Investigation of the formation of poly(ethylene terephthalate) with model molecules: Kinetics and mechanisms of the catalytic esterification and alcoholysis reactions. II. Catalysis by metallic derivatives (monofunctional reactants)

The reactions of model molecules were considered to study the kinetics and the mechanism of the esterification and the alcoholysis reactions catalyzed by various metal compounds (Li, Na, K, Zn, Co, Mn, Ti). Ti was found to be the most active catalyst for both reactions and to be acting via a different mechanism (concerted). The possible structure of the active titanium intermediate was investigated by means of ¹H- and ¹³C-NMR spectroscopy, FT–IR spectroscopy, and electroconductivity.     

Chelate complex Ni(S2C2H2)2 as a molecular model of the hydrodesulfurization active center

Electronic states of Ni atom in a square-planar complex Ni(S₂C₂H₂)₂ and its molecular adduct with H₂S were studied by means ofab initio molecular orbital calculations. H₂S adsorption stabilizes the Ni(IV) state (d⁶) in the complex with the Ni atom shifted from the plane by 0.35 ?.     

Fluorination of multiwall nitrogen-doped carbon nanotubes

A film of oriented nitrogen-doped multiwall carbon nanotubes was grown on a silicon substrate as a result of the thermolysis of an acetonitrile + ferrocene mixture. The fluorination of the film by BrF₃ vapor at room temperature removed the substrate; however, the vertical orientation of the nanotubes was not destroyed. Analysis of micrographs of a fluorinated sample obtained with a high-resolution transmission electron micro-scope showed that only the surface walls of the nanotubes were fluorinated. The fluorine concentration of the product as determined from X-ray photoelectron spectroscopy was about 16%. A comparison of the N1s spectra of the starting and fluorinated samples showed that the nitrogen atoms of CN_x nanotubes changed their electronic state as a result of fluorination. Matching of the X-ray photoelectron spectroscopic data with the results of quantum-chemical calculations for fragments of fluorinated nitrogen-doped nanotubes showed that fluorine atoms preferred to attach to pyridine-like nitrogen atoms or to carbon atoms in the ortho or meta positions relative to a nitrogen atom.     

Effect of the calcination temperature on the properties of Fe2O3/SiO2 catalysts for oxidation of hydrogen sulfide

The effect of the calcination temperature on the properties of supported iron oxide catalysts for hydrogen sulfide oxidation prepared by impregnation of silica with iron(III) nitrate has been studied. An increase in the calcination temperature was found to diminish the catalytic activity of the Fe₂O₃/SiO₂ catalysts in hydrogen sulfide oxidation. This behavior can be explained by the agglomeration of iron oxide particles and by a decrease in the surface concentration of active sites. It has been shown that an increase in the calcination temperature makes the catalyst more stable towards the sulfidation of the active component (Fe₂O₃) to the iron disulfide phase.     

Electrochemical synthesis and physical-chemical characterization of the low-oxidized graphite fluoroxide materials

A process of anodic oxidation of purified natural graphite in aqueous 11-45% HF solutions has been studied at constant voltages 5-7 V. New types of low-oxidized graphite fluoroxide materials (FOG-E) were isolated, prepared under electrochemical synthesis conditions only. Electrochemical oxidation of graphite should include in a first step the reaction of evolving oxygen and O-containing groups with surface carbon atoms on graphite particles and a subsequent formation of C-OH bonds, and in a second step, a partial substitution of the latter groups into C-F groups. Chemical and physical-chemical study of FOG-E showed that they are multiphasic materials. The low-oxidized graphite fluoroxide materials are excellent precursors for the preparation of thermally expanded graphite (TEG).     

Determining the orientations of BrF3 and FeBr3 molecules in graphite fluoride matrices using the XANES and EXAFS polarization dependences

Synchrotron radiation was used to measure the EXAFS and XANES polarization dependences for intercalation compounds of graphite fluoride. An approach is developed which allows one to analyze the orientation of molecules of arbitrary shapes using XANES and EXAFS data. Analyzing the orientation dependences of BrK XANES spectra for the T-shaped BrF₃ molecules, we determined possible combinations and admissible ranges of angles between the normal to the graphite fluoride matrix planes and the Br−F bond directions (α=52–90°, β=27–82°) and between the normal to the matrix planes and the molecular planes (γ=27–53°). The average orientation angles obtained by the combined analysis of the EXAFS and XANES data are as follows: α=62±1.5°, β=58±1.5°, γ=45±1.5°. The interatomic distances Br−F, Br−Br, and Fe−Br are determined. It is established that thermal treatment, which recovers the X-ray diffraction pattern from the unfilled matrix, does not affect the predominant orientation of the BrF₃ molecules. This suggests that the thermally treated graphite fluoride matrix contains thin layers of ordered molecules. The absence of the polarization dependence of the spectra of FeBr₃ in graphite fluoride allows the assumption that the molecular planes are oriented with respect to the normal to the matrix planes at a “magic” angle of 35°. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp. 1020–1029, November–December, 1995. Translated by I. Izvekova     

A survey of predicate realizability logic

The paper contains a survey of the results and methods of studying interpretations of predicate formulas based on constructive semantics of the first-order language of arithmetic and its extensions.