute configuration (by anomalous diffraction). Besides the well-known sofa, a twist-boat conformation of dioxanones appears to be favorable. The solution conformations of the different types of CF3-substituted dioxanones are derived from Nuclear Overhauser NMR measurements and compared with the crystal structures (Figure 3). 相似文献
142.
Sparidans RW Schellens JH López-Lázaro L Jimeno JM Beijnen JH 《Biomedical chromatography : BMC》2004,18(1):16-20
A sensitive bio-analytical assay for the depsipeptide aplidine in plasma has been modified and tested for human whole blood samples. The adapted method is based on reversed-phase liquid chromatography and fluorescence detection of the trans-4'-hydrazino-2-stilbazole derivative of the analyte. Aplidine is isolated from the matrix by solid-phase extraction on an octadecyl modified silica stationary phase. After evaporation of the acetone eluate, the derivatization with the hydrazino reagent is performed in a water-acetonitrile mixture at pH = 4. The reaction mixture is injected directly into the chromatograph and the analyte is quantified by fluorescence detection at 410 and 560 nm for excitation and emission, respectively. The method has been validated in the 2-100 ng/mL range, with 2 ng/mL being the lower limit of quantification. Precision and accuracy both meet the current requirements for a bioanalytical assay. The stability of aplidine in whole blood at ambient temperature and at 37 degrees C is limited; recoveries in the range 60-85% were observed after 7 h. Further, adequate stability of aplidine in plasma at -80 and -20 degrees C for 35 months could now be demonstrated. 相似文献
143.
N2(OH)2光致异构化机理 总被引:1,自引:0,他引:1
偶氮化合物 ,如偶氮芳烃、偶氮冠醚、偶氮环糊精等作为有机光化学开关分子具有重要的应用价值 ,已引起了人们的广泛重视[1] 。这类分子作为光化学开关的基本原理是反式的偶氮化合物在光照射下异构化为顺式构型 ,如下图所示 :长期以来 ,人们对该反应的机理持有两种不同看法[2 ,3] :一是分子平面内按角旋转机制 ,二是绕N =N键旋转的二面角机制。前者受氮上孤对电子的“阻碍” ,而后者将破坏共轭双键 ,显然两种机制在基态都很难发生 ,而如果光激发后发生n→π 或π→π 跃迁 ,则两种机制均有可能发生。本文用量子化学从头算方法研究基态和激… 相似文献
144.
《Electroanalysis》2006,18(22):2263-2268
We present the first electrochemical study in ionic liquid media of cyanocobalamin (vitamin B12), a biologically important molecule, and show its suitability as catalyst in the electroreduction of dibromoalkanes. 相似文献
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A new mechanism of anionic polymerization of butadiene is proposed. In the elementary chemical act, the “living” polymer–monomer complex is excited into the low‐lying triplet state. This state has the character of charge (electron) and cation (Li+ or Na+) transfer from the terminal unit of the active center to the monomer molecule. In the framework of this concept, the probability of chemical bond formation is determined by spin density on radical centers of reagent molecules. Semiempirical and ab initio 6‐31G** quantum‐chemical calculations showed stable interaction between components of the complex in the ground electronic state (9–11 kcal/mol) and low energy levels of triplet excited states (<14 kcal/mol). This new approach is shown to be useful in the analysis of polymerization kinetics and the microstructure of polybutadiene depending on the cation type and the ion pair state. The mechanism of cis‐trans isomerization in the terminal unit of the living polymer consists in concerted rotation about the Cβ? Cγ bond and the migration of Li between Cα and Cγ atoms. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 相似文献
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Tuba Erdogan Eda Gungor Hakan Durmaz Gurkan Hizal Umit Tunca 《Journal of polymer science. Part A, Polymer chemistry》2006,44(4):1396-1403
We prepared a novel miktoarm star copolymer with an azobenzene unit at the core via combination of atom transfer radical polymerization (ATRP) and nitroxide‐mediated free radical polymerization (NMP) routes. For this purpose, first, mikto‐functional initiator, 3 , with tertiary bromide (for ATRP) and 2,2,6,6‐tetramethylpiperidin‐1‐yloxy (TEMPO) (for NMP) functionalities and an azobenzene moiety at the core was synthesized. The initiator 3 thus obtained was used in the subsequent living radical polymerization routes such as ATRP of MMA and NMP of St, respectively, to give A2B2 type miktoarm star copolymer, (PMMA)2‐(PSt)2 with an azobenzene unit at the core with controlled molecular weight and low polydispersity (Mw/Mn < 1.15). The photoresponsive properties of 3 and (PMMA)2‐(PSt)2 miktoarm star copolymer were investigated. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1396–1403, 2006 相似文献