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61.
62.
Andrey Feuerverger Yehuda Vardi 《Annals of the Institute of Statistical Mathematics》2000,52(1):123-138
We further explore the relation between random coefficients regression (RCR) and computerized tomography. Recently, Beran et al. (1996, Ann. Statist., 24, 2569–2592) explored this connection to derive an estimation method for the non-parametric RCR problem which is closely related to image reconstruction methods in X-ray computerized tomography. In this paper we emphasize the close connection of the RCR problem with positron emission tomography (PET). Specifically, we show that the RCR problem can be viewed as an idealized (continuous) version of a PET experiment, by demonstrating that the nonparametric likelihood of the RCR problem is equivalent to that of a specific PET experiment. Consequently, methods independently developed for either of the two problems can be adapted from one problem to the other. To demonstrate the close relation between the two problems we use the estimation method of Beran, Feuerverger and Hall for image reconstruction in PET. 相似文献
63.
It is shown that O(3P) oxidation of aromatic compounds in the liquid phase leading to phenols may involve the intermediacy of triplet diradicals. 相似文献
64.
A new alkaloid designated violatinctamine and four known metabolites were isolated from the tunicate Cystodytes cf. violatinctus collected in Kenya. Violatinctamine has a unique heterocyclic skeleton, which combines a benzothiazole unit and a dihydroisoquinoline unit. The structure of violatinctamine was elucidated by interpretation of MS results as well as 1D and 2D NMR spectra of the alkaloid and of its O,O′-dimethyl derivative. Analysis of the spectral information also implies that violatinctamine exists as a mixture of two tautomers—the imino-phenol and the amino quinone-methide. 相似文献
65.
Three novel compounds, designated kitungolides A (1), B (2), and C (3), were isolated from a soft coral of a new genus collected at Kitungamwe, Kenya. The three new compounds are of a unique heterotricyclic skeleton. The structures and relative stereochemistry of the compounds were elucidated by interpretation of MS, COSY, HMQC, HMBC, and NOESY experiments. [structure: see text] 相似文献
66.
The lowest doublet electronic state of the cyclopentadienyl radical (CPDR) and the lowest singlet state of the cyclopentadienyl cation (CPDC) are distorted from the highly symmetric D(5h) structure due to the Jahn-Teller effect. A valence bond analysis based on the phase-change rule of Longuet-Higgins reveals that in both cases the distortion is due to the first-order Jahn-Teller effect. It is shown that, while for the radical an isolated Jahn-Teller degeneracy is expected, in the case of the cation the main Jahn-Teller degeneracy is accompanied by five satellite degeneracies. The method offers a chemically oriented way for identifying the distortive coordinates. 相似文献
67.
Reaction of cholesterol with silica bound FeCl3 resulted in a mixture of 3β-cholesteryl chloride and dicholesteryl ether.5-Cholestene and hydroxy- and halogeno-substituted cholestane derivatives gave on heating at 100° with this reagent a 1:1 mixture of 20-epimeric diacholestenes. The 20(R)-isomer gave with meta-chloroperbenzoic acid 20(R)-α-epoxide, while the 20(S)-gave a mixture of 20(S)-α- and 20(S)-β-epoxides.5α,6β-Dihydrocholestane reacted with the FeCl3/SiO2 under milder conditions (50°) to give 6β-hydroxy-20(R)-diacholestene, which was converted to the 20(R)-diacholestene. 相似文献
68.
[structure: see text] A novel cyclic peptide, callynormine A, was isolated from the Kenyan marine sponge Callyspongia abnormis and its structure elucidated by interpretation of its NMR data and X-ray diffraction analysis. Callynormine A represents a new class of heterodetic cyclic peptides (designated endiamino peptides) possessing an alpha-amido-beta-aminoacrylamide cyclization functionality. 相似文献
69.
Heinz Falk Günther Vormayr Leon Margulies Stephanie Metz Yehuda Mazur 《Monatshefte für Chemie / Chemical Monthly》1986,117(6-7):849-858
The linear dichroism of several pyrromethene, pyrromethenone and bilatriene-abc derivatives embedded in a polyethylene film and oriented by stretching is investigated. On dissolving these pigments in the polymer matrix no changes in their conformations are observed, as compared to solutions in apolar solvents. Polarisation of the absorption bands of the pigments is deduced from the polarisation spectra. To enhance the solubility and orientation in the polymer, a pyrromethenone and a bilatriene-abc bearing a C18-alkyl chain were synthesised for the first time.
Zum Lineardichroismus von Pyrromethen-, Pyrromethenon- und Bilatrien-abc-Derivaten
Zusammenfassung Der Lineardichroismus einiger Pyrromethen-, Pyrromethenon- und Bilatrienabc-Derivate, die in eine Polyethylenfolie eingebettet vorliegen, wird untersucht. Bei der Lösung dieser Pigmente in der polymeren Matrix wird keine Veränderung der Konformation im Vergleich mit Lösungen in apolaren Lösungsmitteln beobachtet. Aus den Polarisationsspektren wird die Polarisation der Absorptionsbanden dieser Chromophore abgeleitet. Ein Pyrromethenon und ein Bilatrien-abc mit C18-Alkylseitenkette wurden zur Verbesserung der Löslichkeit und Orientierung in der Polymerfolie erstmals dargestellt.相似文献
70.
Alkaline phosphatase, APase, (EC 3.1.31) from calf intestine, after shifting the equilibrium by effector molecules towards
the dimeric form of the enzyme, was coupled (ratio 1:2, protein: copolymer) to a copolymer of ethylene and maleic anhydride,
EMA. The water-soluble APase-EMA was separated from APase and the unbound EMA by DEAE-cellulose ion exchange chromatography.
The specific activity of the APase-EMA, compared to APase, increased 26-fold at pH 7.1 and 10-fold at pH 8.6. The pH optimum
of APase-EMA was shifted down from pH 9.5 (native APase) to 8.6. This change could be interpreted in terms of polyelectrolyte
theory. APase-EMA retained 50–70% of its optimum activity in the pH range 7–8, while APase retained only 5–15% of its optimum
activity within the same pH range. Its isoelectric point, pI, was 4.2 (APase 6.0) and it migrated on polyacrylamide gel electrophoresis
in a single band, anodic movement twice as fast as APase. Parallel with the kinetic measurements, the reactive-enzyme sedimentation
method was used to measure S20,w values. S20,w values obtained for APase-EMA, activated APase, and APase dialyzed against wafer were 6.56S, 6.46S, and 5.17S, respectively.
Molecular weights, Mr, were determined by equilibrium sedimentation: the values obtained were 180,000, 160,000, and 84,500. Mr values of APase-EMA and APase (native) estimated by Sepharose-4B gel filtrations were essentially the same. The above-mentioned
values remained unchanged for APase-EMA after intensive dialysis against water, whereas for the activated APase, separation
from the effector molecules caused the equilibrium to shift back to the monomeric, very slightly active enzyme with concomitant
changes of S20,w to 5.15 and Mr to 82,000. 相似文献