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  • 1
    ISSN: 1588-2837
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Abstract Адсорбция CO, обогащенного изотопом C13, была исследована на катализаторе CoO−MgO, содержащем 3 ат.%-a Co. Дискутируэтся вопрос о том, что сигнал с g 1,2 1 =2,0054±0,0005, g3=2,0020±0,0005, A1=33 эрс, A2=11 эрс и A3=63 эрс, появляющийся в ходе адсорбции, вероятно следует приписывать не анион-радикалам CO 2 − или CO 3 − , а поверхностным комплексам X−CO со структурой, подбной карбониьной.
    Notes: Abstract Adsorption of CO enriched with13C isotope was investigated on a CoO−MgO catalyst containing 3 at.% Co. It is argued that the ESR signal with g1,2=2.0054±0.0005; g3=2.0020±0.0005; A1=33 0e*; A2=11 0e; A3=63 0e, appearing in the course of adsorption may not be ascribed to CO 2 − or CO 3 − anion radicals but should be attributed to surface X−CO complexes of a carbonyl-like structure.
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  • 2
    ISSN: 1588-2837
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Abstract Адсорбция CO на цеолите Rh/NaY была исследована с помощью метода ЭПР, используя окись углерода, обогащенную C13. Неспаренный электрон комплекса делокализуется на ядре родия и, как было показано, одна молекула CO образуется при адсорбции. Плотность неспаренного электрона в молекуле CO равна приблизительно 0,25.
    Notes: Abstract Adsorption of CO on a Rh/NaY zeolite has been studied by the ESR method using carbon monoxide enriched in13C. A complex with one unpaired electron delocalized over a rhodium nucleus and a CO molecule is shown to form upon adsorption. The unpaired electron density on the CO molecule is found to be about 0.25.
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  • 3
    ISSN: 1588-2837
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Conclusion The above developed ideas should not be regarded as criticism of the carbenium ionic conception in heterogeneous acid catalysis. They are aimed rather at its modernization. We consider the present form of this theory as oversimplified. Owing to the absence of solvation effects, surface intermediates of high temperature catalytic conversions of hydrocarbons are of much less ionic character than it is usually believed. Therefore, the carbenium ionic properties are characteristic of transition states but not stable intermediates of these reactions. Another feature of heterogeneous acid catalysis that is not usually taken into account is the bifunctional nature of its active sites. The acidic part protonates adsorbed molecules and the basic one either stabilizes intermediate structures or enables a reversible proton abstraction from transition states. In the case of substrates with a low proton affinity, this results in concerted mechanisms, where simultaneously with a proton transfer a new bond with the surface is formed. Deprotonation of intermediates in the case of catalytic transformations of hydrocarbons with a higher proton affinity supplies them with some features of adsorbed carbenes or ylides. This makes it possible to suggest new mechanisms of such acidic catalytic reactions as synthesis of hydrocarbons from methanol on high silica containing zeolites, olefin oligomerization, etc.
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  • 4
    ISSN: 1588-2837
    Keywords: Hydrolysis ; oxonium ions ; ethyl sulfates ; rapid exchange of protons
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract “In situ” NMR study of ethyl alcohol interaction with sulfuric acid of various concentrations confirmed that in accordance with chemical experience the main reaction product in an excess of concentrated acid is the mono-ethyl sulfate. Only traces of diethyl sulfate are formed in a large excess of 95% acid. In addition, ethyloxonium ions and protonated esters, which rapidly exchange protons with solvating water molecules, are detected as the reaction intermediates. The13C chemical shifts of methylene fragments in protonated esters and ethyl oxonium ions are more than 10 ppm higher than in neutral molecules. The study of reaction products at different sulfuric acid concentrations indicated that similarly to dissociation of concentrated sulfuric acid, hydrolysis of mono-ethyl sulfate requires at least two water molecules.
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  • 5
    ISSN: 1588-2837
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Microporous silica gels containing small amounts of titanium ions have been synthesized. Porous structure and surface active sites of silica gels have been studied. It has been found that the titanium ions are distributed uniformly in silica gel matrix, substituting silicon ions isomorphically.
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  • 6
    ISSN: 1608-3210
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The vibrational and vibrational–rotational spectra of hydrogen adsorbed on zeolite BaX at 77 K were examined by diffuse-reflectance IR spectroscopy over a wide range of wavenumbers. The use of hydrogen enriched in the para isomer and of molecular deuterium allowed us to reliably assign the absorption bands to orthohydrogen and para-hydrogen and compare the experimentally observed band splitting with the theoretically calculated shifts of the corresponding levels resulted from the Stark effect. In the calculations, an adsorption site was simulated as a positive point charge. Although this approximation is rough, the IR spectra of adsorbed hydrogen and deuterium are adequately described in terms of the Stark effect. A decrease in the frequency of stretching vibrations of H–H (D–D) bonds in the molecules coordinated to Ba2+ cations, a decrease in the intensity of vibrational–rotational absorption bands, and the band splitting in the spectra of adsorbed para-hydrogen and orthohydrogen were explained.
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  • 7
    ISSN: 1572-879X
    Keywords: NMR ; tert-butanol ; sulfuric acid ; carbenium ions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract An in situ NMR study of tert-butanol dehydration in 95 and 85% sulfuric acid did not indicate the formation of tert-butyl cations as active reaction intermediates. Instead, only NMR lines from oxonium ions under the condition of fast exchange with water and the acid were observed. It is most likely that, in the concentrated acid, the active intermediates of this reaction are represented by tert-butyl sulfuric ester which probably is a precursor of invisible tert-butyl carbenium ions representing short-lived excited or transition states.
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  • 8
    ISSN: 1572-879X
    Keywords: High silica zeolites ; alkoxy groups ; quantum chemistry ; carbenium ions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract An ab initio quantum chemical study of the mechanism of formation of surface ethoxy groups from ethylene on high silica H-zeolites was carried out using a Gaussian 80 program with the 3–21 G basis set. The obtained results show that the reaction coordinate during ethoxylation of the zeolite surface is rather complicated and that in the transition state the charge distribution and geometry of the ethyl fragment closely resemble those of the classical form of the free ethyl carbenium cation. The mechanism of some acidically catalyzed transformations of olefins over H-zeolites is discussed on the basis of these results.
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  • 9
    ISSN: 1572-879X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract We report in the first IR observation of carbene complex Mo=CH2 on the surface of Mo/SiO2 metathesis catalyst. Mo=CH2 species were produced by cyclopropane chemisorption on catalysts, photoreduced in CO (Mo(IV)/SiO2), and are characterized in IR by C-H stretching vibrations at 3080 and 2945 cm−1.
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  • 10
    ISSN: 1572-879X
    Keywords: CO chemisorption on Pt ; carbon-Pt bond ; IR diffuse reflectance spectroscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The unusual properties of small (1–2 nm) Pt particles within a zeolite support are documented by diffuse reflectance infrared spectroscopy of the metal-carbon bond of linearly adsorbed CO. The wave number of the metal-carbon stretching frequency is found to be about 60 cm−1 higher compared to larger particles (2–3 nm) or to the bulk metal. The stronger bond is attributed to a negative charge on the small metal particles.
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