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921.
922.
A new, simple and general one‐pot method for the preparation of (5‐R‐1,3,4‐oxadiazol‐2‐yl)furoxans has been developed on the basis of the interaction between accessible 3‐methylfuroxan‐4‐carboxylic acid hydrazide and aliphatic, aromatic and heterocyclic carboxylic acids or their chlorides in the presence of POCl3. The synthesis and study of (5‐R‐1,3,4‐oxadiazol‐2‐yl)furoxans reactivity resulted in new polyheterocyclic ensembles incorporating furoxan, 1,3,4‐oxadiazole, pyrrole, triazole, furan, thiophene, pyrimidine, and other heterocycles in different combinations.  相似文献   
923.
The present work examines how substituents affect the interaction of lithium with the benzene ring and considers whether two configurations, “loose” and “tight,” can be found for the investigated set of complexes as in the lithium complexes with polyaromatic hydrocarbons. The two conformations were actually found for the electron-donating substituents, whereas for the electron-withdrawing substituents, CF3 and NO2, only tight conformation could be optimized. For complexes in the loose conformation [with the N(CH3)2, NH2, OH, H, Cl, and CH3 substituents], the transfer of electron density was directed from the region occupied by a disubstituted benzene to that occupied by Li. The amount of transferred density was very similar for all the complexes (0.21–0.25 a.u.). By contrast, in the tight complexes, the density transfer direction was opposite and the amount of the transferred density depended on the electronegativity of the substituents on benzene ring, being ?0.09, ?0.09, ?0.13, ?0.15, ?0.15, ?0.20, and ?0.30 a.u. for N(CH3)2, NH2, OH, H, Cl, CF3, and NO2, respectively. Geometries of the loose and tight conformations differed notably. The six-membered ring was significantly more warped in the tight conformation than in the loose one: In the latter, the ring was nearly planar, whereas in the former, the ring had the boat structure.  相似文献   
924.
It is well established that the formation of transthyretin (TTR) amyloid fibrils is linked to the destabilization and dissociation of its tetrameric structure into insoluble aggregates. Isotope labeling is used for the study of TTR by NMR, neutron diffraction, and mass spectrometry (MS). Here MS, thioflavin T fluorescence, and crystallographic data demonstrate that while the X‐ray structures of unlabeled and deuterium‐labeled TTR are essentially identical, subunit exchange kinetics and amyloid formation are accelerated for the deuterated protein. However, a slower subunit exchange is noted in deuterated solvent, reflecting the poorer solubility of non‐polar protein side chains in such an environment. These observations are important for the interpretation of kinetic studies involving deuteration. The destabilizing effects of TTR deuteration are rather similar in character to those observed for aggressive mutations of TTR such as L55P (associated with familial amyloid polyneuropathy).  相似文献   
925.
Advanced molecular electronic components remain vital for the next generation of miniaturized integrated circuits. Thus, much research effort has been devoted to the discovery of lossless molecular wires, for which the charge transport rate or conductivity is not attenuated with length in the tunneling regime. Herein, we report the synthesis and electrochemical interrogation of DNA‐like molecular wires. We determine that the rate of electron transfer through these constructs is independent of their length and propose a plausible mechanism to explain our findings. The reported approach holds relevance for the development of high‐performance molecular electronic components and the fundamental study of charge transport phenomena in organic semiconductors.  相似文献   
926.
An edge (vertex) colored graph is rainbow‐connected if there is a rainbow path between any two vertices, i.e. a path all of whose edges (internal vertices) carry distinct colors. Rainbow edge (vertex) connectivity of a graph G is the smallest number of colors needed for a rainbow edge (vertex) coloring of G. In this article, we propose a very simple approach to studying rainbow connectivity in graphs. Using this idea, we give a unified proof of several known results, as well as some new ones.  相似文献   
927.
Summary Monodispersed spherical submicron silica particles were obtained by the precipitation of soluble silica on the surface of preliminary obtained smaller particles. Silica was added into the system at low concentrations to prevent both its polymerization in the solution and the formation of new particles. The kinetics of the particle growth is controlled by the diffusion of soluble silica through the double diffusion layer.  相似文献   
928.
The role of the enzyme source was studied in the reaction of hydrogen peroxide decomposition by immobilised catalase in acetonitrile. Enzymes isolated from bacterial and mammalian sources were conveniently immobilised on a spectroscopic graphite to obtain an organic-phase enzyme electrode (OPEE). Amperometry at constant potential was employed as basic analytical approach in this study.  相似文献   
929.
The geometric structure and conformational properties of the saturated five-membered-ring compound 2,2-di-tert-butyl-1,3-diaza-2-silacyclopentane, (t-Bu)(2)Si(NH)(2)(CH(2))(2), was investigated by gas electron diffraction and quantum chemical methods (B3LYP and MP2 with 6-31G basis sets). The compound exists as a mixture of two conformers, both possessing a twist conformation and C(2) symmetry. In the prevailing form (76(6) % at 305 K) the N-H bonds stagger the adjacent CH(2) groups, and in the minor form the N-H bonds eclipse the CH(2) groups. This conformational mixture corresponds to a free energy difference of DeltaG degrees = 0.69(19) kcal/mol. The B3LYP method predicts a preference for the eclipsed conformer. The largest torsion occurs around the C-C bond with tau(NCCN) = 29.2(24) degrees. The degree of puckering in the title compound is considerably smaller than that in silacyclopentane with tau(CCCC) = 49.7(14) degrees. This has been rationalized by larger angle strain in the title compound.  相似文献   
930.
During this study the formation and growth of silver chloride crystals in crazed porous polymeric matrixes of poly(ethylene terephthalate) (PET) and polypropylene (PP) were under investigation. The rate of formation and dispersity and the way AgCl particles aggregate in porous polymers were shown to be dependent on the effective volume porosity, pore dimension, and physical state of the polymer. Methods of the determination of diffusion and distribution constants for low-molecular substances in porous polymers were suggested, and a mechanism of silver chloride crystallization in porous medium was proposed.  相似文献   
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