Spontaneous chirality in simple systems

Two simple examples of spontaneous chiral symmetry breaking are presented. The first is close-packed cylindrically confined spheres. As the cylinder diameter is varied, one obtains a variety of chiral phases. The second example involves unconfined dipolar particles with an isotropic attraction, which also exhibits chiral ground states. We speculate that a dilute magnetorheological fluid film, with the addition of smaller particles to provide an attractive entropic interaction, will exhibit a chiral columnar ground state.     

The structure of prostaglandin B1 dimer

Dimeric derivatives of prostaglandin B₁ were prepared under alkaline conditions and the structures were determined by NMR spectroscopies.     

Cycloheptyne intermediate in the reaction of chiral cyclohexylidenemethyliodonium salt with sulfonates

Reactions of (R)-4-methylcyclohexylidenemethyl(phenyl)iodonium salt and its 3-trifluoromethylphenyl and 4-methoxyphenyl derivatives (1) with tetrabutylammonium mesylate and triflate were carried out in chloroform at 60 degrees C. The products include (S)-4-methylcyclohexylidenemethyl sulfonate (2) and (R)-5-methylcyclohept-1-enyl sulfonate (3) as well as iodoarene. Reactions of (S)-1 were confirmed to provide the counterpart results. The rearranged triflate (R)-3Tf formed in the reaction with triflate maintains mostly the ee (enantiomeric excess) of (R)-1, while the ee of the mesylate product 3Ms is largely lost. The (13)C-labeling at the exocyclic position of 1 results in the isotopic scrambling of C-1 and C-2 of 3Ms in the mesylate reaction. The degree of the scrambling agrees well with that of the loss of ee of (R)-3Ms obtained from (R)-1, implying that the racemization is not due to the intermediate formation of achiral, primary 4-methylcyclohexylidenemethyl cation. Reaction of 1 with mesylate in the presence of CH(3)OD provided the 3Ms deuterated at the 2-position. When tetraphenylcyclopentadienone was added to the mesylate reaction system, the adduct of the 4-methylcycloheptyne intermediate was obtained in 24% yield, but the normal products 2Ms and 3Ms were still formed. The 3Ms obtained here in a low yield maintains the high ee of 1. These results indicate that the cycloheptyne is an intermediate responsible for the formation of racemic product 3Ms in the mesylate reaction. It is also concluded that the unrearranged products 2 are formed via the competitive pathways of in-plane and out-of-plane S(N)2 reactions.     

Highly stereoselective protonation of the enolate of a bicyclic cycloheptatrienyl lactone occurs on the hindered face

[reaction: see text] The H/D exchange of the lactone-fused cycloheptatriene 1 is over 1000 times faster than that of the epimer 2. Interconversion of 1 and 2 provides an equilibrium mixture of 1:0.7, showing a similar stability of the isomers. Protonation of the common enolate 7 occurs far more readily on the more hindered face. Cycloheptatrienide anion is bent and as stable as a divinylcarbanion.     

Enhanced photoluminescence of ZnO-SiO2 nanocomposite particles and the analyses of structure and composition

Stable blue-green photoluminescent ZnO-SiO₂ nanocomposite particles exhibiting quantum efficiency as high as 34.8% under excitation at 360 nm were prepared using a spray-drying process from a feed solution that contained both luminescent ZnO nanoparticles synthesized by a sol-gel method and commercially-available SiO₂ nanoparticles. The effects of silica nanoparticle size and SiO₂-to-ZnO concentration ratio on the PL properties of the composite particles were investigated. The internal structure and chemical composition were investigated in detail using elemental mapping, which revealed that ZnO nanoparticles were well-dispersed within silica nanoparticle matrix. At a LiOH concentration of 0.23 M, the predicted ZnO crystallite diameter before and after spray drying was approximately constant at 3.3 and 3.6 nm, respectively. This result indicates that ZnO particle growth was inhibited and therefore the PL property of ZnO nanoparticles was stably preserved in the composite.     

Highly luminescent silica-coated ZnO nanoparticles dispersed in an aqueous medium

Highly luminescent silica-coated ZnO nanoparticles dispersed in an aqueous medium were synthesized using the sol-gel process. The samples prepared at various temperatures exhibited an emission peak at around 480 nm (blue color) and a quantum efficiency of 60% at maximum by the quantum confinement effect of ZnO nanoparticles, with diameters ranging from 3.1 to 3.5 nm, under ultraviolet excitation. No degradation of the quantum efficiencies and no peak shifting in the emission spectra were observed for 7 days following the preparation, which indicated no growth of ZnO nanoparticles in the aqueous medium.