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721.
Vitamin B(12) (cyanocobalamin) and its biologically active derivatives, methylcobalamin and adenosylcobalamin, are members of the family of corrinoids, which also includes cobinamides. As biological precursors to cobalamins, cobinamides possess the same structural core, consisting of a low-spin Co(3+) ion that is ligated equatorially by the four nitrogens of a highly substituted tetrapyrrole macrocycle (the corrin ring), but differ with respect to the lower axial ligation. Specifically, cobinamides possess a water molecule instead of the nucleotide loop that coordinates axially to Co(3+)cobalamins via its dimethylbenzimidazole (DMB) base. Compared to the cobalamin species, cobinamides have proven much more difficult to study experimentally, thus far eluding characterization by X-ray crystallography. In this study, we have utilized combined quantum mechanics/molecular mechanics (QM/MM) computations to generate complete structural models of a representative set of cobinamide species with varying upper axial ligands. To validate the use of this approach, analogous QM/MM geometry optimizations were carried out on entire models of the cobalamin counterparts for which high-resolution X-ray structural data are available. The accuracy of the cobinamide structures was assessed further by comparing electronic absorption spectra computed using time-dependent density functional theory to those obtained experimentally. Collectively, the results obtained in this study indicate that the DMB → H(2)O lower axial ligand switch primarily affects the energies of the Co 3d(z(2))-based molecular orbital (MO) and, to a lesser extent, the other Co 3d-based MOs as well as the corrin π-based highest energy MO. Thus, while the energy of the lowest-energy electronic transition of cobalamins changes considerably as a function of the upper axial ligand, it is nearly invariant for the cobinamides. 相似文献
722.
Shepherd RF Conrad JC Rhodes SK Link DR Marquez M Weitz DA Lewis JA 《Langmuir : the ACS journal of surfaces and colloids》2006,22(21):8618-8622
The microfluidic assembly of colloid-filled hydrogel granules of varying shape and composition is described. First, drops are formed by shearing a concentrated colloidal microsphere-acrylamide suspension in a continuous oil phase using a sheath-flow device. Both homogeneous and Janus (hemispherically distinct) spheres and disks are produced by confining the assembled drops in microchannels of varying geometry. Next, photopolymerization is carried out shortly after drop breakup to preserve their morphology. Representative wet and dried granules are characterized using fluorescence and scanning electron microscopy, respectively. Our approach offers a facile route for assembling colloid-filled hydrogel granules with controlled shape and composition. 相似文献
723.
Hassan NM Murimboh JD Sekaly AL Mandal R Chakrabarti CL Grégoire DC 《Analytical and bioanalytical chemistry》2006,384(7-8):1558-1566
Kinetic speciation of nickel, aluminium, and iron in fresh water has been investigated by cascade ultrafiltration followed by competing ligand exchange of the ultrafiltered fractions. Graphite furnace atomic absorption spectrometry was used to measure the kinetics of metal complex dissociation. Dissolved metal species were fractionated by cascade ultrafiltration. Metal speciation in each ultrafiltered fraction was then characterized as free metal ions, “labile” metal complexes (with dissociation rate constants ≥10−3 s−1), “slowly labile” metal complexes (with dissociation rate constants >10−6 s−1), and “inert” metal complexes (with dissociation rate constants <10−6 s−1). The experimental results were compared with the predictions of a computer-based equilibrium speciation model, the Windermere humic aqueous model (WHAM) V. Cascade ultrafiltration coupled with kinetic speciation of the metal species in each molecular weight cut-off (MWCO) fraction provided a more comprehensive picture and insight into the physical and the chemical characteristics of the metal species than either ultrafiltration or measurement of dissociation kinetics alone. 相似文献
724.
Isolation of hydroxycinnamoyltartaric acids from grape pomace by high-speed counter-current chromatography 总被引:2,自引:0,他引:2
Maier T Sanzenbacher S Kammerer DR Berardini N Conrad J Beifuss U Carle R Schieber A 《Journal of chromatography. A》2006,1128(1-2):61-67
A method for the isolation of caftaric, coutaric and fertaric acids from grape pomace by high-speed counter-current chromatography (HSCCC) was developed. Using a system of hexane/ethyl acetate/methanol/water 3:7:3:7 (v/v/v/v) and 0.5% trifluoroacetic acid (TFA) in the head-to-tail elution mode, the target compounds were separated from co-extracted polyphenolics and subsequently isolated in a second run (tert-butyl-methyl ether/acetonitrile/n-butanol/water, 2:2:1:5 (v/v/v/v) and 0.5% TFA; tail-to-head elution mode). The concomitant flavonoid quercetin 3-glucuronide was also isolated with the present method. The compounds were characterized by 1H NMR spectroscopy, by LC/electrospray ionization (ESI)-MS/MS in the negative ionization mode, and by UV spectroscopy. A purity of 97.0% (2.0% Z-isomer) for caftaric acid, 97.2% (4.8% Z-isomer) for coutaric acid, and 90.4% (13% Z-isomer) for fertaric acid was obtained from 10 g of grape pomace with yields of 62, 48 and 23%, respectively. Caftaric and coutaric acids may be used for in vitro and in vivo studies and as reference substances for analytical purposes. 相似文献
725.
Electrochemistry and photophysics of donor-substituted triarylboranes: symmetry breaking in ground and excited state 总被引:1,自引:0,他引:1
Stahl R Lambert C Kaiser C Wortmann R Jakober R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(8):2358-2370
We synthesized a series of amino substituted triarylboranes (TABs) 1-3 by copper(I)-catalyzed cross-coupling reactions. The title compounds were investigated by means of cyclic voltammetry (CV) and UV-visible absorption and fluorescence spectroscopy. Electrochemical oxidation of tris(4-carbazolyl-2,6-dimethylphenyl)borane (3) leads to the formation of an electroactive polymer film on the electrode surface. The charge-transfer (CT) absorption band of all three TABs shows a pronounced negative solvatochromism, while the emission is positively solvatochromic. By combining Jortner's theory, AM1 computations, and electrooptical absorption measurements (EOAM), this unexpected behavior was shown to be due to a dipole inversion upon S0-->S1 excitation. Furthermore, polarized steady-state fluorescence spectroscopy and EOAM prove that the ground-state geometry of 3 is of lower symmetry than D3 and that the excitation energy can be transferred from one subchromophore to another within the lifetime of the excited state. Exciton-coupling theory was used to quantitatively analyze this excitation transfer. 相似文献
726.
727.
Arnold A. Feldmann Kai Schmengler Klaus Laue Stefan Marose Kristin Kirschbaum Olaf Conrad Dean M. Giolando 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(11):e531-e531
The crystal structure of the title compound, [Me2NHC2H4NHMe2][SePh]2 or C6H18N22+·2C6H5Se?, reveals hydrogen bonding between the benzeneselenolate anions and the tetramethylethylenediammonium cations. The asymmetric unit contains one formula unit of the title compound. The two Se?H distances are 2.22 (4) and 2.34 (4) Å. 相似文献
728.
729.
Zr6STe2 – a Zirconium‐rich Sulfide Telluride with a Zr3Te Partial Structure of the Re3B Type Zr6STe2 is accessible through the reduction of a mixture of ZrTe2 and ZrS2 with zirconium in fused tantalum tubes at 1520 K. The spatially averaged crystal structure of Zr6STe2 is described in the space group Cmcm, a = 377.81(4), b = 1156.4(1), c = 887.96(8), Z = 2, Pearson symbol oC18, 320 reflexions (I > 2σ(I)), 22 variables, Rw(I) = 0.088. Zr6STe2 crystallizes in a unique structure type, which can be seen as a filled Re3B type structure. The tellurium atoms are surrounded by nine zirconium atoms situated at the vertices of a distorted, tricapped trigonal prism. The Zr9Te tetrakaidecahedra are connected by common triangular prism faces parallel [100], edges approximately along [001] and common vertices along [010], thus forming a three‐dimensional tetrakaidecahedral network [Zr9/3Te], which is decisively stabilized by homonuclear Zr–Zr‐interactions. The tetrakaidecahedra are arranged in such a way, that Zr6 octahedra occur. The octahedra are arranged into layers by sharing edges parallel [100] and vertices along [001]. As a result of a distortion of the structure, every second octahedron is expanded to such an extent as to be able to smoothly accommodate sulfur atoms. According to the modulation of the diffraction intensities, the vacancy ordering in adjacent layers of octahedra occurs independently of each other. 相似文献
730.