The current state of sol-gel technology

The current state of sol-gel technology has been reviewed mainly from the standpoint of microstructures of materials which can be achieved by the sol-gel method. It has been shown that the sol-gel method makes it possible to produce a great variety of high technology materials by providing the existing substances with a significant microstructure and producing the material with a novel microstructure. There are microstructures characterized by micropores, preferential crystal orientation, inorganic-organic and inorganic-inorganic composites of hybrid nature and gradient composition. These characteristic microstructures are related to the properties and applications of the materials.     

Selective matching of catalyst element and carbon source in single-walled carbon nanotube synthesis on silicon substrates

We have studied the compatibility of various catalysts for ethylene and ethanol chemical vapor deposition (CVD) syntheses of single-walled carbon nanotubes (SWNTs) on Si substrates. A strong selectivity between the catalyst elemental species and carbon source was found; SWNT yield for Fe (Co) catalysts was much higher for ethylene (ethanol) CVD than for ethanol (ethylene) CVD. This strong and completely opposite selectivity implies significantly different SWNT growth mechanisms for ethanol and ethylene CVD on Si substrates.     

Stekeospecific transformation of D-sugar to L-sugar in complex aminoglycoside. Synthesis of a kanamycin B analog having 2,6-diamino-2,4,6-trideoxy-L-arabino-hexopyranose

A 4′-ene derivative of kanamycin B ($\text{4}$) was derived from the epoxide ($\text{1}$) by oxidative elimination of the 4′-phenylseleno group into the allylic alcohol ($\text{3}$). The title compound, 0-(2,6-diamino-2,4,6-trideoxy-β-L-arabino-hexopyranosyl)-(1→4)-0-[3-amino-3-deoxy-α-D-glucopyranosyl-(1→6)]-2-deoxystreptamine ($\text{6}$) was obtained from $\text{4}$ by stereospecific hydrogenation followed by removal of the masking groups, changing the D-sugar moiety of the 4-0-glycoside portion into an L-sugar.     

Potentiometric flow-injection determination of trace chlorine based on its redox reaction with an iron(III)/iron(II) buffer

A very sensitive and rapid potentiometric determination of trace chlorine in water is described. The method is based on the transient potential changes which appears during the reduction of dissolved chlorin with an iron(III)/iron(II) potential buffer containing chloride and sulfuric acid. The sample is injected into a water carrier stream and merged with a stream of this potential buffer solution; chlorine is reduced during passage through a short reaction coil. The potential change from the baseline is measured with a flow-through ORP (oxidation-reduction potential) electrode. Potential changes (peak heights) are proportional to chlorine concentrations from 10^?7 M to 10^?5 M. The detection limit is 5 × 10^?8 M (3.5 μgl^?1 as Cl₂). The sample throughput is 45 h^?1. Reproducibility is in the range 2.5–1.1%. Results for potable water agree with those obtained by the o-tolidine method.     

Kinetics and mechanism of the base hydrolysis of cis-eq-[Cr(NCS)(S-pdtra)]−, trans-eq-[Cr(NCS)(edtrp)]− and trans-eq-[Cr(NCS)(R-pdtrp)]− complexes. Strong rate enhancement for NCS− ligand release with an increase of [OH−] only for the trans-equatorial isomers

The [Cr(NCS)(edtrp)]⁻, [Cr(NCS)(R-pdtrp)]⁻ and [Cr(NCS)(S-pdtra)]⁻ complexes, that are derivatives of the trans-equatorial isomers of [Cr(edtrp)(H₂O)]° and [Cr(R-pdtrp)(H₂O)]° and the cis-equatorial isomer of [Cr(S-pdtra)-(H₂O)]° (edtrp = ethylenediamine-N,N,N′-tripropionate, R-pdtrp = R-propane-1,2-diamine-N,N,N′-tripropionate, S-pdtra = S-propane-1,2-diamine-N,N,N′-triacetate) undergo aquation in alkaline media with a strong dependence of the rate on [OH⁻] for the trans-equatorial isomers and a very weak dependence for the cis-equatorial isomer. The thiocyanate ligand release follows a stereoretentive course for all reactants. Based on kinetic data the reaction mechanism has been discussed. Rate differences between the isomers are interpreted in terms of an interchange via a conjugate base (I c.b.) mechanism, assuming an equilibrium between the cis-equatorial-Cr^III-S-pdtra complexes with penta- and tetradentate coordination of the edta-like ligand. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cationic polymerization with p-substituted benzyl p-hydroxyphenyl methyl sulfonium salts: Effect of substituents and mechanistic aspects of initiation reaction

Various p-substituted benzyl p-hydroxyphenyl methyl sulfonium salts ( 2 ) were synthesized and their initiator activities were evaluated in bulk polymerization of glycidyl phenyl ether (PGE). The order of the activity was found to be 2b (X = CH₃) > 2a (X = H) ≈ 2c (X = Cl) > 2d (X = NO₂), indicating that the introduction of an electron-donating group enhanced the activity. In Hammett's plots, the logarithm of the ratio of the polymerization rates (log k_x/k_H) was correlated with σ⁺_ρ better than with σ_p and a negative ρ⁺ value (-1.18) was obtained. Reaction of 2a with benzyl mercaptan mainly gave dibenzyl sulfide and p-hydroxyphenyl methyl sulfide. The obtained results seemed to demonstrate that the OH group of the aryl group yielded no proton as initiator for the polymerization, whereas the benzyl group caused the polymerization, which was initiated by the corresponding benzyl cation formed by C? S bond cleavage. © 1993 John Wiley & Sons, Inc.     

Total synthesis of the disaccharide of bleomycin, 2-0-(α-D-mannopyranosyl)-L-gulopyranose

2-0-(α-$\text{D}$-Mannopyranosyl)-$\text{L}$-gulopyranose, the sugar portion of bleomycin has been synthesized.     

Mechanism of formation of YBa2Cu4O8 superconductor in the sol-gel synthesis

It was shown that a single phase YBa₂Cu₄O₈ (124-phase) could be formed from gels at 1 atm oxygen pressure, and the mechanism of its formation was elucidated. It was found that there are two key routes for the sol-gel formation of the 124-phase, one involving the tetragonal YBa₂Cu₃O _y with a low concentration of oxygen defects (tetra-I phase) and the other involving the Ba₂Cu₃O_5.9 as important intermediates of the 124-phase. The rapid formation of these intermediate compounds from the gel was attributed to the small particle size of the oxides and carbonates precipitating at the initial stage of heating. It was thought that the small particles characteristic of sol-gel processing lead to the rapid formation of the intermediate compounds and subsequent precipitation of the 124-phase.     

Hydrolysis and condensation reactions of Si(OC2H5)4 related to silica fiber drawing

The hydrolysis and condensation reactions of Si(OC₂H₅)₄ (TEOS) at 80°C in the TEOS---H₂O---C₂H₅OH---HCl solutions with H₂O/TEOS molar ratios (r) from 1.0 to 2.0 were followed by gas chromatography (GC) and measurement of molecular weight ( ) of the hydrolyzates, in order to explain the viscosity change of the solutions. It has been found that the siloxane oligomers with average polymerization degree (n) from 2 to 7 are formed in the early stage of reaction and undergo condensation to form higher polymers. The n and the number of silanol groups of the oligomers are increased with increasing r. The fact that the solution viscosity increased once rapidly around the so-called gelation point followed by a sluggish increase with the increase of reaction time for an r of 1.7, while it increased rapidly without a break till gelation for an r of 2.0, was attributed to fewer silanol groups and higher steric hindrance as a result of more −OC₂H₅ groups remaining in the oligomers for an r of 1.7.