Zur Existenz von zwei Kristallmodifikationen der Mellitsäure

Zusammenfassung Es wurde röntgenorgraphisch und mit Hilfe derDTA, festgestellt, daß die Mellitsäure nur in einer Modifikation, der orthorhombischen¹, vorliegt. Die Darstellung der Mellitsäure^2,3 wurde in Einzelheiten modifiziert. Die Reinheit der Präparate wurde spektroskopisch, im IR- bzw. UV-Gebiet, und mit Hilfe der Papierchromatographie geprüft.

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by means of X-ray analysis and DTA it was found that mellitic acid exists only in the orthorhombic modification. The preparation of mellitic acid was modified in part. The purity ot the samples obtained was checked by IR-and UV-spectroscopy and by paper chromatography.

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Zur Kenntnis von Salzen einiger aromatischer Polycarbonsäuren, 3. Mitt.; 2. Mitt.:B. Hájek, E. Kálalová undF. Petr, Z. Chem.5, 230 (1965).

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Gewidmet dem Andenken an Professor Dr.Hans Meyer meinen Lehrer an der ehemaligen Deutschen Universität in Prag. Prof.Meyer geboren am 31. März 1871 in Wien, gestorben am 28. November 1942 in Theresienstadt, beschäftigte sich eingehend (mitH. Raudnitz u. a.) mit der Darstellung und Untersuchung der Mellitsäure und ihrer Derivate.     

Production of oxygen enriched air by rapid pressure swing adsorption

A novel rapid pressure swing adsorption (RPSA) process is described for production of 25–50% oxygen enriched air. The embodiment consists of one or more pairs of adsorbent layers contained in a single adsorption vessel. The layers undergo simultaneous pressurization-adsorption and simultaneous depressurization-purge steps. A total cycle time of 6–20 seconds is used. The process yields a very large specific oxygen production rate and a reasonable oxygen recovery for production of 20–50 mole% oxygen enriched gas.It is demonstrated by a simple mathematical model of isothermal single adsorbate pressure swing ad(de)sorption concept on a single adsorbent particle that the specific production rate of a PSA process cannot be indefinitely increased by reducing the cycle time of operation when adsorbate mass transfer resistances are finite.     

Pmr spectra and stereochemistry of chiral 2-aryl- and 2-hetaryloxazolidines-1,3

PMR spectra of 2-aryl- and 2-hetaryloxazolidines-1,3 are studied. A PMR criterion is revealed according to which the absolute configuration of the C₍₂₎ atom of the oxazolidine ring can be assigned.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 678–680, May, 1992.     

Immunochemical detection of aminoglycosides in milk and kidney

In 1996, the European Union established provisional maximum residue limits (MRL) for gentamicin, neomycin, streptomycin and dihydrostreptomycin in milk and tissue (0.1-5 mg kg-1). For the detection of these four aminoglycosides, three enzyme linked immunosorbent assays (ELISA) for applications in milk and kidney were developed. The screening of defatted and diluted milk resulted in limits of determination (LDM) of < 0.01 mg l-1. Kidney samples were deproteinized with a trichloroacetic acid solution (3%) and after filtration and the addition of buffer, aliquots were used in the ELISA. The LDM of the four aminoglycosides in kidney were < 0.05 mg kg-1. The ELISA were found suitable for the semi-quantitative screening of milk and kidney for the presence of the four aminoglycosides far below the MRL levels. In randomly taken milk samples (n = 776) and in kidneys derived from healthy pigs (n = 124), the aminoglycoside residues found were far below their established MRL. In eight out of the 94 kidney samples obtained from diseased animals after emergency slaughter, aminoglycoside residues were above the MRL.     

Collisional depolarization of NO2 fluorescence as studied by Hanle effect measurements

We report on collisional depolarization of NO₂ fluorescence with use of Hanle effect (zero magnetic field level crossing) experiments. Single fine structure levels of NO₂ in several regions of the visible absorption spectrum predominantly near 593 nm and 514 nm are prepared by selective optical excitation and the depolarization of the fluorescence light versus a magnetic field is investigated. We find that the Hanle signal is in general a superposition of two Lorentzian shaped signals, each with a characteristic dependence on light intensity and NO₂ pressure. For NO₂ pressures >1 µ bar the collisional depolarization follows simple Stern-Volmer kinetics. However, an unusual pressure dependence is observed at NO₂ pressures <1 µ bar. In the same pressure range (<1 µ bar) we see also an unexpected resonance with significantly different properties as the Hanle signal.     

Kinetic isotope effect in proton exchange processes between methanol and water in inert solvents

The kinetic isotope effect in proton exchange between methanol and water in CCl₄ solution was studied by¹H and²H NMR and kinetic IR spectroscopy. The exchange rate increases in the series CD₃OH+H₂O3OD+H₂O3OD+D₂O. It has been concluded that the limiting step of the reaction involves the formation of an H-bonded cyclic intermediate.

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¹H ²H - CCl₄. , CD₃OH+H₂O3OD+H₂O3OD+D₂O. , - -.

     

alpha and piDL helical states of alternating poly(gamma-benzyl D-L-glutamate) in solution.

As in solid state, strictly alternating poly(gamma-benzyl D-L-glutamate) in solution can adopt two different helical conformations. Besides the alpha helix, a second helical conformation is found at higher temperatures in dioxane and chloroform, the properties of which correspond to that of the piDL4 helix. As the molecules have a finite length a screw sense is favored for both helical forms thus giving rise to optical activity allowing the study of the transconformation by optical rotatory dispersion and circular dichroism besides infrared and dielectric measurements. Thus, as the temperature is raised the equilibria right-left handed alpha helices and alpha-piDL helical forms can be followed. The favored screw senses are determined by the number of interacting side chains for the alpha helix and by the number of hydrogen bonds which are formed in the piDL helical conformation. The side chain-side chain interactions in the alpha helix are experimentally shown to be attractive.     

Investigation of the ceriumIV oxidation of poly(vinyl alcohol)

Initiation of ceric ion-induced grafting from poly(vinyl alcohol) (PVOH) was studied by examining the formation and lifetime of colored complexes between cerium^IV and (1) a commercial PVOH, (2) a diol-free PVOH, and (3) several model 1,2-diols. UV/visible spectroscopy was employed using a wavelength of 520 nm. Results showed that the 1,2-diols, which are an inherent defect in PVOH, are the sites most reactive toward ceric ion. The observed rate constant for the reaction was found to increase with decreasing [Ce^IV] until the maximum rate was reached at which time reaction took place exclusively with 1,2-diols. The activation energy for oxidation of commercial PVOH at the maximum rate was 15.4 kcal/mol while that for diol-free PVOH was 22.4 kcal/mol. The 1,2-diols of PVOH displayed a high equilibrium constant for complexation with ceric ion, compared to the model 2,3-butanediol. It was suggested that the PVOH chain segments bring about crowding of the diol group, and this leads to a higher equilibrium constant for complexation. The lowest energy conformation of a PVOH chain segment containing a 1,2-diol linkage was calculated using the CHEMLAB-II molecular design program, and the 1,2-diol was predicted to be at the center of a sharp bend in the chain brought about by its presence.