Influence of substituents on positron annihilation in styrene copolymers

Results of angular correlation of annihilation radiation (ACAR) and positron annihilation lifetime (PAL) measurements are presented for five styrene copolymers: poly(co-styrene-phenylmaleimide) and its three derivatives with chlorine as well as for one with the OH group substituted at the benzene ring. It occurs that the chlorine substituted at three different positions at the benzene ring poly(co-styrene-o (or-m,-p)-chlorophenylmaleimide) inhibits the formation of the positronium to different extent. The greatest effect is observed in case of the chlorine atom substituted at the benzene ring at the ortho-position towards the nitrogen atom. The two long lifetimes (₃ and ₄) observed in the lifetime spectra are connected with the bimodal distributions of the free volume radius in the samples.     

Silicate Xerogels with Dopant-Induced Chirality

A series of silicate xerogels with entrapped chiral amino acids have been obtained via sol-gel technology. The transparent, glassy samples obtained exhibit chirality in the bulk due to the presence of the entrapped asymmetric molecules. Measurements of the optical activity of the doped xerogel samples revealed that the entrapment did not significantly influence the optical activity observed for liquid solutions of the amino acids. Thus, the sol-gel method enables the preparation of amorphous optical materials exhibiting properties of strictly spatially defined molecular systems. Apart from the obvious optical applications, such porous materials with asymmetric centers might find interesting applications in chiral chemical syntheses and separations.     

Monitoring of the Viability of Cells Immobilized by Sol-Gel Process

Three different types of cells, Pseudomonas fluorescens HK44, Saccharomyces cerevisiae strain SP4 and plant cells Nicotiana tabacum L. BY-2, were immobilized by entrapment in tetramethoxysilane prepolymer (TMOS) gel or in composite gel containing prepolymer TMOS and alginate in various ratios. Their growth and viability were monitored by bioluminescence and 2-D fluorescence spectra, which are fast and do not need the dissolution of a matrix. The resulting biocomposite gels were obtained by gelation of the mixtures of TMOS prep. or TMOS/alginate sols and the particular cells in proper media on glass supports to provide films ～1 mm thick. The effect of the following parameters on the growth and viability of the cells was studied: (a) the composition of the biocomposites, (b) the preparation conditions of TMOS and (c) the conditions of the procedure of entrapment. All three types of cells were tested in TMOS gel and the composite TMOS/alginate = 1:1 (v/v). The sensitivity of the cells to the changes of conditions increased in the sequence: P. fluorescence HK44 < S. cerevisiae strain SP4 < N. tabacum L. BY-2. Cell viability decreased with the increasing content of Si in biocomposites. The entrapment into alginate–silica composites resulted in the leakage of microbial and yeast cells. However, it had positive effects on the growth and metabolic activity of plant cells.     

A Robust and Efficient Method for the Computation of Equilibrium Composition in Gaseous Mixtures

This paper presents in detail a robust, efficient and accurate methodology for the computation of equilibrium composition in gaseous mixtures. The methodology is founded on the concept of the chemical basis, which is defined and formalized using a powerful matricial approach. The method is specially designed to be general, thus providing basic thermodynamic data in several areas, such as combustion, plasma chemistry and, more generally speaking, computational fluid dynamics. The performance of the method is given in terms of CPU usage and the computed results are compared with those in the published literature. The method is shown to yield results of very high quality in terms of accuracy and smoothness.     

Palladium-catalyzed carbon-carbon bond-forming 1,2-ligand migration of organoalanes

A one-pot, two-step process that transforms terminal alkynes into ethyl methyl-substituted benzylic quaternary carbon centers is described. (E)-2,2-Disubstituted-1-alkenyldimethylalanes have been shown to participate in 1,2-alkyl migration from aluminum to carbon with concomitant arylation at the 2-position to furnish ethyl methyl-substituted benzylic quaternary carbon centers, when reacted intramolecularly with aryl halides and triflates in the presence of a Pd(0) catalyst. The protocol is initiated with Cp2ZrCl2-catalyzed methylalumination of terminal alkynes followed by palladium-catalyzed intramolecular arylation of the resulting (E)-2,2-disubstituted-1-alkenyldimethylalanes, leading to 1,2-methyl shift from aluminum to carbon. In that sequence, a total of three new C-C single bonds are made, and two of the three alkyl groups on Me3Al transferred to the substrate on vicinal carbons. This method was applied to a variety of substrates, and the mechanism was investigated by deuterium-labeling experiments, which revealed that protodealumination of the final dialkylaluminum triflate or halide intermediates by CH3CN results in the formation of the fourth bond in the course of the transformation.     

Empirische Beziehungen zur Sauerstoffkoordination um Antimon(III) und Tellur(IV) in Antimoniten und Telluriten

The strong variability of bond length between Sb(III) or Te(IV) and oxygen is correlated with distances to more distant oxygen atoms in the trans position. Closer approach of the trans-oxygen atom leads to lengthening of the bond in question. Hyperbolic functions represent suitable approximations for the correlations and give the bond lengths on the average within 0.04 Å. The distance relations are discussed for various limiting coordinations corresponding to 3, 4, and 5.