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661.
Augusto Rivera Dency José PachecoJaime Ríos-Motta Karla FejfarováMichal Dušek 《Tetrahedron letters》2012,53(45):6132-6135
The cyclic aminal 4,9-dimethyl-1,3,6,8-tetraazatricyclo[4.4.1.13,8]dodecane 4c was synthesized by the reaction of commercial rac-1,2-propanediamine with paraformaldehyde in an aqueous solution. 1H NMR analysis clearly revealed that the compound is chiral and racemic with an axis of chirality. To our knowledge, this is the first example of an azaadamantane derivative having axial chirality. This aminal was used in a Mannich type reaction with p-chlorophenol yielding 2,2′-[(4-methylimidazolidine-1,3-diyl)dimethanediyl]bis(4-chlorophenol) 7 as a racemic mixture. The crystal structure of 7 was determined by single X-ray diffraction analysis. 相似文献
662.
The lithium salts of the conjugated bases of 4-methoxy- and 4-acetylamino-2(1H)-pyrimidinones 1-3 undergo highly chemoselective N1-methylation or ethylation when treated with methyl- or ethylsulfate (hard electrophiles) in dry dioxane, while the use of DMF as solvent results in competitive O2-alkylation. Potassium salts of the same bases in DMF undergo prevalent O2-attack. Under the same conditions, a similar but less chemoselective behaviour is observed in alkylation of thymine and uracil, where some N3-attack occurs. This can be rationalised in terms of the HSAB principle. 相似文献
663.
de Oliveira CA Hamelberg D McCammon JA 《The journal of physical chemistry. B》2006,110(45):22695-22701
Our group recently proposed a robust bias potential function that can be used in an efficient all-atom accelerated molecular dynamics (MD) approach to simulate the transition of high energy barriers without any advance knowledge of the potential-energy landscape. The main idea is to modify the potential-energy surface by adding a bias, or boost, potential in regions close to the local minima, such that all transitions rates are increased. By applying the accelerated MD simulation method to liquid water, we observed that this new simulation technique accelerates the molecular motion without losing its microscopic structure and equilibrium properties. Our results showed that the application of a small boost energy on the potential-energy surface significantly reduces the statistical inefficiency of the simulation while keeping all the other calculated properties unchanged. On the other hand, although aggressive acceleration of the dynamics simulation increases the self-diffusion coefficient of water molecules greatly and dramatically reduces the correlation time of the simulation, configurations representative of the true structure of liquid water are poorly sampled. Our results also showed the strength and robustness of this simulation technique, which confirm this approach as a very useful and promising tool to extend the time scale of the all-atom simulations of biological system with explicit solvent models. However, we should keep in mind that there is a compromise between the strength of the boost applied in the simulation and the reproduction of the ensemble average properties. 相似文献
664.
Augusto Visintin 《Annali di Matematica Pura ed Applicata》2006,185(4):487-519
This work deals with an initial- and boundary-value problem for a quasilinear parabolic equation that includes a possibly
discontinuous hysteresis operator,
:
In particular the case of
equal to a so-called relay operator is studied. Well-posedness is proved, as well as regularity of the solution and its robustness w.r.t. perturbations of
. The large-time behaviour is studied; asymptotic stability and compactness are shown. For a time-periodic f, existence of a periodic solution is also established.
Mathematics Subject Classification (2000) 35K60, 35R35, 47J40 相似文献
665.
Augusto Rivera Mauricio Maldonado Jaime Ríos-Motta Miguel Angel Navarro Diego González-Salas 《Tetrahedron letters》2010,51(1):102-104
A one-pot good-yielding synthesis of 1-(alcoxymethyl)-1H-benzimidazoles and 1-((1H-benzimidazol-1-yl)methyl)-1H-benzotriazole from N1,N2-bis((1H-benzotriazol-1-yl)methyl)benzene-1,2-diamine (3) and alcohols is described. The synthesis of 3 from macrocyclic aminal 6H,13H-5:12,7:14-dimethanedibenzo-[d,i][1,3,6,8]tetraazecine (DMDBTA, 1) and benzotriazole is also described. Both these methods are simple, isolation of the products from the reaction mixtures is easy, and the yields are good. 相似文献
666.
José Alberto Fracassi da Silva Nathan Vitorazzi de Castro Dosil Pereira de Jesus Adriana F. Faria Marcus V. N. De Souza Marcone Augusto Leal de Oliveira 《Electrophoresis》2010,31(3):570-574
A method for the determination of ethambutol (EMB), a first‐line drug against tuberculosis, based on CE with capacitively coupled contactless conductivity detection is proposed. The separation of EMB and its main product of degradation were achieved in less than 3 min with a resolution of 2.0 using a BGE composed of 50 mmol/L histidine and 30 mmol/L MES, pH 6.30. By raising the pH to 8.03, the analysis time was reduced to 1.0 min, but with a significant loss of resolution (0.7). Using the best separation conditions, linearity of 0.9976 (R2, five data points), sensitivity of 1.26×10?4 V min μmol?1 L, and LOD and quantification of 23.5 and 78.3 μmol/L, respectively, were obtained. Recoveries at four levels of concentration ranged from 95 to 102% and the concentration range studied ranged from 100 to 500 μmol/L. The results obtained for the determination of EMB in pharmaceutical formulations were compared with those obtained by using CE with photometric detection. 相似文献
667.
Vívian Aparecida Ricardo Teixeira Ernesto Clóvis Augusto Ribeiro Ossamu Hojo Fernando Luis Fertonani Marisa Spirandeli Crespi 《Journal of Thermal Analysis and Calorimetry》2009,97(2):653-656
Bagasse samples from four different sugarcane were directly collected as the residues of milling in a processing plant. The
samples were dried at 105 °C, compressed to small granules and then their TG/DTA and DSC curves in synthetic air were recorded.
Similar thermogravimetric curves were obtained for the different samples and they exhibited four mass loss steps. However,
the analysis of the exotherm DSC peaks showed that the oxidation of the organic matter resulted different enthalpy values
(ΔH/kJ g−1). 相似文献
668.
Leandro M. S. Takata Augusto C. Gonalves Rmulo A. Ando Alcindo A. Dos Santos Pedro H. C. Camargo 《Journal of Raman spectroscopy : JRS》2012,43(6):712-717
This paper describes a surface‐enhanced Raman scattering (SERS) systematic investigation regarding the functionalization of gold (Au) and silver (Ag) nanoparticles with diphenyl dichalcogenides, i.e. diphenyl disulfide, diphenyl diselenide, and diphenyl ditelluride. Our results showed that, in all cases, functionalization took place with the cleavage of the chalcogen–chalcogen bond on the surface of the metal. According to our density functional theory calculations, the molecules assumed a tilted orientation with respect to the metal surface for both Au and Ag, in which the angle of the phenyl ring relative to the metallic surface decreased as the mass of the chalcogen atom increased. The detected differences in the ordinary Raman and SERS spectra were assigned to the distinct stretching frequencies of the carbon–chalcogen bond and its relative contribution to the ring vibrational modes. In addition, the SERS spectra showed that there was no significant interaction between the phenyl ring and the surface, in agreement with the tilted orientation observed from our density functional theory calculations. The results described herein indicate that diphenyl dichalcogenides can be successfully employed as starting materials for the functionalization of Au nanoparticles with organosulfur, organoselenium, and organotellurium compounds. On the other hand, diphenyl disulfide and diphenyl diselenide could be employed for the functionalization of Ag nanoparticles, while the partial oxidation of the organotellurium unit could be detected on the Ag surface. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
669.
Giulia Di Rocco Antonio Ranieri Marco Borsari Marco Sola Carlo Augusto Bortolotti Gianantonio Battistuzzi 《Molecules (Basel, Switzerland)》2022,27(17)
The Met80Ala variant of yeast cytochrome c is known to possess electrocatalytic properties that are absent in the wild type form and that make it a promising candidate for biocatalysis and biosensing. The versatility of an enzyme is enhanced by the stability in mixed aqueous/organic solvents that would allow poorly water-soluble substrates to be targeted. In this work, we have evaluated the effect of dimethylsulfoxide (DMSO) on the functionality of the Met80Ala cytochrome c mutant, by investigating the thermodynamics and kinetics of electron transfer in mixed water/DMSO solutions up to 50% DMSO v/v. In parallel, we have monitored spectroscopically the retention of the main structural features in the same medium, focusing on both the overall protein structure and the heme center. We found that the organic solvent exerts only minor effects on the redox and structural properties of the mutant mostly as a result of the modification of the dielectric constant of the solvent. This would warrant proper functionality of this variant also under these potentially hostile experimental conditions, that differ from the physiological milieu of cytochrome c. 相似文献
670.
Fabiana Inoue Rmulo Augusto Ando Paola Corio 《Journal of Raman spectroscopy : JRS》2011,42(6):1379-1383
Nanocomposites of carbon nanotubes and titanium dioxide (TiO2) have attracted much attention due to their photocatalytic properties. Although many examples in the literature have visualized these nanocomposites by electron microscopic images, spectroscopic characterization is still lacking with regard to the interaction between the carbon nanotube and TiO2. In this work, we show evidence of the attachment of nanostructured TiO2 to multiwalled carbon nanotubes (MWNTs) by Raman spectroscopy. The nanostructured TiO2 was characterized by both full‐width at half‐maximum (FWHM) and the Raman shift of the TiO2 band at ca 144 cm−1, whereas the average diameter of the crystallite was estimated as approximately 7 nm. Comparison of the Raman spectra of the MWNTs and MWNTs/TiO2 shows a clear inversion of the relative intensities of the G and D bands, suggesting a substantial chemical modification of the outermost tubes due to the attachment of nanostructured TiO2. To complement the nanocomposite characterization, scanning electronic microscopy and X‐ray diffraction were performed. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献