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71.
72.
E. Blasius K.-P. Janzen W. Adrian W. Klein H. Klotz H. Luxenburger E. Mernke V.B. Nguyen T. Nguyen-Tien R. Rausch J. Stockemer A. Toussaint 《Talanta》1980,27(2):127-141
Exchangers with cyclic polyethers as anchor groups have a large range of applications such as separations of cations with a common anion, of anions with a common cation, and of neutral organic compounds, and the determination of water by elution chromatography. Some crown ether monomers, especially 4- and 4,4′-alkyl-substituted benzo-derivatives are suitable for extractions and their adducts with heteropoly acids are used as liquid ion-exchangers. The exchangers are also applied in thin-layer chromatography and thin-layer electrophoresis. Furthermore the exchangers are successfully used in preparative chemistry, e.g., in salt conversions in order to isolate salts which are difficult to prepare by other means, in isolation and purification of organic compounds, and for anion activation in organic reactions. 相似文献
73.
Walter Fischer Cyril A. Grob Reinhard Hanreich Georg von Sprecher Adrian Waldner 《Helvetica chimica acta》1981,64(7):2298-2311
The solvolysis rates and products of the 6-exo-substituted 2-exo- 1a - 1u , and 2-endo-norbornyl p-toluenesulfonates 2a - 2u , have been determined. In general, the rate constants for 1 and 2 (log k) correlate well with the inductive constants σ of the substitutents at C(6); however, their sensitivity to σ is much larger in the 2-exo-series 1 than in the 2-endo-series 2 . This differential transmission of polar effects is the cause of decreasing 2-exo/2-endo rate ratios from 2388 for R = t-C4H9 to 0.37 for R = Br, i. e. with increasing electron attraction by the substituent. The high sensitivity of the rate constants for the 2-exo-p-toluenesulfonates 1 to σ indicates an unusually strong inductive interaction between C(6) and the incipient cationic center at C(2). This interaction is ascribed to the participation of the pentacoordinate C(6)-atom, i. e. to 1,3-bridging, a consequence of steric hindrance of nucleophilic solvent participation in norbornanes. Donor substituents enhance 1,3-bridging, lead to faster reactions and to the formation of 2-exo substitution products. Conversely, acceptor substituents reduce 1,3-bridging, decrease rates and facilitate the formation of 2-endo substitution products. Graded 1,3-bridging is discussed in the light of Winstein's nonclassical ion concept. 相似文献
74.
Coleman KS Bailey SR Fogden S Green ML 《Journal of the American Chemical Society》2003,125(29):8722-8723
Single-walled carbon nanotubes have been cyclopropanated under Bingel reaction conditions, and the functionalized nanotubes have been characterized by atomic force microscopy using "chemical tagging" techniques. 相似文献
75.
76.
Alloyed semiconductor quantum dots: tuning the optical properties without changing the particle size 总被引:7,自引:0,他引:7
Alloyed semiconductor quantum dots (cadmium selenium telluride) with both homogeneous and gradient internal structures have been prepared to achieve continuous tuning of the optical properties without changing the particle size. Our results demonstrate that composition and internal structure are two important parameters that can be used to tune the optical and electronic properties of multicomponent, alloyed quantum dots. A surprising finding is a nonlinear relationship between the composition and the absorption/emission energies, leading to new properties not obtainable from the parent binary systems. With red-shifted light emission up to 850 nm and quantum yields up to 60%, this new class of alloyed quantum dots opens new possibilities in band gap engineering and in developing near-infrared fluorescent probes for in vivo molecular imaging and biomarker detection. 相似文献
77.
Gianfranco Denti Mauro Ghedini Jon A. McCleverty Harry Adams Neil A. Bailey 《Transition Metal Chemistry》1982,7(4):222-224
Summary Treatment of [Mo{HB(3,5-Me2C3HN2)3}(NO)I2] with one or two moles of AgPF6 in acetonitrile afforded the paramagnetic (one unpaired electron) complex [Mo{HB(3,5-Me2C3NH2)3}(NO)(NCMe)2][PF6]. The structure of this complex was determined crystallographically, and the six-coordinate geometry of the complex cation confirmed. 相似文献
78.
E Bailey A G Brooks R Purchase M Meakings M Davies D G Walters 《Journal of chromatography. A》1987,421(1):21-31
An improved method has been developed for the determination of the major neutral steroids (cholesterol and 5 beta-cholestan-3 beta-ol) and unconjugated bile acids (deoxycholic acid and lithocholic acid) in human faeces, using capillary gas chromatography with flame ionization detection. The freeze-dried faecal sample was subjected to a two-stage Soxhlet extraction followed by an aqueous alkali-organic solvent partition step to separate neutral steroids from bile acids. The neutral steroids were analysed as their trimethylsilyl ether derivatives on an OV-1 capillary column. The bile acids were further purified on a Sep-Pak C18 cartridge and then fractionated on a Sep-Pak SIL cartridge. Unconjugated bile acids were analysed as their methyl ester-trimethylsilyl ether derivatives also on an OV-1 capillary column. Quantitation of neutral steroids and unconjugated bile acids was achieved by reference to appropriate internal standards, added to the faecal extract immediately after the Soxhlet extraction stage. The method is being used in a study of the effect of diet on the metabolic activity of human gut flora. 相似文献
79.
We propose and test a pair potential that is accurate at all relevant distances and simple enough for use in large-scale computer simulations. A combination of the Rydberg potential from spectroscopy and the London inverse-sixth-power energy, the proposed form fits spectroscopically determined potentials better than the Morse, Varnshi, and Hulburt-Hirschfelder potentials and much better than the Lennard-Jones and harmonic potentials. At long distances, it goes smoothly to the London force appropriate for gases and preserves van der Waals's "continuity of the gas and liquid states," which is routinely violated by coefficients assigned to the Lennard-Jones 6-12 form. 相似文献
80.
When the Hammett-Taft equation log (k/ko)=ρq · σ is applied to the solvolysis of the 3-substituted propyl bromides 6a-6i in ethanol/water 4:1 (v/v) log k correlates linearly with σ except in cases where R exerts an anchimeric effect. The reaction constant ρq for 6 is ? 0.12 and is typical for a nucleophilic solvent-assisted ks process at a primary C-atom. The tertiary halides 1 and 3 , however, which react with little or no nucleophilic solvent assistance, i.e. by kc processes, lead to larger ρq values of ?0.71 and ?1.14, respectively. The reaction constant pq is therefore a sensitive gauge for charge development in the transition state for solvolysis of saturated compounds. 相似文献