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341.
Experimental and computational studies of nJ(77Se, 1H) selenium-proton couplings in selenoglycosides
Kövér KE Kumar AA Rusakov YY Krivdin LB Illyés TZ Szilágyi L 《Magnetic resonance in chemistry : MRC》2011,49(4):190-194
Selenoglycosides are important starting materials in synthetic carbohydrate chemistry and play a role in biological interactions as well. Both aspects are influenced by the conformation around the glycosidic bond. Here, we present a combined experimental and computational approach to measure and evaluate (n)J((77)Se, (1)H) coupling constants for their use in conformational analysis. The measurements were carried out using a modified CPMG-HSQMBC pulse scheme which yields pure absorption antiphase multiplets to allow accurate determination of the (n)J(XH) values regardless of the size of the proton-proton couplings. Theoretical calculations were performed at the Second-Order Polarization Propagator Approach (SOPPA) level. Population-averaged values calculated for geminal and vicinal couplings are in a good agreement with experiment indicating an adequate theoretical level of the calculations. Experimental observations and computations alike have indicated that two-bond (77)Se-(1)H couplings, (2)J((77)Se, (1)H), in a H1-C1-Se-X moiety are very sensitive to the torsion angle around the C1-Se-bond and will, therefore, be useful for conformational studies. 相似文献
342.
Lengyel L Gyóllai V Nagy T Dormán G Terleczky P Háda V Nógrádi K Sebok F Urge L Darvas F 《Molecular diversity》2011,15(3):631-638
Aromatic or heteroaromatic ring precursors with 2–3 identical functionalities are often used in sequential derivatization
depending on the reactivity difference or the selective execution of the reaction such as nucleophilic aromatic substitution.
Continuous flow chemistry offers an enhanced parameter space (pressure and temperature) with rapid parameter optimization
that ensures selectivity in many cases. We developed a flow chemistry procedure to carry out a stepwise aromatic nucleophilic
substitution of difluoro-benzenes having an activating group in meta position to the fluorines. The mono-aminated products were obtained in high yield and selectivity in an extremely short reaction
time, while applying higher temperature, longer reaction zone (or time), and employing higher excess of another amine reactant,
the subsequent introduction of the second amino group was also successfully achieved leading to an unsymmetrically substituted
3,5-diamino-benzonitrile library. 相似文献
343.
Shiro Kubuki Hiroshi Sakka Kanako Tsuge Zoltán Homonnay Katalin Sinkó Ernő Kuzmann Hiroki Yasumitsu Tetsuaki Nishida 《Hyperfine Interactions》2008,185(1-3):115-121
Local structure and thermal durability of semiconducting xBaO·(90?? x)V2O5 · 10Fe2O3 glasses (x = 20, 30 and 40), NTA glass TM, before and after isothermal annealing were investigated by 57Fe-Mössbauer spectroscopy and differential thermal analysis (DTA). An identical isomer shift ( $\mathit{\delta}$ ) of 0.39 ± 0.01 mm s???1 and a systematic increase in the quadrupole splitting (Δ) were observed from 0.70 ± 0.02 to 0.80 ± 0.02 mm s???1 with an increasing BaO content, showing an increase in the local distortion of FeIIIO4 tetrahedra. From the slope of the straight line in the T g–Δ plot of NTA glass TM, it proved that FeIII plays a role of network former. Large Debye temperature (Θ D) values of 1000 and 486 K were respectively obtained for 20BaO · 70V2O5 · 10Fe2O3 glass before and after isothermal annealing at 400°C for 60 min, respectively. This result also suggests that FeIII atoms constitute the glass network composed of tetrahedral FeO4, tetrahedral VO4 and pyramidal VO5 units. The electric conductivity of 20BaO · 70V2O5 · 10Fe2O3 glass increased from 1.6 × 10???5 to 5.8 × 10???2 S cm???1 after isothermal annealing at 450°C for 2,000 min. These results suggest that the drastic increase in the electric conductivity caused by heat treatment is closely related to the structural relaxation of the glass network structure. 相似文献
344.
Jin L Kövér KE Lenoir MR Uhrín D 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2008,190(2):171-182
The sensitivity of cryoprobes, which are rapidly becoming available, have brought about the possibility of measurement of (13)C, (13)C coupling constants at the natural abundance of (13)C using tens rather than hundreds of milligrams of compounds. This relatively recent development lays the foundation for a more routine use of the (13)C, (13)C long-range coupling constants in the conformational analysis of molecules. We have designed novel (1)H-detected INADEQUATE experiments optimized for long-range (13)C, (13)C correlations and the measurement of long-range coupling constants. These experiments incorporate refocusing of (1)J(CH) coupling constants prior to the formation of DQ coherences and (1)H-decoupling during the long carbon-carbon evolution intervals. Such modifications significantly enhance their performance over (1)H-detected INADEQUATE experiments currently in use for mapping the one-bond (13)C, (13)C correlations. (1)H or (13)C polarization is used a starting point in long-range correlation (1)H-detected IPAP DEPT-INADEQUATE or RINEPT-INADEQUATE experiments. These correlation experiments were modified yielding in-phase (IP) or antiphase (AP) (13)C, (13)C doublets in F(1). Procedures were developed for their editing yielding accurate values of small (13)C, (13)C coupling constants. The methods are illustrated using mono- and disaccharide samples and compared with related (13)C-detected experiments by means of the measurement of interglycosidic (13)C, (13)C coupling constants of a disaccharide. 相似文献
345.
Stone cantilever staircases are present in case of both new constructions and reconstructions. The aim of the present paper is to understand the mechanical behaviour of these staircases with the help of discrete element simulations, and to compare the calculated behaviour to the estimations given by the existing manual calculation methods. First a literature review is presented on the statical calculation of cantilevered staircases: manual calculation methods suggested in the 1990s for straight and spiral staircases are introduced, focusing on Heyman’s theory and its improved counterparts. Then the discrete element method is used as a tool to perform virtual experiments, in order to evaluate the mechanical behaviour of the straight and spiral staircases for selfweight, live loads and support movement. The results obtained (internal forces, stresses, deflections) are then compared with the manual calculation results. The most important conclusions are: (1) the term “cantilever stair” is misleading: significant torsion moments occur in the treads, while the bending moments are much smaller than in a free cantilever; (2) the type of the connection between wall and treads (i.e. the end of the tread is simply supported by the wall against translation and torsion, or it is also partly clamped) has a fundamental influence on the internal forces and stress distributions; (3) for simply supported treads the existing manual methods are conservative for straight stairs, but for spiral stairs they dangerously underestimate the torsional moments. 相似文献
346.
Zsuzsa M. Jszay Gyrgy M. Keser Gyrgy Clementis Imre Petnehzy Katalin Kovts Lszl Tke 《Heteroatom Chemistry》2001,12(2):90-96
Starting from a cyclopropane lactone 5 , the synthesis of a 1‐aminocyclopropane‐1‐phosphonic acid derivative 11 is described. The considerable differences in the reactivity of the lactone ring opening in the case of a cyclopropane lactone substituted by a phosphonic acid ester 5 and their carboxylic acid ester analogue 2 toward ammonia or amines have been compared and interpreted by using the map of electrostatic potentials. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:90–96, 2001 相似文献
347.
Krisztina László Katalin Kosik Genovéva Filipcsei Miklós Zrínyi 《Macromolecular Symposia》2003,200(1):181-190
Measurements are reported of the swelling behaviour at 20°C of poly (N isopropylacrylamide) (PNIPA) gels in aqueous solutions of two weak aromatic acids, phenol and resorcinol. For solute concentrations below 45 mmol/l the uptake of these solutions is similar. Due to selective solvation phenol exhibits an excess equilibrium concentration inside the gel of 5% over that in the surrounding bath, while for resorcinol, the excess is found to be 12%. At 50 mmol/l solute concentration, both systems display a volume transition accompanied by expulsion of the solvent. The solubility limits in water of these aromatic compounds, which are significantly different from each other (870 mmol/l and 9080 mmol/l respectively), are far above this critical concentration. In the collapsed condition the expelled liquid spreads on the surface of the phenol treated gel, while an ordered arrangement of separate droplets is generated in the case of resorcinol. In the latter case an acute contact angle was observed. It is also shown by acid-base titration that the PNIPA/water system may exhibit a slight ion exchanging character. 相似文献
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