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Jean M. J. Tronchet Christian Cottet Bernard Gentile Eva Mihaly Jean-Bernard Zumwald 《Helvetica chimica acta》1973,56(5):1802-1806
Several sugars with conjugated unsaturation (dienes or α, β-unsaturated carbonyl compounds) have been synthesised by use of Wittig reactions. Keto-sugars bearing a carbonyl group α to a furanose ring are prone to undergo an elimination leading to conjugated unsaturated systems. This constitutes a novel kind of side-reaction in the application of Wittig reactions to carbonyl sugars. The synthesis of a new kind of acetylenic sugar is also described. 相似文献
44.
We present a microequilibrium analysis of a series of amino acid and peptide derivatives containing the chelating bis(imidazol-2-yl)methyl group (BIP, Gly-BIMA, His-BIMA, alpha-Glu-BIMA, gamma-Glu-BIMA, and Phe-His-BIMA). NMR measurements were performed in D2O to follow the deprotonation steps. The software PSEQUAD and a specialized program written in MATLAB were used to determine the macroscopic and microscopic constants. The method assumes that the effect of pH on the chemical shift of an NMR-active nucleus can be interpreted by adding the independent effects of the protonation of individual sites. For derivatives containing histidine (His-BIMA and Phe-His-BIMA), the deprotonation steps of the second imidazole and the His-imidazole significantly overlap. In the Glu derivatives (alpha-Glu-BIMA and gamma-Glu-BIMA), the amino and the second imidazole pK values are separate; the deprotonation processes of the first imidazole nitrogen and the side-chain carboxyl group, however, significantly overlap. In gamma-Glu-BIMA, the deprotonation sequence is carboxylate-imidazole1-imidazole2-amino, while in the case of alpha-Glu-BIMA, it changes to imidazole1-carboxylate-imidazole2-amino, according to the microscopic pk values. The main advantage of the method is that it does not require the synthesis and NMR microequilibrium analysis of substances modeling the individual parts of the target ligand, in contrast to the methods used by others. The method presented here demands slightly more mathematical and computational power, which is readily available today. 相似文献
45.
P. C. Uden Susan M. Bird Mihaly Kotrebai Paula Nolibos Julian F. Tyson Eric Block Eric Denoyer 《Fresenius' Journal of Analytical Chemistry》1998,362(5):447-456
Consumption of selenium enriched plants or yeast-based nutritional supplements has been reported to provide anticarcinogenic
benefits which are selenium compound dependent. Separation and identification of these selenium compounds is critical to understand
the activity. Plants and yeast convert inorganic selenium in the soil or growth media into organoselenium compounds, probably
following a route similar to the sulfur assimilatory pathway. Non-volatile selenium compounds produced include selenoamino
acids, some of which have shown anticarcinogenic activity. Volatile compounds produced by chemical reaction of involatile
precursors have also been found. An ion pair chromatographic method with ICP-MS detection for the separation of selenoamino
acid standards potentially present in real samples is given. The method allows separation of selenoamino acids including such
analytes as the cis-trans isomers of Se-1-propenyl-dl-selenocysteine. The method also provides the capability of determining the presence of selenoxides and possibly selenones,
and tracking of other functionalities and reactions by selective derivatization. Alternatively, selenoamino acids are treated
with ethylchloroformate to produce stable volatile derivatives which are amenable to GC separation with element specific atomic
emission detection (GC-AED). Results of total selenium determination and speciation of selenium enriched yeast-based nutritional
supplements, selenium enriched allium vegetables and bioremediation samples are presented.
Received: 16 February 1998 / Revised: 4 June 1998 / Accepted: 9 June 1998 相似文献
46.
Treated with 2,3-dimethyl-2,3-bis-(hydroxylamino)-butane, aldehydo-dialdofuranoses ( 1 ) gave a mixture of two compounds: a 1,3-dihydroxyimidazolidine ( 2 ) and a 1-hydroxyimidazoline ( 3 ). Oxidation (PbO2) of compounds 3 gave stable free radicals having the structure of 2-C-Glycosyl-4,4,5,5-tetramethylimidazolines 1-oxyl ( 4 ), whereas 2-C-Glycosyl-4,4,5,5-tetramethylimidazolines 3-oxide 1-oxyl ( 5 ) were formed by oxidation of 2 . The ESR. spectra of compounds 4 and 5 establish the structure of the imidazoline part of these radicals and provide informations on the sugar moiety. 相似文献
47.
The surface potential at the silver chloride aqueous interface was measured by means of a single-crystal silver chloride electrode (SCr-AgCl). The measurements were conducted by titration of the KCl solution with AgNO3, and vice versa. The SCr-AgCl electrode potentials were converted to surface potentials psi(0) by setting zero at the point of zero charge at pCl = 5.2. The psi(0)(pCl) function was linear, with a slope 12% lower with respect to the Nernst equation. It was demonstrated that the surface potential at the silver halide aqueous interface could be interpreted by means of the surface complexation model, originally developed for metal oxides. 相似文献
48.
The effect of temperature on the solubility of ionic surfactants was interpreted in terms of standard enthalpy and entropy of dissolution at reference temperature by considering the change in the heat capacity. The significant value of the latter quantity causes the curvature of the function logarithm of equilibrium constant (or solubility) vs. the reciprocal thermodynamic temperature. The solubility data for several sodium n-alkylsulfonates, published by Saito, Moroi, and Matuura, were interpreted by nonlinear regression analysis. It was found that both the enthalpy and entropy of dissolution decrease with the chain length. The heat capacity increases in the course of the dissolution process. 相似文献
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