On the stabilization and methods for modification of nanosize particles used for the preparation of polymer-inorganic nanocomposites

Data on the methods of stabilization and modification of nanosize particles used in the technology of development of new polymer-inorganic nanocomposites were analyzed and generalized. Advantages of the chemical method using silane-based surface-binding modifiers were discussed. The use of surface-initiated graft polymerization was shown to be promising for the solution of problems of deagglomeration, stabilization, and efficient homogenization of nanosize particles.     

Sonochemical coating of magnetite nanoparticles with silica

Magnetite nanoparticles were coated with silica through the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) under ultrasonic irradiation. The ultrasonic irradiation was used to prevent the agglomeration of the magnetite particles and accelerate the hydrolysis and condensation of TEOS. TEM, DLS, XRF, VSM, TG and sedimentation test were used to characterize the silica-coated magnetite particles. The dispersibility of silica-coated magnetite particles in aqueous solution was improved significantly and the agglomerate particle size was decreased to 110 nm. It was found that the agglomerate particle size of silica-coated magnetite particles was mainly decided by the coating temperature and the pH value in the silica-coating process. The weight ratio of silica in silica-coated magnetite particles was mainly decided by the pH value in the silica-coating process. The dispersibility of silica-coated magnetite particles was mainly decided by the agglomerate particle size of the suspension. The oxidation of magnetite particles in air was limited through the coated silica. The magnetism of silica-coated magnetite particles decreased slightly after silica-coating.     

Problems of the stereoselectivity in the norbornadiene [2+2]-cyclodimerization reactions catalyzed by hydride nickel(i) complexes. Theoretical aspects

Cyclodimerization of norbornadiene (NBD) yielding pentacyclic products of exo-trans(cis)endo-structure in the presence of the model catalytically active complex Ni(H)(η⁴-NBD) has been studied using the DFT/PBE method. The rate-limiting reaction step is the reductive elimination of the metallacycle, the decomposition of the latter yields the norbornadiene dimer.     

Porous V2O5-SnO2/CNTs composites as high performance cathode materials for lithium-ion batteries

Vanadium pentoxide (V₂O₅) exhibits high theoretical capacities when used as a cathode in lithium ion batteries (LIBs), but its application is limited by its structural instability as well as its low lithium and electronic conductivities. A porous composite of V₂O₅-SnO₂/carbon nanotubes (CNTs) was prepared by a hydrothermal method and followed by thermal treatment. The small particles of V₂O₅, their porous structure and the coexistence of SnO₂ and CNTs can all facilitate the diffusion rates of the electrons and lithium ions. Electrochemical impedance spectra indicated higher ionic and electric conductivities, as compared to commercial V₂O₅. The V₂O₅-SnO₂/CNTs composite gave a reversible discharge capacity of 198 mAh·g^?1 at the voltage range of 2.05–4.0 V, measured at a current rate of 200 mA·g^?1, while that of the commercial V₂O₅ was only 88 mAh·g^?1, demonstrating that the porous V₂O₅-SnO₂/CNTs composite is a promising candidate for high-performance lithium secondary batteries.     

New Reactions of N‐tert‐Butylimines; Formation of N‐Heterocycles by Methyl Radical Elimination on Flash Vacuum Thermolysis of N‐Benzylidene‐ and N‐(2‐Pyridylmethylidene)‐tert‐butylamines

Thermal reactions of N‐benzylidene‐ and N‐(2‐pyridylmethylidene)‐tert‐butylamines ( 5 and 13 ) under FVT conditions have been investigated. Unexpectedly, at 800 °C, compound 5 yields 1,2‐dimethylindole and 3‐methylisoquinoline. In the reaction of 13 at 800 °C, 3‐methylimidazo[1,5‐a]pyridine was obtained as the major product. Mechanisms of these reactions have been proposed on the basis of DFT calculations. Furthermore, UV‐photoelectron spectroscopy combined with FVT has been applied for direct monitoring and characterization of the thermolysis products in situ.     

Linear and Cyclic Hybrids of Alternating Thiophene–Amino Acid Units: Synthesis and Effects of Chirality on Conformation and Molecular Packing

The dipeptide isostere 5‐aminothiophene carboxylic acid has been combined with L ‐phenylalanine moieties to provide linear and cyclic hybrid oligopeptides. A suitable protecting group strategy and appropriate coupling methods have been developed to guarantee a high degree of enantiopurity of the resulting amides. Cyclic tetraamides have been efficiently obtained by macrocyclization of the linear derivatives. In the case of racemized cyclization precursors, two diastereomeric macrocycles (S,S/R,R and meso) have been isolated. Their crystal structures show clear effects of the stereogenic centers on the ring conformations and molecular packing.     

Tighter uncertainty principles for periodic signals in terms of frequency

In this study, we propose some new uncertainty principles for periodic signals with sharper lower bounds than those in the existing ones. The improved lower bounds, in particular, are related to the frequency of the signal. Three examples are employed to demonstrate sharpness of the new uncertainty principles. Copyright © 2014 John Wiley & Sons, Ltd.