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161.
Nanogram quantities of heavy metals in 200 ml of water were quantitatively preconcentrated by coprecipitation with magnesium oxinate at pH 9–9.5. The precipitate was collected on a 1-m Nuclepore membrane filter and then dissolved in 5 ml of 1 mol/l nitric acid. After diluting to 10 ml with water, a 10- or 20-l aliquot of the solution was directly analyzed by electrothermal atomic absorption spectrometry using deuterium background correction. The background absorption due to the collector magnesium oxinate was negligibly small. The optimized coprecipitation technique has been applied to the determination of cobalt, nickel, copper, cadmium and lead at the ppt level in river and seawater. The relative standard deviations were within 10% and the detection limits were 0.025 (for Co), 0.019 (for Ni), 0.006 (for Cu), 0.001 (for Cd) and 0.017 g/l (for Pb). Blanks through the whole procedure were not detectable. 相似文献
162.
An efficient catalytic (2 + 2)-cycloaddition reaction leading to the formation of cyclobutane rings has been devised. The process transforms silyl enol ethers and alpha,beta-unsaturated esters into polysubstituted cyclobutanes with a high degree of trans-stereoselectivity. Both the rate and stereoselectivity of the process can be controlled by the choice of the ester group and silyl substituents. The results of stereochemical studies show that the cycloaddition step in this reaction proceeds in a nonstereospecific manner and, thus, by a pathway involving sequential nucleophilic additions via a short-lived zwitterionic intermediate. 相似文献
163.
164.
The photochemically initiated reactions and the atmospherically initiated reactions of alkyl aryl tellurides with electron-deficient heteroaromatic bases in the presence of N-acetoxy-2-thiopyridone and triethylborane, respectively, have been investigated. These reactions were applied to the preparation of carbocyclic four-membered C-nucleoside analogs. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8: 411–419, 1997 相似文献
165.
A water-in-oil type emulsion containing oxine has been used for the discrimination of copper(II) ions and copper-humic complexes
in aqueous solutions. A toluene solution containing oxine and nonionic surfactant (Span-80) was vigorously mixed with 1 mol/L
HCl by ultrasonic irradiation. The resulting emulsion was added to water and dispersed by stirring as numerous small globules.
Copper(II) ions were quantitatively permeated across the oil layer and incorporated in the tiny droplets of HCl, whereas copper-humic
complexes remained in the sample solution. After collecting the dispersed emulsion globules, they were destroyed by heating
to segregate the aqueous (HCl) and organic (toluene) phases. The copper in the aqueous phase was determined by graphite-furnace
atomic absorption spectrometry (GFAAS). The analytical results agreed with those obtained by the adsorption method, where
negatively charged humic complexes were selectively collected on a macroreticular anion exchanger Sephadex A-25 column. The
conventional liquid-liquid extraction did not offer a chemical speciation because copper(II) ions and humic complexes were
simultaneously extracted into the organic phase. The proposed emulsion method was successfully applied to the analysis of
river water samples.
Received: 14 April 1998 / Revised: 22 July 1998 / Accepted: 27 July 1998 相似文献
166.
Masataka Nagaoka Naoto Yoshida Tokio Yamabe 《International journal of quantum chemistry》1996,60(1):287-295
The generalized Langevin equations are presented by considering such microscopic motions of molecules described by the microscopic Hamiltonian whose potential function is quadratic and internal degree of freedom is multidimensional. Considering the long time behavior of the reactive mode, the Grote-Hynes equation has been derived from the generalized Langevin equations. Furthermore, we have proved that solving the Grote-Hynes equation is equivalent to solving the eigenvalue problem for the whole system, and then the Grote-Hynes treatment coincides with the transition-state theory for the whole system. © 1996 John Wiley & Sons, Inc. 相似文献
167.
Tomoko Ikeda‐Fukazawa Naohiro Ikeda Mayu Tabata Masataka Hattori Mamoru Aizawa Shunji Yunoki Yurina Sekine 《Journal of Polymer Science.Polymer Physics》2013,51(13):1017-1027
To investigate the effects of crosslinker density on the properties of hydrogels, compression tests, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and Raman measurements were performed on poly‐N,N‐dimethylacrylamide hydrogels. The results of the compression tests showed that the Young's modulus increases as the crosslinker density increases. To understand the mechanism of the change in the mechanical properties, the structures of the polymer networks and water and the molecular vibrations were analyzed using SEM, DSC, and Raman methods. From the SEM images, it was found that the porosity estimated from the mesh size and cell density increases with increasing crosslinker density. In addition, the DSC and Raman results show that the thickness of the bound water increases as the porosity increases, although the density of the polymer chains in the porous wall remains nearly constant. The increase in the number density of polymer chains can be one of the mechanisms contributing to the increase in the mechanical strength of the hydrogels at lower crosslinker density below 5 mol %, as proposed by previous studies. At higher crosslinker density, however, the number density of polymer chains does not increase with increasing crosslinker density. The present results suggest that the bound water plays an important role in strengthening the hydrogel. The water structure may be one of the dominant factors governing the chemical and physical properties of hydrogels. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1017–1027 相似文献
168.
Takeshi Nagai Takahiro Uno Masataka Kubo Takahito Itoh 《Journal of polymer science. Part A, Polymer chemistry》2012,50(3):466-479
Asymmetric anionic polymerizations of 7‐cyano‐7‐alkoxycarbonyl‐1,4‐benzoquinone methides ( 1 ) with various alkoxy groups were performed using chiral initiators such as lithium isopropylphenoxide (iPrPhOLi)/(S)‐(–)‐2,2′‐isopropylidene‐bis(4‐phenyl‐2‐oxazoline) ((–)‐PhBox) and lithium isopropylphenoxide (iPrPhOLi)/(–)‐sparteine ((–)‐Sp) to investigate the effect of the alkoxy groups of alkoxycarbonyl substituent in the monomers 1 and chiral ligands of chiral initiators on the control of chiral center in the formation of polymers. Molar optical rotation values of the polymers were significantly dependent upon alkoxy groups, and the polymers with higher molar optical rotation were obtained in monomers with primary alkoxy groups. The asymmetric anionic oligomerizations of the quinone methides having methoxy( 1a ), ethoxy( 1b ), and n‐propoxy( 1c ) groups with chiral initiators were carried out. Both 1‐mers and 2‐mers were isolated and their optical resolutions were performed to determine the extent of stereocontrol. High stereoselectivity was observed at the propagation reaction, but not at the initiation reaction. The effect of the counterion on the control of chiral center in the formation of the polymer was investigated in the asymmetric anionic polymerizations of 1b with iPrPhOM(M = Li, Na, K)/(–)‐Sp and iPrPhOM(M = Li, Na, K)/(–)‐PhBox initiators and discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011 相似文献
169.
Hideo Nemoto Hiroshi Kurobe Masataka Ihara Keiichiro Fukumoto Tetsuji Kametani 《Tetrahedron letters》1984,25(29):3095-3098
A convergent and stereoselective synthesis of vitamin D3 was achieved via 3,5-cyclovitamins D3 (20) which were prepared from the chiral aldehyde (2) and the vinyl bromide (12) derived from Grundmann's ketone. 相似文献
170.