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251.
Cation-pi interactions are increasingly recognized as important in chemistry and biology. Here we investigate the cation-pi interaction by determining its effect on the helicity of model peptides using a combination of CD and NMR spectroscopy. The data show that a single Trp/Arg interaction on the surface of a peptide can make a significant net favorable free energy contribution to helix stability if the two residues are positioned with appropriate spacing and orientation. The solvent-exposed Trp-->Arg (i, i + 4) interaction in helices can contribute -0.4 kcal/mol to the helix stability, while no free energy gain is detected if the two residues have the reversed orientation, Arg-->Trp (i, i + 4). The derived free energy is consistent with other experimental results studied in proteins or model peptides on cation-pi interactions. However in the same system the postulated Phe/Arg (i, i + 4) cation-pi interaction provides no net free energy to helix stability. Thus the Trp-->Arg interaction is stronger than Phe-->Arg. The cation-pi interactions are not sensitive to the screening effect by adding neutral salt as indicated by salt titration. Our results are in qualitative agreement with theoretical calculations emphasizing that cation-pi interactions can contribute significantly to protein stability with the order Trp > Phe. However, our and other experimental values are significantly smaller than estimates from theoretical calculations. 相似文献
252.
Müller R Knapp M Heckmann K von Ruthendorf M Boden G 《Langmuir : the ACS journal of surfaces and colloids》2004,20(7):2598-2606
To suppress the reactivity of nanoscaled non-oxidic powders of titanium nitride (TiN) and silicon carbonitride (SiCN) against hydrolysis and oxidation, chemical surface modification with nitrogen-containing surfactants was investigated. Among these surfactants, long-chain primary amines, ethylenediamines, guanidines, nitriles, isocyanates, and succinimides were examined. Thermogravimetry, elemental analysis, and behavior against the water-vapor adsorption of the modified particles were used as methods to estimate the protective capacity of the organic coating material. The best results were obtained by using the long-chain amines and octadecylisocyanate, which were indicated by a significant shift of the powder oxidation toward the higher temperatures and an increase of the particle hydrophobicity. A long-chain succinimide was found to be the most effective in dispersing nanoscaled TiN in organic media. Preparation of a stable aqueous dispersion without significant changes in the elemental composition of the powder was achieved by the application of an ionic surfactant to the surface-modified particles. 相似文献
253.
The tricyclic skeleton of lycoricidine has been prepared by a route involving an intramolecular [4 + 2]cycloaddition of a trimethylsilyloxydienamide. 相似文献
254.
Thomas K. Halstead Neville Boden Leslie D. Clark Carl G. Clarke 《Journal of solid state chemistry》1983,47(2):225-230
Deuterium NMR spectra have been obtained by the solid echo technique for polycrystalline samples of DUO2XO4, 4D2O, with X = P (DUP) and As (DUAs). Transitions (II → I) were found, by DSC measurements, to occur at 260 and 290 K for DUP and DUAs, respectively. In the high temperature tetragonal phase I, the NMR lineshapes are consistent with motionally averaged axially symmetric electric field gradient tensors, with values for of 39.2 ± 0.2 and 38.2 ± 0.2 kHz for DUP and DUAs, respectively. This requires fast chemical exchange of the deuterium atoms between all of the hydrogen-bond sites in a water layer, and participation of all of the deuterium atoms in the diffusion process. There are no discernible discontinuities in the spectra at the transition temperature suggesting that there is little change in the local structure of the water layers at the transition. The spectra disappeared at about 10 K below Tc and for DUP a distorted broader spectrum became discernible below about 200 K. 相似文献
255.
The proton dipolar echo responses to both in-phase and in-quadrature rf pulse sequences in nematic 4,4′-bis(hexyloxy)azoxybenzene are shown to be characteristic of a spin-1/2 pair system. This result is consistent with the chemical structure of this mesogen. The echo technique has therefore been used to measure the interpair second moment M2(inter) as a function of temperature over the entire nematic range. The total second moment M2 has been extracted from the FID signal. The intra- and interpair second moments are discussed in relation to the order parameter S and are shown to be averaged in distinctly different ways by the internal motions of the mesogen. 相似文献
256.
257.
G. A. Neville H. D. Beckstead J. D. Cooney 《Analytical and bioanalytical chemistry》1994,349(10-11):746-750
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies have been performed on a commercial preparation of spironolactone and on five solvates (acetonitrile, ethyl acetate, ethanol, methanol, and benzene) prepared from it by recrystallization at room temperature. The results of these studies are discussed and compared with those recently reported in the literature for similar preparations. The variable behaviour of these solvates in their release (and partial retention) of solvent before the onset of melting, meant that their stoichiometry had to be measured by elemental microanalyses. Solvent loss from solvate decomposition was followed by thermogravimetric analysis (TGA). Solvate decomposition and spironolactone melting temperatures and enthalpies were determined by differential scanning calorimetry (DSC). The parent substance and two of the solvates (methanol and ethanol) exhibited small exotherms in addition to the expected endotherms. 相似文献
258.
Alexopoulos T Antoniazzi L Arenton M Ballagh HC Bingham H Blankman A Block M Boden A Bonomi G Borodin SV Budagov J Cao ZL Cataldi G Chen TY Clark K Cline D Conetti S Cooper M Corti G Cox B Creti P Dukes EC Durandet C Elia V Erwin AR Evangelista E Fortney L Golovatyuk V Gorini E Grancagnolo F Hagan-Ingram K Haire M Hanlet P He M Introzzi G Jenkins M Jennings J Judd D Kononenko W Kowald W Lau K Lawry T Ledovskoy A Liguori G Lys J Mazur PO McManus A Misawa S Mo G Murphy CT Nelson K Panareo M 《Physical review letters》1996,77(12):2380-2383
259.
Hysteretic Four‐Step Spin Crossover within a Three‐Dimensional Porous Hofmann‐like Material
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Dr. John E. Clements Dr. Jason R. Price Dr. Suzanne M. Neville Prof. Cameron J. Kepert 《Angewandte Chemie (International ed. in English)》2016,55(48):15105-15109
Materials that display multiple stepped spin crossover (SCO) transitions with accompanying hysteresis present the opportunity for ternary, quaternary, and quinary electronic switching and data storage but are rare in existence. Herein, we present the first report of a four‐step hysteretic SCO framework. Single‐crystal structure analysis of a porous 3D Hofmann‐like material showed long‐range ordering of spin states: HS, HS0.67LS0.33, HS0.5LS0.5, HS0.33LS0.67, and LS. These detailed structural studies provide insight into how multistep SCO materials can be rationally designed through control of host–host and host–guest interactions. 相似文献
260.
Concistrè M Gansmüller A McLean N Johannessen OG Marín Montesinos I Bovee-Geurts PH Verdegem P Lugtenburg J Brown RC DeGrip WJ Levitt MH 《Journal of the American Chemical Society》2008,130(32):10490-10491
The 13C chemical shifts of the primary visual photointermediate bathorhodopsin have been observed by performing double-quantum magic-angle-spinning NMR at low temperature in the presence of illumination. Strong isomerization shifts have been observed upon the conversion of rhodopsin into bathorhodopsin. 相似文献