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61.
An efficient non-aldol convergent synthesis of the C14-C25 polyketide fragment of bafilomycin A(1) was completed in 16% overall yield and 8 steps in its longest linear sequence. This synthesis highlights the formation of the key fragments using a three-step sequence of epoxide cleavage, alkyne reduction, and epoxidation developed in our laboratory; starting from suitably protected enantiomeric epoxides of trans-2,3-epoxybutanol. This chemistry represents a quick asymmetric and diastereoselective construction of the polyketide chain of bafilomycin A(1), in which every stereogenic center was constructed using solely epoxide chemistry. 相似文献
62.
Victoria Muñoz–Iglesias Laura Jiménez Bonales David Santamaría-Pérez Mercedes Taravillo Valentín García Baonza Olga Prieto-Ballesteros 《光谱学快报》2013,46(6):407-412
Clathrate hydrates are particular solids that planetologists study in detail because those solids may be present in several bodies of the solar system, such as Mars, comets, and the icy satellites. The solids are formed by solid H2O, like common water ice, but adopt open structures with cavities containing gas molecules. Clathrate hydrates are usually stable at relatively low temperature and high pressure, which are the typical conditions present inside these planetary objects. Their interest for astrobiology is that they represent potential sources of liquid water and gases when they decompose. The present work is focused on the crystallization of clathrates in Europa's (icy satellite of Jupiter) interior conditions. We postulate that clathrate hydrates may play an important role in its crust mineralogy and that it can explain some features of the satellite's surface due to their formation/destabilization. An in situ kinetic study by Raman Spectroscopy of the clathrate formation from salty solutions was performed in our laboratory. The chemical composition that we used mimics those obtained from Europa's surface during the Galileo mission. An effect of the salting-out process in the solution was monitored through the clathrate formational path. Our results demonstrate that this process may have geological consequences on Europa and confirm the suitability of Raman spectroscopy for planetary detection of clathrate hydrates and other ices. 相似文献
63.
Galliano Valent 《Communications in Mathematical Physics》2010,299(3):631-649
A few years ago Selivanova gave an existence proof for some integrable models, in fact geodesic flows on two dimensional manifolds,
with a cubic first integral. However the explicit form of these models hinged on the solution of a nonlinear third order ordinary
differential equation which could not be obtained. We show that an appropriate choice of coordinates allows for integration
and gives the explicit local form for the full family of integrable systems. The relevant metrics are described by a finite
number of parameters and lead to a large class of models mainly on the manifolds
\mathbb S2{{\mathbb S}^2} and
\mathbb H2{{\mathbb H}^2} . Many of these systems are globally defined and contain as special cases integrable systems due to Goryachev, Chaplygin,
Dullin, Matveev and Tsiganov. 相似文献
64.
Synthesis and Characterization of Vanadium-Containing ZrSiO4 Solid Solutions from Gels 总被引:1,自引:0,他引:1
Valentín Carla Muñoz Mari Carmen Alarcón Javier 《Journal of Sol-Gel Science and Technology》1999,15(3):221-230
A procedure is reported for the preparation of vanadium-doped zircon pigmenting system with different vanadia loadings which enabled their complete formation and further characterization. Vanadium-zircon solid solutions were prepared by gelling mixtures of ZrO2 and V2O5 colloidal sols and tetraethylorthosilicate and studied over the temperature range up to the formation of zircon. The reaction sequence of gels was evaluated by X-ray powder diffraction (XRD) and ultraviolet-visible (UV-Vis) diffuse reflectance. It was found that the first crystalline phase detected was a vanadium-containing tetragonal ZrO2 solid solution where vanadium was stabilized in the reduced V+4 state. The formation of the V-ZrSiO4 solid solution occurred by the reaction between the monoclinic form of V+4-ZrO2 solid solution and the amorphous silica phase. Energy dispersive X-ray microanalysis (SEM/EDX) data, measurements of lattice parameters and UV-Vis diffuse reflectance of V-ZrSiO4 solid solutions revealed that vanadium was dissolved as V+4 replacing Si+4 in tetrahedral sites in the crystal structure of zircon. The solubility limit of vanadium in ZrSiO4 was about 0.01 mole of vanadium per mole of zircon (0.5 wt% as V2O5). 相似文献
65.
66.
Ohne Zusammenfassung 相似文献
67.
Grotjahn DB Kragulj EJ Zeinalipour-Yazdi CD Miranda-Soto V Lev DA Cooksy AL 《Journal of the American Chemical Society》2008,130(33):10860-10861
The secondary structure of a bifunctional catalyst positions a crucial reactive proton in the final intermediate of anti-Markovnikov alkyne hydration to give an aldehyde. NMR coupling and isotopic labeling studies elucidate the location of this proton and its involvement in hydrogen bonding. 相似文献
68.
Miranda-Soto V Grotjahn DB DiPasquale AG Rheingold AL 《Journal of the American Chemical Society》2008,130(40):13200-13201
Loss of chloride ion from imidazol-2-yl complex 4a activates the H-H bond of dihydrogen or the C-H bond of acetylene, forming an Ir(III) N-heterocyclic carbene (NHC) complex (3b or 9). Deprotonation of Ir(III) hydride 4b gives one new species, formulated as Ir(I) carbene complex 5. Protonation or alkylation of 5 occurs at the metal, returning the Ir(III) core of 6a,b. Deprotonation of cationic NHC complex 3a gives neutral imidazol-2-yl analogue 4a; as seen by X-ray diffraction, the Ir-C bond in 3a is shorter than that in 4a. These and other comparisons and interconversions of NHC complexes with an NH function and related imidazol-2-yl species expand the potential of NHC complexes by showing their bifunctional character. 相似文献
69.
Paula Guedes de Pinho Patrícia Valentão Rui F. Gonçalves Carla Sousa Paula B. Andrade 《Rapid communications in mass spectrometry : RCM》2009,23(15):2292-2300
Volatile and semi‐volatile components of internal and external leaves of Brassica oleracea L. var. costata DC, grown under different fertilization regimens, were determined by headspace solid‐phase microextraction (HS‐SPME) combined with gas chromatography/ion trap mass spectrometry (GC/ITMS). Forty‐one volatiles and non‐volatile components were formally identified and thirty others were tentatively identified. Qualitative and quantitative differences were noticed between internal and external leaves. In general, internal leaves exhibited more aldehydes and sulfur volatile compounds than external ones, and less ketone, terpenes and norisoprenoid compounds. The fertilization regimens influenced considerably the volatile profile. Fertilizations with higher levels of sulfur produced Brassica leaves with more sulfur volatiles. In opposition, N and S fertilization led to leaves with lower levels of norisoprenoids and terpenes. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
70.