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Ab initio Hartree–Fock band structure and molecular calculations have been performed to study the electronic structure of LiN3 in a monoclinic C 2/m crystal structure. The total energy, band structure, density of states, and charge densities are computed. The calculated lattice energy (energy to separate the ions infinitely apart) of 8.6 eV agrees very well with 8.45 eV deduced from Madelung and London polarizability energies. The calculated split of the N 1s core bands of 5.0 eV compares favorably with the experimental X-ray photoelectron value of 4.4 eV. This good agreement is not contributed to crystalline environment effects as proposed in earlier MO studies of N where the best values obtained were 5.1, 5.8, and 6.3 eV, but to the quality of the nitrogen core basis set. The calculated valence density of states supports one of two competing interpretations that peak III observed in the X-ray photoelectron spectrum arises from contaminations or other extrinsic states. 相似文献
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W. Kunz J. Barthel L. Klein T. Cartailler P. Turq B. Reindl 《Journal of solution chemistry》1991,20(9):875-891
A variety of methods has been used for the study of lithium bromide solutions in acetonitrile yielding by their combination reliable information on different levels of approximation. Osmotic coefficients based on precise vapor pressure measurements are reproduced by CM (chemical model) and HNC (hypernetted chain) calculations and by BD (brownian dynamics) simulations. The results of neutron scattering experiments are treated with the help of HNC and BD methods. Hartree-Fock calculations on isolated LiBr pairs and solvated lithium ions yield reliable particle distances and reveal the geometry of the lithium solvation sphere. 相似文献
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Okamura WH Zhu GD Hill DK Thomas RJ Ringe K Borchardt DB Norman AW Mueller LJ 《The Journal of organic chemistry》2002,67(5):1637-1650
Isotope-labeled drug molecules may be useful for probing by NMR spectroscopy the conformation of ligand associated with biological hosts such as membranes and proteins. Triple-labeled [7,9,19-(13)C(3)]-vitamin D(3) (56), its 25-hydroxylated and 1 alpha,25-dihydroxylated metabolites (58 and 68, respectively), and other labeled materials have been synthesized via coupling of [9-(13)C]-Grundmann's ketone 39 or its protected 25-hydroxy derivative 43 with labeled A ring enyne fragments 25 or 26. The labeled CD-ring fragment 39 was prepared by a sequence involving Grignard addition of [(13)C]-methylmagnesium iodide to Grundmann's enone 28, oxidative cleavage, functional group modifications leading to seco-iodide 38, and finally a kinetic enolate S(N)2 cycloalkylation. The C-7,19 double labeling of the A-ring enyne was achieved by the Corey-Fuchs/Wittig processes on keto aldehyde 11. By employing these labeled fragments in the Wilson-Mazur route, the C-7,9,19 triple-(13)C-labeled metabolites 56, 58, and 68 as well as other (13)C-labeled metabolites have been prepared. In an initial NMR investigation of one of the labeled metabolites prepared in this study, namely [7,9,19-(13)C(3)]-25-hydroxyvitamin D(3) (58), the three (13)C-labeled carbons of the otherwise water insoluble steroid could be clearly detected by (13)C NMR analysis at 0.1 mM in a mixture of CD(3)OD/D(2)O (60/40) or in aqueous dimethylcyclodextrin solution and at 2 mM in 20 mM sodium dodecyl sulfate (SDS) aqueous micellar solution. In the SDS micellar solution, a double half-filter NOESY experiment revealed that the distance between the H(19Z) and H(7) protons is significantly shorter than that of the corresponding distance calculated from the solid state (X-ray) structure of the free ligand. The NMR data in micelles reveals that 58 exists essentially completely in the alpha-conformer with the 3 beta-hydroxyl equatorially oriented, just as in the solid state. The shortened distance (H(19Z))-H(7)) in micellar solutions as compared to that in the solid state is most easily rationalized on the basis that the 5(10)-torsion angle in 58 is decreased in micellar solutions as compared to that in the solid state. 相似文献
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Michael Karas Hanno Ehring Eckhard Nordhoff Bernd Stahl Kerstin Strupat Franz Hillenkamp Matthias Grehl Bernt Krebs 《Journal of mass spectrometry : JMS》1993,28(12):1476-1481
Selected benzoic acid derivatives and related substances were used as additives to 2,5-dihydroxybenzoic acid (2,5DHB) and the performance of the mixtures in matrix-assisted laser desorption/ionization mass spectrometry was investigated. Using benzoic acid derivatives substituted at position 2 and/or 5 or related substances as a co-matrix in the 1–10% range with 2,5DHB results in improved ion yields and signal-to-noise ratio of analyte molecules, especially for the high-mass range. The enhanced performance is prominent for 2-hydroxy-5-methoxybenzoic acid and exists for both proteins and oligosaccharides. It is suggested that the improvement is caused by a disorder in the 2,5DHB crystal lattice allowing ‘softer’ desorption. Charge transfer from matrix ions to additive molecules at the expense of analyte ionization gives a simple explanation for the deteriorating effects of some tested additives. 相似文献
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G. Schwedt Abu Baker Tawali Kerstin Koch 《Analytical and bioanalytical chemistry》1998,360(5):589-594
After the application of simulated digestive fluids (gastric and intestinale fluid) as extraction solutions, AAS and differential pulse anodic-stripping voltammetry (DPASV) were used as analytical methods in combination with ion-exchange procedures for the determination of zinc species and total zinc contents. The ion exchange procedure and the shifting of electrochemical potentials as well as changes of the calibration slope after standard addition in the polarographic analysis allowed discriminating statements with regard to a possible resorption of zinc. On the basis of these methods, a strategy is presented for in vitro investigations of the bioavailability of zinc in foodstuffs. 相似文献
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The synthesis of the octasaccharide [p-(trifluoroacetamido)phenyl]ethyl 4-O-[2-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-beta-D-glucopyranosyl]-6-O-[2-O-[4-O-(4-O-alpha-D-galactopyranosyl-beta-D-galactopyranosyl)-alpha-D-glucopyranosyl]-beta-D-glucopyranosyl]-3-O-beta-D-glucopyranosyl-alpha-D-glucopyranoside, representing the outer part of the lipooligosaccharide from Moraxella catarrhalis serotype A, is described, together with a hepta-, a hexa-, and a pentasaccaride, composing parts thereof with shorter oligosaccharide chains substituted in the 6-position of the central 3,4,6-branched glucose moiety. The versatility of the use of thioglycosides in oligosaccharide synthesis is shown, since throughout the synthesis thioglycosides are used as glycosyl donor precursors, either directly in dimethyl(methylthio)sulfonium triflate (DMTST)-promoted coupling reactions or after conversion to the corresponding glycosyl bromide in silver triflate-promoted couplings. The effects of different protecting groups, anomeric leaving groups, and solvents used in the various coupling reactions are often substantial, which necessitates the use of easily convertible intermediates. 相似文献