Die Erscheinung der Fraktionierung von Kristallgemischen bei der Kataphorese

Zusammenfassung Wir haben eine bei der kataphoretischen Herstellung von Oxydkathoden aufgetretene Erscheinung untersucht und mit Hilfe von Modellversuchen festgestellt, da? bei der Kataphorese von Suspensionen mit K?rnchen von inhomogener Zusammensetzung eine Fraktionierung der einzelnen Kristallsorten eintritt. Bei den Karbonaten des Bariums, Strontiums und Kalziums besteht diese Fraktionierung darin, da? die kataphoretische Abscheidung der K?rnchen mit Ca-Gehalt langsamer ist als die Abscheidung der K?rnchen mit Ba- und Sr- Gehalt.     

Kinetische Untersuchung der basenkatalysierten Chalkonbildung

Zusammenfassung Es wurde die mit Benzaldehyd, 4-Chlor- und 4-Nitrobenzaldehyd in Gegenwart von Natriumhydroxyd ablaufende Kondensation des 5-Nitro-2-hydroxy-acetophenons kinetisch untersucht. Dabei stellte sich heraus, daß die Reaktion reversibel und ferner ihre Teilordnung für alle drei Komponenten 1 ist. Die an dem p-C-Atom zur Aldehydkomponente stehenden elektronenanziehenden Substituenten bewirken Steigerung der Reaktionsgeschwindigkeit und beeinflussen die Werte der Gleichgewichtskonstante und der Aktivierungsenergie.Mit 8 Abbildungen     

A reinterpretation of the mechanism of the simplest reaction at an sp3-hybridized carbon atom: H + CD4 --> CD3 + HD

A comparison between theory and experiment for the benchmark H + CD4 --> HD + CD3 abstraction reaction yields a reinterpretation of the reaction mechanism and highlights the unexpected contribution of a stripping mechanism. Whereas the best analytic surface fails to reproduce experiment, a first-principles direct-dynamics (on the fly) treatment is in good agreement, showing that the H + CD4 reaction exhibits extreme sensitivity to modest differences in the potential energy surface. We find that bent H-D-C transition state geometries play an important role in the dynamics. A simple model that relates the scattering angle impact parameter and cone of acceptance accounts well for the overall reaction dynamics.     

Structural characterization of artificial self-assembling porphyrins that mimic the natural chlorosomal bacteriochlorophylls c, d, and e

We report two crystal structures of a synthetic porphyrin molecule which was programmed for self-assembly. The same groups which ensure that bacteriochlorophylls c, d, and e can self-assemble into the chlorosomal nanorods, the photosynthetic antenna system of some green bacteria, have been engineered into desired positions of the tetrapyrrolic macrocycle. In the case of the 5,15-meso-substituted anchoring groups, depending upon the concentration, by using the same crystallization solvents, either a tetragonal or a layered structure of porphyrin stacks were encountered. Surprisingly, pi-pi interactions combined with extensive dispersive interactions, which also encompass cyclohexane, one of the crystallization solvents, win over putative hydrogen bonding. We are aware that our compounds differ considerably from the natural bacteriochlorophylls, but based upon our findings, we now question the hydrogen-bonding network, previously proposed to organize stacks of bacteriochlorophylls. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS) on various isomeric compounds support our challenge of current models for the chlorosomal antenna as these show structures, astonishingly similar to those of chlorosomes.     

Thin-layer chromatography-densitometry and liquid chromatography analysis of alkaloids in leaves of Papaver somniferum under stress conditions

The effect of stress conditions on the concentrations of secondary metabolites were examined during various developmental stages of Papaver somniferum plants. P. somniferum plants were grown in laboratory conditions (Budakalász). The experiment consisted of 22 treatments. Significantly different alkaloid contents can be observed under different stress conditions. In general, the alkaloid contents of plants are very low; therefore, a highly sensitive and reliable method has to be developed for analysis. The amount of alkaloids was measured by 2 separation and detection techniques. Accuracy of the thin-layer chromatography method for quantitative analysis is limited. Without purification of samples the background is too noisy. Column liquid chromatography is a sensitive and relatively inexpensive method that allows precise quantitative determination of the alkaloid content.     

Effect of the type of ion-pairing reagent in reversed-phase ion-pair chromatography

Summary The retention of positively and negatively charged and neutral solutes was studied in an aqueous phosphate buffer eluent, with constant inorganic-counterion concentration, on ODS-Hypersil stationary phase in the presence of various sulfonic acid ion-pairing reagents. The adsorption isotherms of the ion-pairing reagents d-camphor-10-sulfonic acid, sodium cyclohexylsulfamic acid and sodium alkylsulfonates (butyl-, hexyl- and octyl-) were determined by the breakthrough method.Dedicated to Professor J. F. K. Huber on the occasion of his 60th birthday.