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151.
Microwave induced delayed phosphorescence (MIDP) measurements have been performed in the 00 bands of the phosphorescence spectrum of benzene dissolved in two phases of cyclohexane. From the relative radiative rates for decay of the three zero-field levels it is concluded that on the average the symmetry of the electronic structure is lower than D2h. In the monoclinic low temperature stable phase of cyclohexane (phase II) the structure is approximately antiquinoidal, and in the metastable phase III it is approximately quinoidal. 相似文献
152.
Pieter Van der VekenIstván Kertèsz Kristel SentenAchiel Haemers Koen Augustyns 《Tetrahedron letters》2003,44(33):6231-6234
(E)- and (Z)-fluoro-olefin analogues of potent dipeptidyl peptidase IV inhibitors were synthesized. A Wadsworth-Horner-Emmons reaction, followed by amide formation and reduction of the amide were used for the construction of the α-fluoro-α,β-unsaturated amine functionality. 相似文献
153.
Electrophoretic mobilities of DNA molecules ranging in length from 100 to 10 000 base pairs (bp) were measured in gels of eleven concentrations of agarose from 0.5 to 1.5%. Excellent fits of the dependence of mobility on DNA length were obtained with the relationship [equation: see text] showing an e(-L/gamma) crossover, where L is the length of a DNA fragment and gamma is a crossover length ranging from 8000 to 12000 bp. The other parameters in the fit are mu(s) the mobility of short DNA with unit charge in the limit as length is extrapolated to zero, and muI, the mobility of long DNA as length is extrapolated to infinity. This exponential relationship should be a useful interpolation function for determining DNA lengths over a wide range. The simplicity of this relationship may be of more fundamental significance and suggests that some common feature dominates the electrophoresis of double stranded DNA fragments in agarose gels, regardless of length. 相似文献
154.
155.
156.
The use of monoenergetic exciting radiation of molybdenum obtained with a secondary target set-up, a conventional water-cooled tungsten anode x-ray tube and a Si(Li) semiconductor detector permits many trace and minor constituents from chlorine to strontium to be determined in particulate material collected on cellulose filters. The method is rapid and economical compared to other multielement techniques. The use of one or more single-element thin-film standards to calibrate for the analysis of many elements appears to be possible, but some systematic errors must be prevented. Absorption of the fluorescent radiation is remedied by a simple correction method. The precision and the accuracy of the method were critically examined by independent analysis of a number of samples with instrumental neutron activation analysis and by interlaboratory comparisons on the same samples. 相似文献
157.
Jean-Claude Grenier Grard Schiffmacher Paul Caro Michel Pouchard Paul Hagenmuller 《Journal of solid state chemistry》1977,20(4):365-379
In order to check the extended defect model previously proposed for the nonstoichiometric perovskite AnMnO3n−1 (n 2), the Ca2Ti2−2xFe2xO6−x solid solution has been studied by means of X-ray diffraction and electron microscope investigations. The results show the existence of a continuous evolution (0 x 1) with two different regions. Close to the CaTiO3 composition (0 x 0.40) the oxygen vacancies seem to be randomly distributed and the symmetry remains pseudocubic. In the composition range 0.55 x 1, the electron microscope patterns show an ordering of the defects: the oxygen vacancies are ordered in the (0k0) planes in strings parallel to the [101] direction. It is concluded from these observations that perpendicular to the Oy direction isolated planes of tetrahedra arranged in rows separate planes of corner-linked octahedra. The n = 2; 2.5; 3, et 4 terms of the AnMnO3n−1 series have been identified. C.S. planes characteristic of classical nonstoichiometric oxides with small cations do not appear: instead, tetrahedra planes succeed to the previous octahedra planes as the degree of nonstoichiometry increases.
Résumé
Afin de vérifier le modèle de non-stoechiométrie proposé dans un travail précédent pour les perovskites lacunaires AnMnO3n−1 (n 2), la solution solide Ca2Ti2−2xFe2xO6−x a été, étudiée par diffraction X et microscopie électronique. Les résultats montrent l'existence d'une évolution continue (0 x 1) avec deux domaines différents. Pour les compositions proches de CaTiO3 (0 x 0.40), les lacunes oxygénées semblent être distribuées statistiquement et la symétrie demeure pseudocubique. Dans le domaine de compositions 0.55 x 1, les images de microscopie électronique montrent un ordre des défauts: les lacunes d'oxygène sont ordonnées dans les plans (0k0) en rangées parallèles à la direction [101]. Ces observations ont permis de conclure que perpendiculairement à la direction Oy des plans isolés de files de tétraèdres séparent des plans d'octaèdres liés par leurs sommets. Les termes n = 2; 2.5; 3 et 4 des séries AnMnO3n−1 ont été identifiés. Des plans de cisaillement rencontrés dans les oxydes non-stoechiométriques classiques de petits cations n'ont pas été mis en évidence: par contre, des plans de tétraèdres se substituent aux plans d'octaèdres initiaux lorsque le degré de non-stoechiométrie augmente. 相似文献158.
159.
The title compounds were obtained by addition of a glycine enolate to a N-(tetrazol-5-yl) imine followed by cyclisation. This route provided both the and the substituted azetidinones. The compounds possessed improved antibiotic activity as compared to the counterparts. 相似文献
160.
C. H. Stam El Sayed El Ashry Yeldez El Kilany H. C. Van DerPlas 《Journal of heterocyclic chemistry》1980,17(3):617-619
X-ray crystallographic data show that the product obtained in the reaction of 4-(2-acetoxy-ethylidene)-4-hydroxy-2,3-dioxobutyro-1,4-lactone-2-(p-bromophenylhydrazone) with methyl-hydrazine is the bicyclic compound 2,6-dimethyl-3,4-dioxo-2,3,4,6,7,8-hexahydropyridazino-[4,3-c]pyridazine 4-(p-bromophenylhydrazone) ( 10 ) and not as originally suggested 1-methyl-3-(1-methylpyrazolin-3-yl)-4,5-pyrazoledione 4-(p-bromophenylhydrazone) ( 8 ). 相似文献