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541.
Ramos CI Santana-Marques MG Ferrer Correia AJ Tomé JP Alonso CM Tomé AC Neves MG Cavaleiro JA 《Journal of mass spectrometry : JMS》2008,43(6):806-813
The solutions of four meso-tetrakis(N-alkylpyridinium-4-yl)porphyrin salts and of the p-toluenesulfonate salt of meso-tetrakis(4-trimethylammoniumphenyl)porphyrin, in methanol, were studied by electrospray mass spectrometry, in order to investigate the influence of the type of counter ion, the length of the substituent N-alkyl groups of the four (N-alkylpyridinium-4-yl)porphyrins and the presence of an aromatic (alkylpyridinium) or aliphatic (trimethylammonium) nitrogen, in ion formation.In our experimental conditions, adducts with the counter ions were formed only for the meso-tetrakis(4-trimethylammoniumphenyl)porphyrin and were not observed for the other porphyrins, even when the counter ion was the same. In contrast, formation of reduced species, such as the [M(4+) + e(-)]3+, [M(4+) + 2e(-)]2+, [M(4+) + 4e(-) + 2H(+)]2+, and [M(4+) + 5e(-) + 2H(+)]+ ions was observed only for the (N-alkylpyridinium-4-yl)porphyrins and the appearance of these species is apparently solvent related and may occur via counter ion/solvent adducts. 相似文献
542.
In this paper we have proved several approximation theorems for the family of minimal surfaces in that imply, among other things, that complete minimal surfaces are dense in the space of all minimal surfaces endowed with
the topology of C
k
convergence on compact sets, for any .
As a consequence of the above density result, we have been able to produce the first example of a complete proper minimal
surface in with uncountably many ends.
This research is partially supported by MEC-FEDER Grant no. MTM2004 - 00160. 相似文献
543.
The literature has shown numerous contributions on the synthesis and physicochemical properties of persistent organic radicals but there are a lesser number of reports about their use as building blocks for obtaining molecular magnetic materials exhibiting an additional and useful physical property or function. These materials show promise for applications in spintronics as well as bistable memory devices and sensing materials. This critical review provides an up-to-date survey to this new generation of multifunctional magnetic materials. For this, a detailed revision of the most common families of persistent organic radicals-nitroxide, triphenylmethyl, verdazyl, phenalenyl, and dithiadiazolyl-so far reported will be presented, classified into three different sections: materials with magnetic, conducting and optical properties. An additional section reporting switchable materials based on these radicals is presented (257 references). 相似文献
544.
545.
The systematic analysis of the luminescence of a series of alkynyl gold derivatives with general formulas [(diphos)(AuC≡Cpy)(2)] (diphosphane =2,2'-bis(diphenylphosphanyl)propane or dppip (1), bis(diphenylphosphanyl)acetylene or dppa (2), 1,2-bis(diphenylphosphanyl)ethane or dppe (3) and 1,4-bis(diphenylphosphanyl)butane or dppb, (4), has shown a straightforward correlation between the Au(I)···Au(I) distance and the emission quantum yields and decaytimes. The analysis of the decaytimes, quantum yields and thus, the corresponding calculated rate constants demonstrated the existence of a correlation between Au(I)···Au(I) distance and the radiative rate constant for the deactivation of the emissive triplet states. It was concluded that the increased emission of these compounds results from the increase in spin-orbit coupling that favors the spin forbidden transition to the singlet ground state. 相似文献
546.
547.
Manetti F Tintori C Armand-Ugón M Clotet-Codina I Massa S Ragno R Esté JA Botta M 《Journal of chemical information and modeling》2006,46(3):1344-1351
The HIV-1 entry process is an important target for the design of new pharmaceuticals for the multidrug therapy of AIDS. A lot of polyanionic compounds, such as polysulfonated and polysulfated, are reported in the literature for their ability to block early stages of HIV-1 replication. Several studies have been performed to elucidate the mechanism of the anti-HIV-1 activity of sulfated polysaccharides and polyanions in general, including binding to cell surface CD4 and interfering with the gp120-coreceptor interaction. Here, we show molecular modeling investigations on ADS-J1, a polyanionic compound with anti-HIV activity that is able to interfere with gp120-coreceptor interactions. Agreeing with experimental data, computer simulations suggested that the V3 loop of gp120 was the preferential binding site for ADS-J1 onto HIV-1. Moreover, mutations induced by the inhibitor significantly changed the stereoelectronic properties of the gp120 surface, justifying a marked drop in the affinity of ADS-J1 toward an ADS-J1-resistant HIV-1 strain. 相似文献
548.
Aprile C Boronat M Ferrer B Corma A García H 《Journal of the American Chemical Society》2006,128(26):8388-8389
Organogold compounds arising from the trapping of carbon-centered radicals by gold chlorides (AuCl, AuCl3, and HAuCl4) and colloidal gold have been characterized by different spectroscopic techniques. Laser flash photolysis has provided kinetic evidence of the occurrence of radical trapping in the microsecond time scale, while theoretical calculations give the energetics of the elementary steps. The trapping of carbon radicals by gold species observed here may explain why the presence of radical initiators enhance dramatically the activity of gold catalysts for some reactions occurring through radical intermediates. 相似文献
549.
Ferrer S Tuñón I Martí S Moliner V Garcia-Viloca M Gonzalez-Lafont A Lluch JM 《Journal of the American Chemical Society》2006,128(51):16851-16863
In some enzymatic systems large conformational changes are coupled to the chemical step, in such a way that dispersion of rate constants can be observed in single-molecule experiments, each corresponding to the reaction from a different reactant valley. Under this perspective here we present a computational study of pyruvate to lactate transformation catalyzed by lactate dehydrogenase. The reaction consists of a hydride transfer and a proton transfer that seem to take place concertedly. The degree of asynchronicity and the energy barrier depend on the particular starting reactant valley. In order to estimate rate constants we used a free energy perturbation technique adapted to follow the intrinsic reaction coordinate for several possible reaction paths. Tunneling effects are also obtained with a slightly modified version of the ensemble-averaged variational transition state theory with multidimensional tunneling contributions. According to our results the closure of the active site by means of a flexible loop can lead to the formation of different reactant complexes displaying different features in the disposition of some key residues (such as Arg109), interactions with the substrate and number of water molecules in the active site. The chemical step of the reaction takes place with a different reaction rate from each of these complexes. Finally, primary kinetic isotope effects for replacement of the transferring hydrogen of the cofactor with a deuteride are in good agreement with experimental observations, thus validating our methodology. 相似文献
550.