Formation of α-Mn2O3 nanorods via a hydrothermal-assisted cleavage-decomposition mechanism

A hydrothermal cleavage-decomposition mechanism was used to synthesize single-crystal α-Mn₂O₃ nanorods at 160 °C for 16 h using KMnO₄ as manganese source and CTAB as reducing regent. The as-synthesized products were characterized by powder X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy and infrared spectrum. The results indicate that the reaction temperature is a crucial factor for the formation of α-Mn₂O₃ nanorods. These nanorods exhibit single-crystal nature, and have an average diameter of 36 nm and lengths of up to 1 μm. Based on our experimental results, a hydrothermal cleavage-decomposition mechanism has been proposed on the formation of α-Mn₂O₃ nanorods.     

Electron Probe Microanalysis of HfO2 Thin Films on Conductive and Insulating Substrates

Quantitative electron probe microanalysis of highly insulating materials is a complicated problem, partially solved by coating samples with grounded thin conductive layers or using novel scanning electron microscopy (SEM) techniques, such as low-voltage and/or variable pressure SEM. In this work, some problems of quantitative X-ray microanalysis of thin HfO₂ films, in particular the possibility to determine mass thickness correlated to the density of the layer material, are discussed. For comparison, Al₂O₃, Ta₂O₅ and TiO₂ films grown onto both semiconductive Si and insulating quartz substrates were also analysed. All the films studied were synthesized by atomic layer deposition method.     

Role of the surface heterogeneity in adsorption of hydrogen ions on metal oxides: theory and simulations

In this article we study the effect of energetic heterogeneity of a crystalline surface on the adsorption of hydrogen ions (protons) from the liquid phase. In particular, we examine the influence of the shape of the adsorption energy distribution on the equilibrium isotherms of hydrogen ions. To that purpose, a few popular distribution functions, including rectangular, exponential, and asymmetric Gaussian are considered. Additionally, multimodal distribution functions, which may correspond to the adsorption on different crystal planes of the oxide, are also used. Lateral interactions between adsorbed charges are modeled using the potential function proposed by Borkovec et al., which accounts also for polarization of the liquid medium. The results presented here are obtained using both Monte Carlo (MC) simulations and theoretical calculations involving Mean Field Approximation (MFA). They indicate that increased energetic heterogeneity of the adsorbing surface may, in general, considerably change the behavior of the adsorption isotherms, regardless of the assumed distribution function. It is also shown that the predictions of the proposed theory are consistent with the data obtained from the MC simulations.     

A new method for the synthesis of diorganylvinylphosphine oxides

Diorganylvinylphosphine oxides were synthesized in 31–38% yields on heating (50°C) diorganylphosphine oxides with vinyl sulfoxides or divinyl sulfone in the presence of KOH. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1895–1896, October, 1997.     

The surface and materials science of tin oxide

The study of tin oxide is motivated by its applications as a solid state gas sensor material, oxidation catalyst, and transparent conductor. This review describes the physical and chemical properties that make tin oxide a suitable material for these purposes. The emphasis is on surface science studies of single crystal surfaces, but selected studies on powder and polycrystalline films are also incorporated in order to provide connecting points between surface science studies with the broader field of materials science of tin oxide. The key for understanding many aspects of SnO₂ surface properties is the dual valency of Sn. The dual valency facilitates a reversible transformation of the surface composition from stoichiometric surfaces with Sn⁴⁺ surface cations into a reduced surface with Sn²⁺ surface cations depending on the oxygen chemical potential of the system. Reduction of the surface modifies the surface electronic structure by formation of Sn 5s derived surface states that lie deep within the band gap and also cause a lowering of the work function. The gas sensing mechanism appears, however, only to be indirectly influenced by the surface composition of SnO₂. Critical for triggering a gas response are not the lattice oxygen concentration but chemisorbed (or ionosorbed) oxygen and other molecules with a net electric charge. Band bending induced by charged molecules cause the increase or decrease in surface conductivity responsible for the gas response signal. In most applications tin oxide is modified by additives to either increase the charge carrier concentration by donor atoms, or to increase the gas sensitivity or the catalytic activity by metal additives. Some of the basic concepts by which additives modify the gas sensing and catalytic properties of SnO₂ are discussed and the few surface science studies of doped SnO₂ are reviewed. Epitaxial SnO₂ films may facilitate the surface science studies of doped films in the future. To this end film growth on titania, alumina, and Pt(1 1 1) is reviewed. Thin films on alumina also make promising test systems for probing gas sensing behavior. Molecular adsorption and reaction studies on SnO₂ surfaces have been hampered by the challenges of preparing well-characterized surfaces. Nevertheless some experimental and theoretical studies have been performed and are reviewed. Of particular interest in these studies was the influence of the surface composition on its chemical properties. Finally, the variety of recently synthesized tin oxide nanoscopic materials is summarized.     

Novel synthesis of 4-halo-3-hydroxy-2-pyrone: one pot rearrangement-cyclization reaction by magnesium halide

Treatment of acetonide protected 4,5-dihydroxy-2-chloroglycidic ester or its rearrangement product, the acetonide protected 4,5-dihydroxy-3-chloro-2-oxo ester, with magnesium halides gave 4-halo-3-hydroxy-2-pyrone in excellent to reasonable yields in one pot. The mechanism of this novel one pot rearrangement-cyclization reaction is also proposed.     

An efficient route to regioselective opening of N-tosylaziridines with zinc(II) halides

An efficient route for the regio- and stereoselective ring opening of N-tosylaziridines with zinc dihalides (ZnX₂, X = Cl, Br, I) is described. Depending on the solvent and Zn(II) halide, β-halo amines or imidazolines are obtained selectively in good to excellent yields.     

Towards supramolecular fixation of NOX gases: encapsulated reagents for nitrosation

The use of simple calix[4]arenes for chemical conversion of NO2/N2O4 gases is demonstrated in solution and in the solid state. Upon reacting with these gases, calixarenes 1 encapsulate nitrosonium (NO+) cations within their cavities with the formation of stable calixarene-NO+ complexes 2. These complexes act as encapsulated nitrosating reagents; cavity effects control their reactivity and selectivity. Complexes 2 were effectively used for nitrosation of secondary amides 5, including chiral derivatives. Unique size-shape selectivity was observed, allowing for exclusive nitrosation of less crowded N-Me amides 5 a-e (up to 95 % yields). Bulkier N-Alk (Alk>Me) substrates 5 did not react due to the hindered approach to the encapsulated NO+ reagents. Robust, silica gel based calixarene material 3 was prepared, which reversibly traps NO2/N2O4 with the formation of NO+-storing silica gel 4. With material 4, similar size-shape selectivity was observed for nitrosation. The N-Me-N-nitroso derivatives 6 d,e were obtained with approximately 30 % yields, while bulkier amides were nitrosated with much lower yields (<8 %). Enantiomerically pure encapsulating reagent 2 d was tested for nitrosation of racemic amide 5 t, showing modest but reproducible stereoselectivity and approximately 15 % ee. Given high affinity to NO+ species, which can be generated by a number of NOX gases, these supramolecular reagents and materials may be useful for NOX entrapment and separation in the environment and biomedical areas.     

Carbenium ion pairs on silica: UV-spectroscopic and electrokinetic measurements of triarylmethylium/halide-aerosil in 1,2-dichloroethane

The interaction of carbenium ion pairs with silica is studied by means of UV-spectroscopy, electrokinetic and adsorption measurements using triphenylmethyl derivatives, (RC₆H₄)₃CX, (X=F, Cl, Br, SCN, OH; R=Cl, I, H, CH₃, C(CH₃)₃, OCH₃, N(CH₃)₂) in interaction with silica particles suspended in 1,2-dichloroethane. The adsorption of triarylmethyl-halides onto silica is accompanied by the heterolytic dissociation of the tertiary carbonhalogen bond. The degree of ionization depends on the basicity of the counter anion and acidity of the cation, respectively. The influences of both concentration and structure of triarylmethyl halides on the zeta-potential are discussed with regard to steric and electronic factor. The zeta-potential values of the adsorbates decrease significantly, as compared with the free silica surface, from a certain triarylmethyl halide concentration where ionization takes place, to a constant level which is characteristic of the carbenium ion pair. These constant zeta-potential values of the adsorbates depend on the basicity of the counter ion and the _p Hammett constants of the ring substitutents, with exclusion of sterically hindered substituents and salt derivatives, e.g., crystal violet and malachite green.     

[Rb2(H2O)2][Re3(μ-Cl)3Br7(H2O)2]2 · H2O,ein gemischtes Halogenid-Hydrat mit anionischen Dimeren {[Re3(μ-Cl)3Br7(H2O)2]2 · H2O}2−

[Rb₂(H₂O)₂][Re₃(μ-Cl)₃Br₇(H₂O)₂]₂ · H₂O, a Mixed Halide-Hydrate with the Anionic Dimer {[Re₃(μ-Cl)₃Br₇(H₂O)₂]₂ · H₂O}^2? [Rb₂(H₂O)₂][Re₃(μ-Cl)₃Br₇(H₂O)₂]₂ · H₂O crystallizes as dark redbrown single crystals from an hydrobromic-acid solution of ReCl₃ and RbBr at 0°C. An important feature of the crystal structure (monoclinic, C2/c; a = 1494.61(8); b = 835.71(4); c = 3079.96(19) pm; β = 97.801(4)°; V_m = 573.9(4) cm³mol^?1; R = 0.060; R_w = 0.038) is the connection of two anions [Re₃(μ-Cl)₃Br₇(H₂O)₂]^? via a water molecule to dimers, {[Re₃(μ-Cl)₃Br₇(H₂O)₂]₂ · H₂O}^2?. These dimeric units are contained in slabs that are stacked in the [001] direction and held together by Rb⁺ cations and crystal water.