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排序方式: 共有613条查询结果,搜索用时 15 毫秒
121.
Godinez CE Zepeda G Mortko CJ Dang H Garcia-Garibay MA 《The Journal of organic chemistry》2004,69(5):1652-1662
We report a highly convergent synthesis for the preparation of molecular gyroscopes consisting of para-phenylene rotors linked by triple bonds to methyl-substituted triptycenes acting as pivots and encapsulating frames. The desired 1,4-bis[2-(2,3,6,7,12,13-hexamethyl-10-alkyl-9-triptycyl)ethynyl]benzenes were prepared from 2,3-dimethyl-1,3-butadiene using Diels-Alder cycloadditions and Pd(0)-catalyzed coupling as the key reactions. The main challenge in the synthesis came about in the preparation of 9-alkynyl-triptycenes by Diels-Alder reaction of benzynes and 9-alkynyl-2,3,6,7-tetramethylanthracenes. These reactions occurred with chemical yields and regioselectivities that were strongly influenced by steric and electronic effects of substituents at C10 of the anthracene core. Anthracenes with methyl, propyl, and phenyl substituents were utilized to complete the synthesis of their corresponding molecular gyroscopes, and their solid-state structures were determined by single-crystal X-ray diffraction analysis. Examination of these results indicated that, as expected, the bulky triptycyl groups encourage crystallization motifs that create more free volume around the phenylene rotor, as needed to facilitate fast gyroscopic motion in the solid state. 相似文献
122.
W. Bernd Schweizer Jack D. Dunitz Rolland A. Pfund Gerardo M. Ramos Tombo Camille Ganter 《Helvetica chimica acta》1981,64(8):2738-2740
Crystallographic and spectroscopic evidence for an intramolecular (OH … π)-interaction in anti9,10-10endo-hydroxy, 10exo-butyltricyclo [4.2.1.12.5]deca-3, 7-dien-9-one ( 1 ) is presented. 相似文献
123.
124.
Luis Gerardo Pedraza Saavedra 《Journal of Russian Laser Research》2011,32(5):476-485
We complete and justify the Bohr and Rosenfeld’s extension of their seminal work on the measurability of the free electromagnetic-field
components to their pioneering paper on the measurement of charge–current uncertainty relations and the quantum control of
vacuum fluctuations. We also show that it is possible to complete the aforementioned article with pseudo-realistic graphics
of the thought experiments they designed. 相似文献
125.
The thermal conductivity (κ(T)) in a lattice is studied as a function of rigidity by using the Kubo-Greenwood formula. The rigidity is modulated by changing the second neighbour interactions. The results show that κ(T) is strongly determined by the rigid character of the network through the low frequency vibrational modes. The transition from an isostatic to overconstrained lattices is thus reflected in the behavior of κ(T). 相似文献
126.
L. Gerardo Zepeda Eleuterio Burgueño‐Tapia Nury Pérez‐Hernández Gabriel Cuevas Pedro Joseph‐Nathan 《Magnetic resonance in chemistry : MRC》2013,51(4):245-250
Complete assignment of the 1H NMR chemical shift and coupling constant values of perezone (1), O‐methylperezone (2) and 6‐hydroxyperezone (3) was carried out by total‐line‐shape‐fitting calculations using the PERCH iterative spectra analysis software (PERCH Solutions Ltd., Kuopio, Finland). The resulting simulated spectra for the three compounds showed strong similarity to their corresponding experimental spectra. Particularly, all vicinal, allylic and homoallylic coupling constant values for the side chain of the three compounds were very similar, thus revealing that the conformation of these three molecules in solution is indeed almost identical. This fact is in agreement with extended side chain conformations over folded chain conformations because 1, 2 and 3 undergo completely different intramolecular cycloaddition reactions. In addition, results of double pulsed field gradient spin echo NOESY 1D experiments performed on perezone (1) were unable to provide evidence for folded conformers. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
127.
Ali Liazid Gerardo F. Barbero Miguel Palma Jamal Brigui Carmelo G. Barroso 《Chromatographia》2010,72(5-6):417-424
The development of a rapid, reliable and reproducible LC method for the determination and quantification of 13 polyphenols (gallic acid, protocatechuic aldehyde, gentisic acid, catechin, vanillinic acid, caffeic acid, vanillin, epicatechin, syringaldehyde, p-coumaric acid, ferulic acid, sinapic acid and resveratrol) in grapes and derived products is reported. The polyphenols were separated in less than 8 min. Employed was an RP-18e (100 mm × 4.6 mm) monolithic type column. A gradient method with the following solvents was utilised for the chromatographic separation: A: 90% water, 2% acetic acid in methanol, and B: 90% methanol, 2% acetic acid in water. Two detectors in series were employed: a UV–Vis detector and a fluorescence excitation/emission detector. Influence of temperature (15–40 °C) and solvent flow rate (2–5 mL min?1) on the separation were studied, and 25 °C and 2.5 mL min?1 were found to be the optimum conditions. The relative standard deviations of the resulting peak areas, for both intra- and inter- experiments, were less than 2.4 and 2.6%, respectively. Finally, the developed method has been utilised for the quantification of the polyphenols in real samples. 相似文献
128.
129.
Gilberto Gonzalez Avalos Gerardo Ayala Noe Barrera Gallegos Aaron Padilla Jose 《Mathematical and Computer Modelling of Dynamical Systems: Methods, Tools and Applications in Engineering and Related Sciences》2019,25(3):284-332
Based on non-linear systems described by multibond graphs, a procedure designed to present symbolic linearization of these multibond graphs, is presented in this paper. Firstly, a junction structure of a multibond graph with multiport gyrators that represent Eulerian junction structures is proposed. In addition, non-linear multiport resistors are considered. By knowing the non-linear causal paths and loops of the non-linear multibond graph, the linearization is obtained by two steps: (1) The original multibond graph on the nominal operating point is evaluated; (2) New and additional paths based on the non-linear causal paths and loops are included. The state space representation of the linearized multibond graph using the corresponding junction structure is presented. An advantage of this methodology is its ability to allow the user to define a nominal operating point in which the linearization will be carried out.
In order to apply the proposed methodology, two physical systems are modelled and linearized by multibond graphs: a synchronous generator and a two degrees of freedom PUMA. Simulation results of these non-linear and linearized systems are shown. 相似文献
130.
Nadia G. Boetti Joris Lousteau Alessandro Chiasera Maurizio Ferrari Emanuele Mura Gerardo C. Scarpignato Silvio Abrate Daniel Milanese 《Journal of luminescence》2012,132(5):1265-1269
Er3+ doped niobic-tungsten–tellurite glasses doped with concentration of Er3+ ion up to 3 wt% were fabricated. The effect of Er3+ doping concentration on thermal stability and optical properties was investigated in order to obtain the most suitable rare earth content for developing 1.5 μm compact fiber amplifier pumped with a commercial telecom 980 nm laser diode. The maximum doping concentration allowed was found to be around 1.77×1020 ions/cm3, for which a broad 1.5 μm emission spectra of 65 nm FWHM and a lifetime of 3.4 ms for the 4I13/2 level was measured. 相似文献