Efficient epoxidation of electron-deficient alkenes with hydrogen peroxide catalyzed by [γ-PW10O38V2(μ-OH)2]3-

A divanadium‐substituted phosphotungstate, [γ‐PW₁₀O₃₈V₂(μ‐OH)₂]^3? ( I ), showed the highest catalytic activity for the H₂O₂‐based epoxidation of allyl acetate among vanadium and tungsten complexes with a turnover number of 210. In the presence of I , various kinds of electron‐deficient alkenes with acetate, ether, carbonyl, and chloro groups at the allylic positions could chemoselectively be oxidized to the corresponding epoxides in high yields with only an equimolar amount of H₂O₂ with respect to the substrates. Even acrylonitrile and methacrylonitrile could be epoxidized without formation of the corresponding amides. In addition, I could rapidly (≤10 min) catalyze epoxidation of various kinds of terminal, internal, and cyclic alkenes with H₂O₂ under the stoichiometric conditions. The mechanistic, spectroscopic, and kinetic studies showed that the I ‐catalyzed epoxidation consists of the following three steps: 1) The reaction of I with H₂O₂ leads to reversible formation of a hydroperoxo species [γ‐PW₁₀O₃₈V₂(μ‐OH)(μ‐OOH)]^3? ( II ), 2) the successive dehydration of II forms an active oxygen species with a peroxo group [γ‐PW₁₀O₃₈V₂(μ‐η²:η²‐O₂)]^3? ( III ), and 3) III reacts with alkene to form the corresponding epoxide. The kinetic studies showed that the present epoxidation proceeds via III . Catalytic activities of divanadium‐substituted polyoxotungstates for epoxidation with H₂O₂ were dependent on the different kinds of the heteroatoms (i.e., Si or P) in the catalyst and I was more active than [γ‐SiW₁₀O₃₈V₂(μ‐OH)₂]^4?. On the basis of the kinetic, spectroscopic, and computational results, including those of [γ‐SiW₁₀O₃₈V₂(μ‐OH)₂]^4?, the acidity of the hydroperoxo species in II would play an important role in the dehydration reactivity (i.e., k₃). The largest k₃ value of I leads to a significant increase in the catalytic activity of I under the more concentrated conditions.     

Two kinds of positive holes in YBa2Cu3O7-δ around Tc through the analysis of the hall effect in weak field

Abstract

To obtain information about the mechanism of high T_c superconductivity in YBa₂Cu₃O_7-δ (YBCO) the dependence of Hall coefficient R_H and electrical resistivity ρ on the temperature were studied in range from 77 to 300K. An inflexion point θ was found to exist around 130K in R_H(T) and ρ(T) curves. Both the mobility μ and the concentration of the holes were evaluated above and below θ, separately. The μ of the holes dominant below θ was ten times greater than that above θ. The concentration of the heavy holes above θ was, however, one hundred times larger than that below θ. A discussion will be given in terms of the transport process of the holes drifting through P_[sgrave]- and P_σ-orbits on the CuO₂ planes in YBCO.     

Effect of restart at signals on traffic flow through a series of signals

We study the effect of restart at signals on the vehicular traffic controlled by a series of signals. The Nagel–Schreckenberg model (NS model) and Fukui–Ishibashi model (FI model) are applied to the vehicular motion. In the FI model, the step-by-step acceleration is not taken into account but the acceleration effect is included in the NS model. It is shown that the difference between both models results in the restart effect at signals. The extended version of the NS model with signals is formulated by the difference equation. The restart at signals has an effective effect on the traffic flow. The fundamental diagram changes highly by the restart effect. The dependences of mean speed on the cycle time are shown.     

Characterization of acoustic cavitation in water and molten aluminum alloy

High-intensive ultrasonic vibrations have been recognized as an attractive tool for refining the grain structure of metals in casting technology. However, the practical application of ultrasonics in this area remains rather limited. One of the reasons is a lack of data needed to optimize the ultrasonic treatment conditions, particularly those concerning characteristics of cavitation zone in molten aluminum.The main aim of the present study was to investigate the intensity and spectral characteristics of cavitation noise generated during radiation of ultrasonic waves into water and molten aluminum alloys, and to establish a measure for evaluating the cavitation intensity. The measurements were performed by using a high temperature cavitometer capable of measuring the level of cavitation noise within five frequency bands from 0.01 to 10 MHz. The effect of cavitation treatment was verified by applying high-intense ultrasonic vibrations to a DC caster to refine the primary silicon grains of a model Al–17Si alloy. It was found that the level of high frequency noise components is the most adequate parameter for evaluating the cavitation intensity. Based on this finding, it was concluded that implosions of cavitation bubbles play a decisive role in refinement of the alloy structure.     

Dose equivalents inside the MIR Space Station measured by the combination of CR-39 plates and TLDs and their comparison with those on Space Shuttle STS-79, -84 and -91 missions

In 1997, four dosimeter packages, each of which contains two CR-39 plates and 18 TLDs (Mg2SiO4:Tb), were placed inside the MIR Space Station and flew on an orbit with an inclination angle of 51.6 degrees and an altitude of approximately 400 km for 40 days. We estimated the absorbed doses, dose equivalents and effective quality factors during the flight by combining CR-39 data and TLD data. We then compared these results to those obtained with the same analysis method from the dosimeter packages on board Space Shuttle missions STS-79, -84 and -91 that flew along the same orbit. Finally, the differences between our results and those obtained by another group using passive dosimeters on the MIR are discussed.     

Dynamic instability of a wheel moving on a discretely supported infinite rail

The parametric instability of a wheel moving on a discretely supported rail is discussed. To achieve this, an analysis method is developed for a quasi-steady-state problem which can represent an exponential growth of oscillation. The temporal Fourier transform of the rail motion is expanded by a Fourier series with respect to the longitudinal coordinate, and then the response of the rail deflection due to a quasi-harmonic moving load is derived. The wheel/track interaction is formulated by the aid of this function and reduced to an infinite system of linear equations for the Fourier coefficients of the contact force. The critical velocities between the stable and unstable states are calculated based on the nontrivial condition of the homogeneous matrix equation. Through these analyses the influences of the modeling of rail and rail support on the unstable speed range are examined. Moreover, not only the first instability zone but also other zones are evaluated.     

Generation of vacuum ultraviolet radiation by intracavity high-harmonic generation toward state detection of single trapped ions

Vacuum ultraviolet (VUV) radiation around 159 nm is obtained toward direct excitation of a single trapped \(^{115}\hbox {In}^{+}\) ion. An efficient fluoride-based VUV output coupler is employed for intracavity high-harmonic generation of a Ti:S oscillator. Using this coupler, where we measured its reflectance to be about 90 %, an average power reaching 6.4  \(\upmu\) W is coupled out from a modest fundamental power of 650 mW. When a single comb component out of 1.9  \(\times\)  10 \(^{5}\) teeth is resonant to the atomic transition, 100s of fluorescence photons per second will be detectable under a realistic condition.     

Effect of signals on two-route traffic system with real-time information

We study the effect of signals on the vehicular traffic in the two-route system at the tour-time feedback strategy where the vehicles move ahead through a series of signals. The Nagel–Schreckenberg model is applied to the vehicular motion. The traffic signals are controlled by both cycle time and split. The tour times on two routes fluctuate periodically and alternately. The period increases with decreasing the split. Also, the tour time on each route varies with time by synchronizing with the density. The dependences of tour times and densities on both split and cycle time are clarified.     

Fabrication and proton conductivity of sulfonated silica composites prepared by postoxidization of mercaptomethoxysilane

Sol–gel derived organic–inorganic hybrids containing phosphotungstic acid (PWA) have been prepared previously to obtain proton conductive membranes. However, leaking of PWA was a serious problem to achieve the higher proton conductivity. In this study, polyelectrolyte membranes functionalized with sulfonic acid groups were fabricated by the sol–gel method. Proton conductivity measurements were performed on an impedance analyzer at 80°C/95% RH. The functionalized polyelectrolyte membranes exhibited the proton conductivity σ ～ 0.9 (S/cm) which was much higher than the previously reported hybrids containing PWA. Although the hybrids exhibited fairly high proton conductivity irrespective to the catalysts used, that under the low relative humidity strongly depends on the catalysts. Among the hybrids prepared in this study, the membrane synthesized with HCl showed outstanding proton conductive properties even at the low humidity thanks to the proton transport channel formed by the swelling of ionic clusters. This fact was confirmed by measuring the ion exchange capacity, water uptake, swelling rate, Fourier transform infrared spectroscopy, atomic force microscopy, and thermogravimetric analysis. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis and Hierarchical Self‐Assembly of Diphenylalanine‐Based Homopolymer and Copolymers By RAFT Polymerization

Dipeptide diphenylalanine has attracted significant research interests because of its ability to self‐assemble into various nanostructures such as nanotubes, nanowires, and nanoribbons. In this article, we present the synthesis and self‐assembly of a novel diphenylalanine‐based homopolymer and block/random copolymers by the reversible addition–fragmentation chain transfer (RAFT) polymerization of an acrylamide having a dipeptide moiety. The RAFT polymerization of N‐acryloyl‐l ,l ‐diphenylalanine (A‐Phe‐Phe‐OH) afforded novel amino acid‐based polymers with predetermined molecular weights and relatively narrow‐molecular weight distributions. The hierarchical self‐assembled structures of poly(A‐Phe‐Phe‐OH), which involve nanorods, larger nanofiber‐like microcrystals, and fiber bundles, were characterized by atomic force microscopy (AFM), transmission electron microscopy, scanning electron microscopy, and dynamic light scattering measurements. The circular dichroic measurements of poly(A‐Phe‐Phe‐OH) revealed its characteristic chiroptical property, which is affected by the nature of the solvents and the addition of urea and salts via hydrophobic, hydrogen bonding, and electrostatic interactions. Thermo‐ and pH‐responsive block and random copolymers composed of A‐Phe‐Phe‐OH and N‐isopropylacrylamide were synthesized by RAFT polymerization, and the thermoresponsive properties and assembled structures of the resulting copolymers were investigated by AFM and turbidity measurements. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2562–2574