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11.
Vedejs E Naidu BN Klapars A Warner DL Li VS Na Y Kohn H 《Journal of the American Chemical Society》2003,125(51):15796-15806
An enantiocontrolled route to aziridinomitosenes had been developed from l-serine methyl ester hydrochloride. The tetracyclic target ring system was assembled by an internal azomethine ylide cycloaddition reaction based on silver ion-assisted intramolecular oxazole alkylation and cyanide-induced ylide generation via a labile oxazoline intermediate (62 to 66). Other key steps include reductive detritylation of 26, methylation of the N-H aziridine 56, oxidation of the sensitive cyclohexenedione 68 to quinone 70, and carbamoylation using Fmoc-NCO. Although the aziridinomitosene tetracycle is sensitive, a range of protecting group manipulations and redox chemistry can be performed if suitable precautions are taken. A study of DNA alkylation by the first C-6,C-7-unsubstituted aziridinomitosene 11a has been carried out, and evidence for DNA cross-link formation involving nucleophilic addition to the quinone subunit is described. 相似文献
12.
A kinetic study of Diels-Alder additions between norbornene derivatives acting as dienophiles and the dienes tropone, perchlorocyclopentadiene and 9,10-dimethylanthracene is presented. The results suggest that tropone thereby acts as an electron acceptor, classifying these reactions then as Diels-Alder additions with ‘inverse electron demand’. The role of non-reacting double bonds was investigated using 7-alkylidenorbornene derivatives as dienophiles. These show comparable reactivity in ‘normal’ but enhanced reactivity in ‘inverse electron demand’ Diels-Alder additions. This outcome is interpreted in terms of frontier orbital interactions between the reactants using photoelectron spectroscopical results as a basis for the qualitative perturbation treatment. It is suggested that the enhanced reactivity in the case of the 7-alkylidene derivatives does not necessitate the consideration of a direct (‘through-space’) participation of the non-reacting double bond. 相似文献
13.
Bockstaller MR Lapetnikov Y Margel S Thomas EL 《Journal of the American Chemical Society》2003,125(18):5276-5277
Dependent on the relative particle core size, two distinct types of particle topologies in block copolymer/nanocrystal blends have been identified, that is, the localization of particles along the intermaterial dividing surface or at the center of the respective polymer domain. In ternary systems consisting of block copolymer and two different-sized nanocrystal species, the distinct morphological types are conserved, resulting in autonomous size-selective separation and organization of the respective nanocrystals within alternating arrays and sheets. 相似文献
14.
Computer least-squares methods are used to conduct an automatic reverse search of a library of complete mass spectra for the analysis of the output data matrix from a g.c.—m.s. system. The search is limited to those mass spectra falling within a narrow retention index window about the retention index corresponding to each scan in the datamatrix. The least-squares estimated parameters obtained are proportional to the concentrations of the substances present at each scan and are displayed graphically as the profiles of resolved overlapping g.c. peaks. The precision of the parameters is estimated. The root mean square Values of the residuals of the least-squares fits are used successfully to indicate the presence of substances not in the library. Examples are given of the results produced with data from this laboratory. Possible elaborations and improvements of this computer program are discussed. 相似文献
15.
Treatment of 11a with methyllithium affords the destannylated product 12 together with a small amount of tetracyclic product derived from intramolecular Michael addition. The same procedure from the deuterated analogue 11b gives the tetracyclic 18 as the major product, the result of a substantial kinetic deuterium isotope effect that favors formation of 16 and 17 by suppressing indole ring lithiation to the undesired 15. When the product mixture is quenched with phenylselenenyl chloride, 17 is converted into the aziridinomitosene 19 in 80% yield. Conversion into the aziridinomitosene alcohol 21 and the deprotected aziridine 20 is also demonstrated. 相似文献
16.
17.
Fichele S Paley MN Woodhouse N Griffiths PD van Beek EJ Wild JM 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,167(1):1-11
Finite difference simulations have been used to model (3)He gas diffusion in simulated lung tissue. The technique has the advantage that a wide range of structural models and diffusion-sensitizing gradient waveforms can be investigated, for which analytical methods would otherwise be virtually impossible. Results from simulations and in vivo pulsed-gradient-spin-echo (PGSE) experiments show that the apparent diffusion coefficient (ADC) is a function of diffusion time and gradient strength, and suggests diffusion is locally anisotropic. The simulations have been compared to recent work on an analytical model that characterizes lung tissue as a series of independent cylinders. The results presented may have clinical implications for (3)He ADC measurements in assessing lung diseases such as chronic-obstructive-pulmonary-disease. 相似文献
18.
Ch.V. Subba Reddy Edwin H. Walker Jr. S.A. Wicker Sr. Quinton L. Williams Rajamohan R. Kalluru 《Current Applied Physics》2009,9(6):1195-1198
Vanadium dioxide nanorods were synthesized through a hydrothermal reaction from V2O5 xerogel, poly(vinyl pyrrolidone) (PVP) and lithium perchlorate (LiClO4). The prepared samples were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical discharge–charge cycling in lithium battery. SEM images reveal the nanorods to have dimensions on the order of 1–3 μm in length and 10–50 nm in diameter. The measured initial discharge capacity of the lithium battery with a cathode made of VO2 (B) nanorods was 152 mA h/g. 相似文献
19.
20.
We study the one-loop effective potentials of the four-dimensional Lifshitz scalar field theory with the particular anisotropic scaling z=2, and the mass and the coupling constants renormalization are performed whereas the finite counterterm is just needed for the highest order of the coupling because of the mild UV divergence. Finally, we investigate whether the critical temperature for the symmetry breaking can exist or not in this approximation. 相似文献