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671.
Large-scale quantum and molecular mechanical methods (QM/MM) and QM calculations were carried out on the soluble Δ(9) desaturase (Δ(9)D) to investigate various structural models of the spectroscopically defined peroxodiferric (P) intermediate. This allowed us to formulate a consistent mechanistic picture for the initial stages of the reaction mechanism of Δ(9)D, an important diferrous nonheme iron enzyme that cleaves the C-H bonds in alkane chains resulting in the highly specific insertion of double bonds. The methods (density functional theory (DFT), time-dependent DFT (TD-DFT), QM(DFT)/MM, and TD-DFT with electrostatic embedding) were benchmarked by demonstrating that the known spectroscopic effects and structural perturbation caused by substrate binding to diferrous Δ(9)D can be qualitatively reproduced. We show that structural models whose spectroscopic (absorption, circular dichroism (CD), vibrational and M?ssbauer) characteristics correlate best with experimental data for the P intermediate correspond to the μ-1,2-O(2)(2-) binding mode. Coordination of Glu196 to one of the iron centers (Fe(B)) is demonstrated to be flexible, with the monodentate binding providing better agreement with spectroscopic data, and the bidentate structure being slightly favored energetically (1-10 kJ mol(-1)). Further possible structures, containing an additional proton or water molecule are also evaluated in connection with the possible activation of the P intermediate. Specifically, we suggest that protonation of the peroxide moiety, possibly preceded by water binding in the Fe(A) coordination sphere, could be responsible for the conversion of the P intermediate in Δ(9)D into a form capable of hydrogen abstraction. Finally, results are compared with recent findings on the related ribonucleotide reductase and toluene/methane monooxygenase enzymes. 相似文献
672.
2-Vinylchromanes (1), 2-vinyl-1,4-benzodioxanes (2), and 2,3-dihydro-2-vinyl-2H-1,4-benzoxazines (3) can be prepared in high yields (90-98%) and excellent enantiomeric purities (87-98% ee) by [COP-OAc](2)-catalyzed cyclization of phenolic (E)-allylic trichloroacetimidate precursors. Deuterium-labeling and computational experiments are consistent with these cyclization reactions taking place by an anti-oxypalladation/syn-deoxypalladation mechanism. 2-Vinylchromanes can also be prepared in good yields and high enantiomeric purities from analogous (E)-allylic acetate precursors, which constitutes the first report that acetate is a competent leaving group in COP-catalyzed enantioselective S(N)2' substitution reactions. 相似文献
673.
674.
Photonic crystal slab cavities were investigated for increased light-matter interaction based on selective placement of sublattice hole sized defect holes inside L3 cavities. A multiple-hole defect (MHD) consisting of three defect holes placed in the regions of highest cavity mode field intensity were demonstrated through finite-difference time-domain simulations and experiments to exhibit the strongest light-matter interaction without introducing significant scattering losses. Compared to an L3 cavity without defect holes, these strategically designed three-hole MHD cavities presented higher quality factor and more than double the resonance wavelength shift upon exposure to a thin oxide and two small chemical molecules. 相似文献
675.
676.
677.
This paper explores the changes in public expenditures for Medicare and Medicaid that would result between 1980 and 2040 under the ceteris paribus assumption, under the age, sex, and family structure projections of John Wilkin of the Social Security Administration. Inflation aside, costs are shown to approximately double as a percent of the payroll of active workers. The most dramatic increases occur in nursing home costs: Costs for welfare recipients under age 65 change little except for medical care inflation. This latter results from the assumed low birth rates assumed in the population forecast. 相似文献
678.
Solomon M. Jekel 《manuscripta mathematica》1977,21(2):189-203
We describe a subcomplex S* (G) of a K(G,1) which depends on a given presentation of the group G. We prove that under a certain condition S* (G) is a K(G,1). Complexes of the type S* (G) arise in the homotopy theory of the classifying space for foliations.This research was partially supported by the N.S.F. 相似文献
679.
K.L. Chopra K. Solomon Harshvardhan S. Rajagopalan L.K. Malhotra 《Solid State Communications》1981,40(4):387-390
Large photocontraction of up to 26% has been observed in some chalcogenide films. Conditions necessary for the occurrence of large photocontraction are indicated to be: ability to form a glass, high bond ionicity, strong electron-phonon coupling, low density of the bulk material, and a large density difference between the bulk and the thin-film. These studies suggest that contraction (densification) is due to the mechanical collapse of the columns resulting from the large internal stresses caused by the interaction of the photogenerated carriers and the charged dangling bonds. 相似文献
680.
Gideon Fleminger Beka Solomon Tamar Wolf Eran Hadas 《Applied biochemistry and biotechnology》1990,26(3):231-238
A single step periodate oxidative binding of glycoproteins to synthetic polymeric matrices is proposed. By this procedure,
the glycoprotein to be immobilized is simultaneously mixed with sodium periodate and a hydrazide-containing matrix. The carbohydrate
moieties of the protein are oxidized and react with matrix. The single step reaction is faster than the commonly-used multistep
procedure and it allows conjugation of minute amounts of glycoproteins to the matrix. 相似文献