Decay kinetics of the green emission in tungstates and molybdates

Luminescence characteristics of a number of undoped and variously doped PbWO₄ crystals were studied at 0.4–400 K by the time-resolved spectroscopy and compared with those of ZnWO₄,CdWO₄ and PbMoO₄ crystals. Two types of green emission centres are detected in PbWO₄ crystals. The centres of the first type are responsible for the low-temperature 2.3–2.4 eV emission observed under excitation around 3.90–3.95 eV. The structure and parameters of their relaxed excited states were determined. It was concluded that the origin of defects responsible for the green emission of the first type could vary for different crystals. The centres of the second type with the emission around 2.5 eV appear in crystals containing oxygen vacancies after the thermal destruction of Pb⁺-WO₃ centres at T>180 K. Decomposition of the exciton and various defect-related states was also studied, and activation energies of this process were calculated.     

The world according to Rényi: thermodynamics of multifractal systems

We discuss basic statistical properties of systems with multifractal structure. This is possible by extending the notion of the usual Gibbs-Shannon entropy into more general framework—Rényi’s information entropy. We address the renormalization issue for Rényi’s entropy on (multi)fractal sets and consequently show how Rényi’s parameter is connected with multifractal singularity spectrum. The maximal entropy approach then provides a passage between Rényi’s information entropy and thermodynamics of multifractals. Important issues such as Rényi’s entropy versus Tsallis-Havrda-Charvat entropy and PDF reconstruction theorem are also studied. Finally, some further speculations on a possible relevance of our approach to cosmology are discussed.     

Preparation and properties of isobutylene–isoprene rubber–clay nanocomposites

Isobutylene isoprene rubber (IIR)‐clay nanocomposites have been prepared successfully by melt intercalation with maleic anhydride‐grafted IIR (Ma‐g‐IIR) and organophilic clay. In IIR‐clay nanocomposites, the silicate layers of the clay were exfoliated and dispersed into the monolayer. The nanocomposites exhibited greater gas barrier properties compared with those of Ma‐g‐IIR. When 15 phr clay was added, gas barrier properties were 2.5 times greater than those of Ma‐g‐IIR. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1182–1188, 2006     

Preparation and characterization of mesoporous silica thin films from polyethoxysiloxanes

Tetraethoxysilane (TEOS) and polyethoxysiloxanes (PEOSs; prepared by the acid‐catalyzed hydrolytic polycondensation of TEOS) were subjected to the sol–gel process in the presence of cetyltrimethylammonium bromide (CTAB), respectively. The PEOSs with M_w 700–26,000, as prepared by sol–gel coating of TEOS and PEOS under various conditions, were used. Uniform and crack‐free thin films of thickness 276–613 nm were prepared by spin‐coating of a PEOS solution containing CTAB. When the coating films were sintered at 400 °C, the combustion of ethoxy groups and CTAB took place to provide porous silica thin films. The structure of the thin films was found to be dependent on the molecular weight of PEOS and the molar ratio of CTAB/Si: lamellar or hexagonal phase was observed for M_w less than 15,000 and for CTAB/Si molar ratios greater than 0.10. Honeycomb structures were observed for M_w less than 5000 and for CTAB/Si molar ratios of 0.15. The honeycomb structure was also observed by atomic force microscopy and transmission electron microscope. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2542–2550, 2006     

Hydration Structure Around the Carboxyl Group Studied by Neutron Diffraction with 12C/13C and H/D Isotopic Substitution Methods

Time-of-Flight (TOF) neutron diffraction measurements have been carried out on aqueous 8 mol% sodium acetate solutions in D₂O. Scattering cross sections that were observed for sample solutions involving ¹²C/¹³C and H/D isotopically substituted acetate ions were used to derive the first-order difference functions, Δ_H(Q) and Δ_C(Q), and corresponding distribution functions, G _H(r;r) and G _C(r;r), which describe the environmental structure around the methyl and the carboxyl groups within the acetate ion, respectively. Structural parameters concerning the first hydration shell of the carboxyl group within the acetate ion were obtained through the least squares fit to the observed intermolecular difference function, Δ_C ^inter(Q). The nearest neighbor C _O...D _W1 (C_O: carboxyl carbon atom, D_W1: water deuterium atom) distance, r(C _O...D _W1 ), and the angle, ∠ C _O ...D _W1 -O _W (O _W : water oxygen atom), were determined to be 2.63(1) Å and 120(1)°, respectively. The coordination number, n(C _O ...D _W1 ), was obtained to be 4.0(1). These results are consistent with the hydration structure in which water molecules in the first hydration shell of the carboxyl group are hydrogen-bonded with oxygen atoms of the carboxyl group.     

Reversible electro-optical switching of a memory type PDLC using two-frequency-addressing liquid crystals

Abstract

A memory type PDLC has been prepared by sandwiching a mixture of two-frequency-addressing liquid crystals and acrylate monomers with a hydoroxy group between two glass substrates with ITO electrodes followed by UV irradiation. This PDLC can be electrically switched between a transparent state and a light scattering state, which are maintained over several months without electric fields.     

Numerical simulation of 2D granular particles and its analyses by means of the micropolar fluid model

Numerical simulations of two-dimensional granular flows under uniform mean shear and external body torque were performed following the setting of the authors’ previous study [10]. Convergence of the stresses with the increase of coarse-graining length is investigated. Difference R between vorticity field and spin field is controlled by the external torque and the stresses for the region R > 0 is obtained as well as those for R < 0. The symmetry of the stresses under the change of the sign of R is discussed.     

Unusual annulation reaction of electron-deficient alkenes with enamines: an easy access to stereocontrolled 4-fluoroalkylated 3,4-dihydro-2H-pyrans

The reaction of β-fluoroalkylated α,β-unsaturated ketones with various enamines gave 4-fluoroalkylated 3,4-dihydro-2H-pyrans as a major product in good yields by a one-pot operation, and these products display the high diastereoselection just after single recrystallization. This unexpected result is rationalized by the unique reactivity of β-fluoroalkylated α,β-unsaturated ketones. As the synthetic application, 4-trifluoromethyl tetrahydropyran was synthesized in moderate isolated yield with high diastereoselectivity.     

Improved synthesis of (R)-7-hydroxycarvone from (S)-α-pinene

A three-step synthesis of (R)-7-hydroxycarvone (1), which is anticipated to be a valuable building block for the versatile preparation of natural products, is described. Optimization of the reaction conditions for photooxygenation and migration of (S)-α-pinene 2, tetrapropylammonium perruthenate (TPAP) oxidation of the generated alcohol, and a subsequent ring-opening reaction in the presence of Cu(OTf)₂ led to the synthesis of 1 with good reproducibility. The desired product 1 was thus obtained in 46% yield over three steps.     

Neutron Diffraction Study on the Structure of Aqueous LiNO3 Solutions

Neutron diffraction measurements were carried out at 25 °C for aqueous LiNO₃ heavy water solutions, (*LiNO₃) _x (D₂O)_1?x where x = 0.1, 0.05 and 0.01, in which the ⁶Li/⁷Li isotopic ratios were varied. Structural information on intermolecular nearest neighbor Li⁺···D₂O interactions in the extensive concentration range was derived from the first-order difference function, ?_Li(Q), obtained from the difference in scattering cross sections between ⁶Li- and ⁷Li-enriched sample solutions. The nearest neighbor Li⁺···O distance and coordination number for sample solution with x = 0.1 were determined to be r _LiO = 1.969 (8) Å and n _LiO = 4.12 (6), respectively, corresponding to the four-coordinated Li⁺ ion in the solution. On the other hand, those obtained for the solution with x = 0.01 are r _LiO = 2.00 (2) Å and n _LiO = 6.0 (2), respectively, indicating that hexaaqua Li⁺ is dominant in the dilute solution. These results clearly indicate that a concentration dependence of the hydration number of Li⁺ occurs in the aqueous solutions.