Reductive cleavage of the C-O bond of acetals and orthoesters: reduction by silane in the presence of a Rh-PPh3 complex

Reductions of acetals to ethers and of orthoester to acetal by hydrosilane using rhodium catalyst are described.     

High-Q concentrated directional emission from egg-shaped asymmetric resonant cavities

We propose the use of egg-shaped asymmetric resonant cavities (ARCs), each of which consists of a half-circular part and a half-deformed part, as promising candidates in obtaining desirable whispering-gallery-mode resonances. According to numerical analysis based on a ray-optics model, more than an order-of-magnitude higher Q and more-concentrated emission from the tip of the egg region were obtained for egg-shaped ARCs than for the previously studied quadrupolar ARCs.     

Mechanism of the solvent extraction of pertechnetate with tetraphenylarsonium chloride

Based on the fundamental parameters governing the distribution equilibrium of tetraphenylarsonium chloride (TPAC) between then-hexane-chloroform and aqueous solutions, the extraction mechanism of pertechnetate with TPAC has been systematically investigated. By the analysis of a relation of the distribution ratio of pertechnetate and the hydrogen ion concentration, the dissociation constant of pertechnetic acid was determined to beK _a=2.09±0.18 at 25 °C.     

Synthesis of tris(β-diketonato) technetium(III) and its adsorption behaviour on silica gel

Tris (benzoylacetonato) technetium(III) and tris (2-thenoyltrifluoroacetonato) technetium(III) were synthesized by the direct reduction of pertechnetate with dithionite in the presence of excess ligands. The geometrical isomers of the benzoylacetonate were further separated by means of a silica gel column chromatography. In the silica gel system, the adsorption distribution coefficient of the tris (-diketonato) technetium(III) decreases in the order of Tc(acac)₃>fac-Tc(bzac)₃>mer-Tc(bzac₃)>Tc(tta)₃.     

Unexpected reactivity of the burgess reagent with thiols: synthesis of symmetrical disulfides

Reaction of the Burgess reagent with a series of aliphatic and aromatic thiols led to the corresponding symmetrical disulfides in high yields. No olefins were detected in the reactions of aliphatic thiols.     

Liquid structure of the urea-water system studied by dielectric spectroscopy

Dielectric spectroscopy measurements for aqueous urea solutions were performed at 298 K through a concentration range from 0.5 to 9.0 M with frequencies between 200 MHz and 40 GHz. Observed dielectric spectra were well represented by the superposition of two Debye type relaxation processes attributable to the bulk-water clusters and the urea-water coclusters. Our quantitative analysis of the spectra shows that the number of hydration water molecules is approximately two per urea molecule for the lower concentration region below 5.0 M, while the previous molecular dynamics studies predicted approximately six water molecules. It was also indicated by those studies, however, that there are two types of hydration water molecule in urea solution, which are strongly and weakly associated to the urea molecule, respectively. Only the strongly associated water was distinguishable in our analysis, while the weakly associated water exhibited the same dynamic feature as bulk water. This implies that urea retains the weakly associated water in the tetrahedral structure and, thus, is not a strong structure breaker of water. We also verified the model of liquid water where water consists of two states: the icelike-ordered and dense-disordered phases. Our dielectric data did not agree with the theoretical prediction based on the two-phase model. The present work supports the argument that urea molecules can easily replace near-neighbor water in the hydrogen-bonding network and do not require the presence of the disordered phase of water to dissolve into water.     

Efficient propagation of polarization from laser photons to positrons through compton scattering and electron-positron pair creation

We have demonstrated for the first time the production of highly polarized short-pulse positrons with a finite energy spread in accordance with a new scheme that consists of two-quantum processes, such as inverse Compton scattering and electron-positron pair creation. Using a circularly polarized laser beam of 532 nm scattered off a high-quality, 1.28 GeV electron beam, we have obtained polarized positrons with an intensity of 2 x 10(4) e+ /bunch. The magnitude of positron polarization has been determined to be 73 +/- 15(stat) +/- 19(syst)% by means of a newly designed positron polarimeter.