Ionic conductivity in three crystalline phases of LiBH4 under pressure

The AC electrical conductivity of LiBH₄ was investigated below 2 GPa between 1 Hz and 1.6 MHz. The high-temperature phase has an ionic conductivity of up to 0.01 S cm^?1, while the low-temperature phases have conductivities two orders of magnitude lower. All phases show an Arrhenius behaviour with activation energies E _a between 0.5 and 0.7 eV, in good agreement with earlier data except for phase III, which is found to have the highest activation energy of the phases studied. The high-temperature phase has a minimum in E _a near 1 GPa, close to the triple point, correlated with a sudden change in activation volume. These features may indicate an isostructural phase transition. The conductivities of the ambient temperature phases increase temporarily by an order of magnitude after transitions between these phases, probably due to new diffusion channels via structural defects. The phase diagram agrees well with earlier results.     

Thiophene‐cored 2,2‐bis(methylol)propionic acid dendrimers for optical‐power‐limiting applications

The synthesis and characterization of dendron‐coated 2,5‐bis(phenylethynyl)thiophene chromophores are described. The dendrimers were grown divergently on the arylthiophene core with the versatile anhydride of 2,2‐bis(methylol)propionic acid. The arylthiophene core was synthesized with Sonogashira coupling reactions. Structurally well‐defined dendrimers up to the fourth generation were grown, as confirmed by size exclusion chromatography, NMR, and matrix‐assisted laser desorption/ionization time‐of‐flight analysis. The different dendritic substitution did not influence the absorption spectra of the compounds in or near the visible region. Solutions of arylthiophenes had good transparency at wavelengths greater than 400 nm. The dendritic thiophenes exhibited an optical‐power limit at the laser wavelength of 532 nm. However, the magnitude of the optical‐power limit of these compounds was slightly lower than that of a nondendritic arylthiophene with n‐pentyl substituents. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1177–1187, 2005     

Cyclic division algebras

A general example of cyclic division algebra is given, based on a construction of Brauer, yielding examples of division algebras of arbitrary prime exponent without proper central subalgebras, and also noncrossed products of arbitrary exponent. This research was supported in part by the U.S.-Israel Binational Science Foundation. An erratum to this article is available at .     

Trace metal determinations in estuarine waters by electrothermal atomic absorption spectrometry after extraction of dithiocarbamate complexes into freon

Cadmium, copper, iron, lead, nickel and zinc are determined. The dithiocarbamate complexes of the metals are extracted into Freon-TF and back-extracted into dilute nitric acid solution. Portions of the back-extracts are injected into a graphite furnace. The method gives complete separation from the matrix irrespective of salinity. It is therefore useful throughout the full salinity range of an estuary, 0–35‰.The effect of high iron concentrations on the extraction is eliminated by using a mixed acetate buffercomplexing agent solution.     

Carbon-13 NMR studies of twenty-four methyl-substituted morpholines

Carbon-13 NMR spectra of all the isomers of monomethyl-, 2,3-, 2,5-, 2,6-, 3,5-dimethyl-, 2,3,5-, 2,3,6-trimethyl- and 2,3,5,6-tetramethylmorpholine have been obtained at both ambient (25 °C) and low temperature (～ ?100 to ?120 °C). The ring carbon shifts appear to be additive with respect to the position of the methyl groups. A good correlation between predicted and experimental shift values was obtained (r = 0.998₉). The values were used in an attempt to assign, conformationally, the ‘all cis’ isomer 2,3,5,6-tetramethylmorpholine, which from ¹HNMR spin–spin coupling studies has been unsuccessful. Methyl carbon shifts to high field were found for axially oriented carbons. The extracted ‘steric shift’ values for such carbons were compared to their corresponding proton shift data.     

Swelling of PVC latex particles by VCM

The uptake of VCM by PVC particles prepared by emulsion polymerization at various temperatures, the solubility of VCM in water and the density of VCM vapour have been studied between 15 and 60°. The swelling of PVC by VCM was almost independent of temperature. The solvent interaction parameter, x, calculated according to the Flory-Huggins equation, was strongly dependent on polymer concentration and showed a maximum value. The maximum uptake of VCM depended on the temperature of polymerization. PVC samples prepared at 11, 50 and 90° showed maximum uptakes of 0.22, 0.29 and 0.37 kg VCM/kg PVC respectively. The influences of particle size and emulsifier concentration have been studied. The results indicate that VCM is solubilized in the surface layer of adsorbed emulsifier.     

Über die Verteilung organischer Verbindungen zwischen Wasser und organischen Lösungsmitteln

Zusammenfassung Verteilungskoeffizienten einer Anzahl von organischen Verbindungen zwischen Wasser und organischen Lösungsmitteln wurden bestimmt. Die Papierchromatographie aromatischer Verbindungen zeigte, daß der chromatographische Prozeß hauptsächlich ein Verteilungsvorgang ist und für die Schätzung von Verteilungskoeffizienten verwendet werden kann. Mit Hilfe von Papierchromatographie und Verteilungskoeffizienten sind Klassifizierungen von Lösungsmitteln und gelösten Verbindungen ausgeführt worden, welchen die wasserstoffbindenden Eigenschaften der erwähnten Stoffe zugrunde gelegt sind.     

FLASH PHOTOREDUCTION OF FLAVIN MONO NUCLEOTIDE IN NEUTRAL SOLUTIONS ABSOLUTE ABSORPTIVITY OF THE SEMIQUINONE

Abstract— Flash photoreduction of FMN, in aqueous solutions (pH 5–9) leads to the semiquinone (FMNH or FMN). The yield of semiquinone was determined by measuring the amount of leuco-FMN (FMNH₂ or FMNH-) formed in the subsequent disproportionation. The absorption spectrum of the semiquinone, previously known in relative units, has been converted to an absolute basis. Revised values are given for the protolysis constant for FMNH, for the disproportionation rate, and for the formation constant K _F. It is found that K _F is much lower for the anionic than for the neutral form of the semiquinone.