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131.
Gerhard Holxe Titus A. Jenny Petr Nesvadba Albert Gossauer Ludger Ernst Walter Keller Christoph Kratky 《Helvetica chimica acta》1991,74(6):1287-1295
On treatment with H2SO4/MeOH, epimerization of hexamethyl cis-5,6-dihydroxycobyrinate c,8-lactam ( 3 ) takes place quantitatively at C(6), yielding the corresponding trans-diol 4 . The corresponding lactone 7 , whose structure has been established by X-ray analysis, is obtained from xanthocorrinoids 5 and 6 under similar conditions. 相似文献
132.
The first examples of sequential photocycloaddition-rearrangement reactions of naphthalene-tethered resorcinol ethers are described. Bichromophoric aromatic compounds with naphthalene and resorcinol ether moieties were irradiated in the presence/absence of a small amount of acid to give the corresponding cycloaddition-rearrangement products. From the determination of quantum yields, steady-state fluorescence spectral studies, and fluorescence lifetime measurements, the mechanism of this novel photoinduced multistep reaction was elucidated to involve the initial intramolecular exciplex formation, followed by the intramolecular [2 + 2] photocycloaddition between the two aromatic rings and the subsequent acid-catalyzed skeletal rearrangement of the resulting cyclobutane derivative leading to the final products. 相似文献
133.
E. Hoffmann 《Fresenius' Journal of Analytical Chemistry》1962,185(5):372-376
Summary The use of chloranilic acid salts is recommended for the micro determination of aluminium salts, sulphides, thiosulphates, cyanides and fluorides. The methods are based on the reaction of an insoluble chloranilate with the ion to be determined to form an insoluble compound of this ion and an equivalent of liberated chloranilic acid which can be measured. The procedures are simple and rapid.
Zusammenfassung Zur Mikrobestimmung von Aluminiumsalzen, Sulfiden, Thiosulfaten, Cyaniden und Fluoriden wird die Verwendung von Salzen der Chloranilsäure empfohlen. Die Methoden beruhen auf der Reaktion unlöslicher Chloranilate mit dem zu bestimmenden Ion unter Bildung einer unlöslichen Verbindung dieses Ions und Freisetzung einer äquivalenten Menge Chloranilsäure, die gemessen wird. Die Bestimmungen lassen sich einfach und schnell durchführen.相似文献
134.
Most reported examples of transition metal-mediated pericyclic reactions have been shown to proceed by nonpericyclic mechanisms. However, the conversion of Fe(CO)(4)-cyclobutenes to Fe(CO)(3)-butadienes and CO is thought to proceed through a mechanism involving an electrocyclic ring-opening step. We have examined this and possible alternative mechanisms for the parent Fe(CO)(4)-cyclobutene complex using hybrid HF-DFT calculations. In this system there is an inherent preference for a pathway that avoids metal-templated electrocyclic ring-opening, but the pericyclic pathway is predicted to dominate for more elaborate substrates and under certain experimental conditions. 相似文献
135.
Prässler F Hoffmann V Schumann J Wetzig K 《Analytical and bioanalytical chemistry》1996,355(7-8):840-846
An algorithm is presented as a concept for the quantification in direct current and radiofrequency glow discharge (GD) modes for GD optical emission spectroscopy. The algorithm is divided into excitation and sputtering part and thus it is possible to distinguish between the different excitation processes and to consider equivalent sputtering crater formations in both modes. Intensity-time profiles are affected corresponding to the method by several effects. One important effect is that sputtering occurs at a single time in different depths because of curved crater bottoms, this is usually called crater effect. The main purpose is to introduce an iterative deconvolution technique which for the quantification numerically takes into account the curved sputtering crater bottom. Input data for the deconvolution technique are the calibrated mass-time profile, the partial densities of the sample constituents and the measured final shape of the sputtering crater. Using a relatively simple model for ion sputtering the deconvolution technique improves iteratively the calculated layer structure by means of information on crater formation. The mathematical handling is illustrated for the quantification of a depth profile of a multilayer sample of ten 100 nm layers. The resulting concentration-depth profile reflects excellently the real elemental distribution of the multilayer system. 相似文献
136.
A practicable and selective liquid chromatography-mass spectrometry assay for the determination of two cyclooxygenase-2 inhibitors, etoricoxib and valdecoxib, in human plasma is presented. The analytical technique is based on reversed-phase high-performance liquid chromatography (HPLC) coupled to atmospheric pressure chemical ionisation (APCI) mass spectrometry (Finnigan Mat LCQ ion trap). Mass analysis was performed in the positive ion mode. The ion trap was operated in the tandem MS mode (MS2) and the transitions of etoricoxib (m/z 359.2 --> 280.3) and valdecoxib (m/z 315.1 --> 235.1) were followed by selected reaction monitoring. Retention times of etoricoxib and valdecoxib were 1.05 and 1.08 min, respectively. The method was validated over a linear range 10-2500 and 5-1000 microg/L using the other substrate as internal standard. After validation, the method was used to study the pharmacokinetic pro fi le of etoricoxib or valdecoxib in a healthy volunteer after administration of a single oral dose (valdecoxib, 20 mg; etoricoxib, 90 mg). The presented method was suf fi cient to cover more than 90% of the area under the plasma concentration time curve. 相似文献
137.
Regulation of polar cap formation in the life cycle of Escherichia coli 总被引:48,自引:0,他引:48
138.
Stephan Weinbruch Michael Wentzel Manfred Kluckner Peter Hoffmann Hugo M. Ortner 《Mikrochimica acta》1997,125(1-4):137-141
In this paper procedures for the characterization of individual aerosol particles by element mapping in the electron microprobe are presented. The number, size and qualitative chemical composition of particles is derived from a combination of secondary or backscattered electron images and element distribution maps. Accuracy of the size distribution and reliability of the qualitative analysis procedure were checked with silicate samples. In order to obtain a semi-quantitative estimate of the chemical composition of individual particles the count rates taken from element distribution maps are corrected for matrix and geometric effects using particle ZAF procedures.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday 相似文献
139.
The structure of gilmicolin and mycorrhizinol, two new metabolites isolated from culture filtrates of Gilmaniella humicola BARRON , have been shown to be 3 and 15 respectively by spectral and chemical studies. The X-ray analysis of gilmicolin ( 3 ) is also reported. 相似文献
140.