Novel examples of three-dimensional aromaticity: 1,3-dehydro-silaadamantane dications. A theoretical (DFT,GIAO NMR,NICS) study

A DFT study of the hitherto elusive 1,3-dehydro-silaadamantane dications 2(2+)-5(2+) has been carried out. Computed nucleus independent chemical shift (NICS) values are strongly indicative of three-dimensional heteroaromaticity in the resulting caged dications (via 2-electron, 4-center homoconjugation). In the optimized structures, silicon is pyramidalized. Although charge calculations (NPA and MKS) indicate significant positive charge build-up at silicon(s), the (29)Si GIAO NMR chemical shifts are unusually shielded. The latter finding agrees with the recent DFT calculations on 7-silanobornadien-7-ylium monocation 10(+), suggesting that silicon shielding is a consequence of unusual bonding and homoconjugation in the dications. Both NICS values and silicon shielding decrease in going from 2(2+) to 5(2+). Natural bond orbital (NBO) analysis was utilized to shed light on the origin of the three-dimensional heteroaromaticity in these dications.     

Microrheology of polysaccharide nanogel-integrated system

The viscoelastic behavior of a cholesterol-modified pullulan (CHP) nanogel at various concentrations was measured using passive particle-tracking microrheology. Microrheology measures stress–strain relationships in small volumes of material by monitoring the response of probes embedded in the medium. Although microrheology is a useful way to overcome sample volume limitations, the application of the method to CHP nanogel systems has not been reported. The viscoelastic spectra of the CHP nanogels obtained from the microrheological measurements were in good agreement with the bulk rheological measurements for each sample, demonstrating that microrheological measurement is effective in CHP nanogel systems. The gelation behavior of CHP nanogel dispersions containing pullulans of different molecular weights was also investigated by microrheology. CHP nanogels made from 1.0 or 4.0?×?10⁵ molecular weight pullulans formed a macrogel at around 3.0 wt%, whereas the CHP nanogel consisting of 0.55?×?10⁵ molecular weight pullulan did not form a macrogel. This suggests that the mechanical properties of the system can be controlled by the molecular weight of the pullulan used. These insights into gelation behavior should be useful in predicting the most favorable conditions for developing novel materials.     

Influence of Monovalent Electrolytes on Rheological Properties of Gelled Colloidal Silica Suspensions

Rheological responses of the gelled (G) Snowtex 20 silica suspensions in the presence of LiCl, NaCl, and KCl have been investigated as functions of concentrations of silica and salt at pH 9.8 in terms of the Hofmeister series effect. The primary silica particle is isolated, and it is coagulated to form a gel above at 0.1, 0.3, and 0.5 M concentrations of LiCl, NaCl, and KCl, respectively, when the silica volume fraction φ is beyond 1.0%. The resulting G silica suspensions are classified into a strong‐link gel and their power‐law dependences of the critical strain as well as the storage modulus on the silica volume fraction were compared with the predictions by the fractal gel model. The resulting power‐law exponents of the critical strain are negative, this is in agreement with that predicted by the fractal model, and their magnitudes decrease with an increase in salt concentration, irrespective of the salt. Moreover, the magnitude of the power‐law exponent for the critical strain is well related to the Hofmeister series effect, namely it decreases in the order Li⁺, Na⁺, and K⁺, and the least hydrated K⁺ adsorbs in great enough in amount to create a less flexible network structure in the G silica suspension due to stronger attraction between the silica particles. Moreover, the stronger attraction, on the other hand, should be responsible for both the larger storage modulus and the higher power‐law exponent of the silica volume fraction dependence on the storage modulus. Therefore, the Hofmeister series effect is useful to understand the rheological properties of the G Snowtex 20 silica suspensions.     

Formation of Oxoborane and Thioxoborane from a Dithiastannaboretane Derivative

Reaction with, an oxygen-donating reagent such as DMSO and thermolysis of a 1,3,2,4-dithiastannaboretane derivative bearing 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) group led to the formation of novel boron–group 16 element double bond compounds, oxoborane (Tbt–B=0) and thioxoborane (Tbt–B=S). The oxoborane and thioxoborane underwent cycloaddition reactions to give the corresponding adducts in good yields.     

Common origin of the circular-dichroism pattern in angle-resolved photoemission spectroscopy of SrTiO3 and Cu(x)Bi2Se3

Circular dichroism in the angular distribution of photoelectrons from SrTiO(3):Nb and Cu(x)Bi(2)Se(3) is investigated by 7-eV laser angle-resolved photoemission spectroscopy. In addition to the well-known node that occurs in the circular dichroism pattern when the incidence plane matches the mirror plane of the crystal, we show that another type of node occurs when the mirror plane of the crystal is vertical to the incidence plane and the electronic state is two-dimensional. The flower-shaped circular dichroism patterns in the angular distribution occurring around the Fermi level of SrTiO(3):Nb and around the Dirac point of Cu(x)Bi(2)Se(3) are explained on equal footings. We point out that the penetration depth of the topological states of Cu(x)Bi(2)Se(3) depends on momentum.     

Ion-specific swelling behavior of poly(vinyl alcohol) gel in aqueous solutions containing some divalent ions

Swelling behaviors of poly(vinyl alcohol) (PVA) gel were investigated in solutions of salts containing certain divalent ions, namely, alkaline-earth-metal halides and sulfates of alkali-metals, alkaline-earth-metals, and transition metals. The gel deswelled in the solutions of the alkaline-earth-metal chlorides but swelled in those of the bromides. However, the cation specificities of the swelling for both of the halides were roughly the same: the degrees of swelling are Sr²⁺2+2+. The gel deswelled for all sulfate solutions with specificities for alkali-metal and alkaline-earth-metal cations: the swelling degrees are K⁺;Na⁺2++. There was no order for transition metals.