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941.
Y Miura N Hayashi S Yokoshima T Fukuyama 《Journal of the American Chemical Society》2012,134(29):11995-11997
A first asymmetric total synthesis of (-)-isoschizogamine has been accomplished. Our synthesis features the facile construction of the carbon framework of the natural product through a Wagner-Meerwein rearrangement, a tandem metathesis, a stereoselective rhodium-mediated 1,4-addition of an arylboronic acid, and a ring-closing metathesis via a hemiaminal ether. 相似文献
942.
Hayashi T Caranto JD Matsumura H Kurtz DM Moënne-Loccoz P 《Journal of the American Chemical Society》2012,134(15):6878-6884
Flavodiiron proteins (FDPs) play important roles in the microbial nitrosative stress response in low-oxygen environments by reductively scavenging nitric oxide (NO). Recently, we showed that FMN-free diferrous FDP from Thermotoga maritima exposed to 1 equiv NO forms a stable diiron-mononitrosyl complex (deflavo-FDP(NO)) that can react further with NO to form N(2)O [Hayashi, T.; Caranto, J. D.; Wampler, D. A; Kurtz, D. M., Jr.; Mo?nne-Loccoz, P. Biochemistry 2010, 49, 7040-7049]. Here we report resonance Raman and low-temperature photolysis FTIR data that better define the structure of this diiron-mononitrosyl complex. We first validate this approach using the stable diiron-mononitrosyl complex of hemerythrin, Hr(NO), for which we observe a ν(NO) at 1658 cm(-1), the lowest ν(NO) ever reported for a nonheme {FeNO}(7) species. Both deflavo-FDP(NO) and the mononitrosyl adduct of the flavinated FPD (FDP(NO)) show ν(NO) at 1681 cm(-1), which is also unusually low. These results indicate that, in Hr(NO) and FDP(NO), the coordinated NO is exceptionally electron rich, more closely approaching the Fe(III)(NO(-)) resonance structure. In the case of Hr(NO), this polarization may be promoted by steric enforcement of an unusually small FeNO angle, while in FDP(NO), the Fe(III)(NO(-)) structure may be due to a semibridging electrostatic interaction with the second Fe(II) ion. In Hr(NO), accessibility and steric constraints prevent further reaction of the diiron-mononitrosyl complex with NO, whereas in FDP(NO) the increased nucleophilicity of the nitrosyl group may promote attack by a second NO to produce N(2)O. This latter scenario is supported by theoretical modeling [Blomberg, L. M.; Blomberg, M. R.; Siegbahn, P. E. J. Biol. Inorg. Chem. 2007, 12, 79-89]. Published vibrational data on bioengineered models of denitrifying heme-nonheme NO reductases [Hayashi, T.; Miner, K. D.; Yeung, N.; Lin, Y.-W.; Lu, Y.; Mo?nne-Loccoz, P. Biochemistry 2011, 50, 5939-5947 ] support a similar mode of activation of a heme {FeNO}(7) species by the nearby nonheme Fe(II). 相似文献
943.
A palladium-catalyzed decarboxylative cyclopropanation of γ-methylidene-δ-valerolactones with aromatic aldehydes has been developed to give 4-oxaspiro[2.4]heptanes with high selectivity. The site of nucleophilic attack to a π-allylpalladium intermediate has been controlled with a sterically demanding phosphine ligand. The course of the reaction is highly dependent on ligands and solvents, and selective formation of methylenetetrahydropyrans has also been realized. 相似文献
944.
Ordered nanofacet structures on vicinal 6H-SiC(0 0 0 1) surfaces, consisting of pairs of a (0 0 0 1) basal plane and a facet, are investigated in terms of stable surface stacking of the (0 0 0 1) basal planes. The surface termination of S3 (or S3*), i.e., ABC (or A*C*B*), was suggested by a structural model based on quantized step-bunching, which typically gives a one-unit-cell bunched step configuration at the facet. Here, we evaluate the surface termination at basal planes covered with a layer of silicon oxynitride by means of quantitative low-energy electron diffraction (LEED) analysis combined with scanning tunneling microscopy (STM), and show the validity of the structural model proposed. 相似文献
945.
A. Ya. Faenov T. A. Pikuz Y. Fukuda M. Kando H. Kotaki T. Homma K. Kawase T. Kameshima A. Pirozhkov A. Yogo M. Tampo M. Mori H. Sakaki I. Hayashi T. Nakamura S. A. Pikuz Jr. V. Kartashev I. Yu. Skobelev A. Giulietti C. A. Checchetti A. S. Boldarev V. A. Gasilov A. I. Magunov S. Kar M. Borghesi P. Bolton H. Daido T. Tajima I. Kato S. V. Bulanov 《JETP Letters》2009,89(10):485-491
It has been shown that a femtosecond plasma of cluster targets is an almost isotropic source of fast ions and, hence, can be used to obtain ionographic images with a wide field of view. The spatial resolution of the resulting ionographic images is no worse than 600 nm, which corresponds to a uniquely high value of about 105 of the ratio of the field of view to the resolution. The use of 100–300-keV ion fluxes ensures the sensitivity of the method to the sample thickness of no worse than 100 nm even for samples consisting of light chemical elements (C, H). The proposed method can be used to obtain images of low-contrast biological objects, thin films, membranes, and other nanostructured objects. 相似文献
946.
H. Nakamura N. Hayashi N. Nakai M. Okumura M. Machida 《Physica C: Superconductivity and its Applications》2009,469(15-20):908-911
In order to resolve a discrepancy of the magnetic moment on Fe between the experimental and calculation results, we perform first-principle electronic structure calculations for iron-based superconductors LaFeAsO1-xFx in which x=0.0 and x=0.125 by using the LSDA + U framework. Consequently, we confirm in both the mother and doped compounds that negative U correction is crucial in matching the calculated magnetic moment with the observed one. A reason of the negative correction is that the Coulomb interaction on Fe orbitals is unexpectedly screened than LSDA’s expectation. We discuss which type of situation emerges when the negative U is a good correction in these compounds. 相似文献
947.
948.
Two new Lycopodium alkaloids, lycopladines F (1) and G (2), have been isolated from the club moss Lycopodium complanatum, and the structures and relative stereochemistries of 1 and 2 were elucidated on the basis of spectroscopic data. Lycopladine F (1) is a rare C16N2-type Lycopodium alkaloid possessing an amino acid residue (C4N). 相似文献
949.
Molecular dynamics (MD) simulations of cellulose IIII crystal models have been carried out. The crystal models were composed by either 24 or 48 cellooligomers consisting of either
20 or 40 residues and were surrounded by waters in a periodic boundary box. Two base plane types differing in a constituent
crystal lattice plane, (0 −1 0) × (0 1 0) and (1 0 0) × (0 1 0), were additionally considered. Among the resulting eight crystal
models, an overall structure conversion was observed for the seven models. The final structures had a triclinic-like chain
arrangement involving one-quarter staggering chains with respect to its axis. The successive, local transformation involving
cooperative bends in cellooligomers was observed during the structure conversion. Only the 48 × 20-mer model having the (0 −1 0)
lattice plane retained the original crystal structure throughout a 2.5-ns simulation. The MD simulations with an implicit
solvent system and a vacuum system were also performed to asses a solvent effect on the structure conversion. 相似文献
950.
This paper studies Nash implementation in the job-matching market where each worker works for only one firm and a firm hires
as many workers as it wishes. We show that the competitive equilibrium correspondence (CEC) is the smallest Nash implementable
correspondence satisfying individual rationality and Pareto indifference. Furthermore, the CEC is the minimal monotonic extension of the worker-optimal and firm-optimal subcorrespondences. We offer
two “good” mechanisms that implement this correspondence in Nash equilibrium. 相似文献