Stereoselective effects in the solid-phase hydrogenation of unsaturatedl-hydroxyproline derivatives

The solid-phase catalytic hydrogenation of (R-4tert-butoxy-Δ¹-pyrroline-2-carboxylic) acid under the action of hydrogen spillover was studied. The reaction proceeds stereoselectively with the predominant formation of thel-amino acid. The configuration of the asymmetric center formed is determined by that of the asymmetric C(4) atom. The major portion of the isotope label is incorporated into the allylic C(3) and C(5) positions, and the β-H atoms are more mobile. Using quantum-chemical calculations, the geometric structure of thel-hydroxyproline molecule was calculated, and the spin-spin coupling constants for this tritium-labeled amino acid were determined. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1820–1823, October, 1997.     

Flotation-photometric determination of silicon in the form of chromopyrazole II molybdosilicate

The process of concentrating chromopyrazole II alpha- and beta-molybdosilicates by flotation with mixtures of toluene with polar solvents (acetone, ethanol, isoamyl alcohol, cyclohexanol or cyclohexanone) has been studied. It was found that the composition of the complex depends on the acidity of the medium. The complex was isolated from 1.5M sulphuric acid. On the basis of elemental analysis the ratio of chromopyrazole II to molybdosilicate was found to be 2:1. Infrared spectroscopy was used to establish that an ion-association complex is formed. The results were used to establish a method for determination of ultratrace levels of silica (2 mug/l.) in high-purity water. The relative standard deviation is 7-11%.     

Incompressible electron phase in field-effect transistors

The formation of regions of compressible and incompressible phases in the quantum Hall effect regime has been considered for a two-dimensional (2D) electron system that is created in a field-effect transistor. This effect arises from long-period fluctuations of the density of ionized donors supplying electrons to the 2D system. It is shown that the motion of these regions caused by variations of the average electron density gives rise to minima in the capacitance of the capacitor formed by the 2D electron system and the transistor gate. When the corrections to the capacitance are small, the shape of the minima reproduces the donor density distribution function. Experimental data are presented that demonstrate good agreement with the predictions of the model.     

Study of the products of platinum-catalyzed hydrogenation of chlorinated nitro and amino aromatic compounds at high degree of dehalogenation

The kinetics of formation and consumption of the dehalogenation products during hydrogenation of 3,4-dichloronitrobenzene under static and gradient-free flow conditions and hydrogenation of p-chloroaniline under static conditions were studied in the presence of platinum supported on active carbon. In the former case, p-chloroaniline and aniline are the primary dehalogenation products; in the presence of the catalyst, they can react with each other giving biphenyl derivatives, whose synthesis and accumulation in the reaction mixture occur with a considerable delay and have not been investigated before.     

Synthesis and structure of new thorium phosphate CaGdTh(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Phosphate CaGdTh(PO₄)₃ was prepared by thermal treatment of a mixture of oxides. The final temperature was 1400°C. The phosphate was characterized by powder X-ray diffraction analysis and IR spectroscopy. The crystal structure was studied by the Rietveld method. The compound crystallizes in the monazite structure type (sp. gr. P2₁/n). A comparative analysis of the structures of this phosphate and cerium orthophosphate CePO₄ was carried out.     

Liquid-phase catalytic hydrogenation of 3,4-dichloronitrobenzene over Pt/C catalyst under gradient-free flow conditions in the presence of pyridine

Experimental data on nitro compound uptake, the intermediate product accumulation, and the corresponding amine compound generation were obtained on hydrogenating 3,4-dichloronitrobenzene over Pt/C catalyst in the gradient-free flow regime in the presence and absence of pyridine. In addition, a side reaction of dehalogenation was investigated. The role of pyridine admixture on every step of the process was analyzed and the rate of hydrogenation of the nitro compound was determined both in the presence and in the absence of inhibitor.

     

Recrystallization of natural chrysoberyl in multicomponent melts

Chrysoberyl and alexandrite crystals have been grown from solutions in melts based on the Li₂CO₃-MoO₃, Bi₂O₃-MoO₃, PbO-V₂O₅, Na₂B₄O₇, and K₂MoO₄-MoO₃ systems using natural alexandrite and chrysoberyl debris as the initial BeAl₂O₄ compound. An analysis of the morphology and homogeneity of the crystals grown has revealed the Bi₂O₃-MoO₃ solvent to be the most appropriate. The optimal color characteristics (??quality?? of alexandrite effect) manifest themselves when adding about 5 mol % Cr₂O₃. The largest crystals (up to 10 mm in size) were obtained from a solution in melt based on PbO-V₂O₅ at a ratio of the crystal-forming component to the solvent of 9: 91 wt %; These characteristics, along with a relatively low operating temperature (970°C), give grounds to consider this type of solvent promising.     

Simulation of soliton solutions for the propagation of a femtosecond pulse in a medium with a cubic nonlinearity

Soliton solutions are constructed numerically for the problem of propagation of a femtosecond pulse in a medium with a cubic nonlinearity. The problem is posed as an eigenvalue problem with an operator nonlinear in the eigenfunctions. For given values of the propagation parameter we find the real eigenvalue λ and the corresponding eigenvector. This eigenvector is a soliton, i.e., a solution that does not vary in the coordinate of propagation of the light pulse. An algorithm is proposed to find the minimum eigenvalue and the corresponding eigenfunctions that satisfy given conditions. Translated from Prikladnaya Matematika i Informatika, No. 2, pp. 63–68, 1999.     

Study of the solid-state hydrogen isotope exchange ofl-alanine

The solid-state reaction of isotope exchange ofl-alanine (l-Ala) with spillover-hydrogen activated on a Rh(Pd)-supported catalyst was studied. The reactivity of the carbon atoms and the activation energies of isotope exchange of the hydrogen at the C(2) and C(3) atoms of thel-Ala molecule were determined using tritium NMR. Theab initio calculations of the activation energy of a model reaction between the alanine molecule and a hydroxonium cation were carried out. The mechanism and plausible structures of the transition states of this reaction were proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 757–762, April. 1997.     

Synthesis of biotinylated pentasaccharide structurally related to a fragment of glucomannan from Candida utilis

Russian Chemical Bulletin - The polysaccharide mannan is the main surface antigen of the cell wall of Candida fungi, playing an important role in the pathogenesis of diseases caused by these...