Nanostructured Colloidal Solutions of Malachite Green Formulated: Nonlinear Optical Properties and Simulation

Journal of Fluorescence - Malachite green (MG) is a dye that has been presented to use as photosensitizers for photodynamic therapy (PDT). Nonlinear absorption coefficient (β) and nonlinear...     

Chain “stationary” insertion mechanism and production of isotactic polypropylene with C 1 symmetric catalyst systems

In this contribution, the stereochemistry of propylene insertion/propagation reactions with a variety of C ₁ symmetric metallocene catalysts, containing bridged cyclopentadienyl-fluorenyl ligand for the preparation of highly stereoregular polypropylene is presented. The impact of the distal substitutent’s size and composition and changes that the catalytic sites undergo upon such substitution is elaborated. A comprehensive mechanism is proposed to explain the resulting catalytic changes that bring about the irreversible C _s/C ₁ site transformation and tactic behavior inversion. Furthermore the cyclopentadienyl’s combined distal/proximal and fluorenyl’s frontal substituent effects on molecular weight, regio-, and stereoregularity of the final polymers are discussed. Finally, stereoselectivities of C ₂ and C ₁ symmetric catalyst systems are compared. It is shown that current high performance C ₁ symmetric catalyst systems with central site chirality can be isotactic selective as well or even better in certain aspects than the C ₂ symmetric bridged bisindenyl-based metallocene catalysts. The text was submitted by the authors in English.     

Metallocene–methylaluminoxane catalyst for olefin polymerization. II. Bis-η5-(neomenthyl cyclopentadienyl)zirconium dichloride

Bis(neomenthyl cyclopentadienyl)zirconium dichloride/methyl aluminoxane (η⁵-(NMCp)₂ZrCl₂/MAO) catalyst has been investigated for ethylene polymerization. About 51% of the Zr forms active sites more or less instantaneously according to quenching with tritiated methanol. There is an initial drop of rate of polymerization, R_p, of about 30% which remains constant thereafter. The catalytic activity increases monotonically with temperature; it is proportional to [MAO]^1.75 at a constant [Zr] = 1.5 μM and proportional to [Zr]^?1.2 at a constant [MAO] = 64.5 mM. At very large [MAO]/[Zr], the catalyst has extremely high activity; κ_p = 5 × 10³ (Ms)^?1 at 50°C. There is also facile chain transfer to aluminum, κ = 0.14 s^?1 at 50°C. Both κ_p and κ are about 30 times greater than the corresponding rate constants for MgCl₂ supported TiCl₃ catalysts. The TiCl₃/MgCl₂ and (NMCp)₂/MAO catalysts have nearly the same activation energy for propagation (ca. 7 kcal/mol^?1). The higher activity of the latter is due to its larger preexponential factor in κ_p. The dependence of catalytic activity on the [MAO]/[Zr] ratio may be explained by rapid association-dissociation equilibria of MAO involving acid-base and/or electron deficient bridge complexation.     

Investigation of hydrogen and methane adsorption/separation on silicon nanotubes: a hierarchical multiscale method from quantum mechanics to molecular simulation

A combination of ab initio quantum mechanical (QM) calculations and canonical Monte Carlo (CMC) simulations are employed to investigate possible usage of single-walled silicon nanotubes (SWSiNTs) as a novel media for hydrogen and methane adsorption as well as their separation from each other. By fitting the force field, a Morse potential model is selected as an efficient potential to describe the binding energies between both hydrogen-SiNTs and methane-SiNTs obtained from ab initio calculations. Then CMC simulations are performed to evaluate the adsorption and separation behaviors of H₂ and CH₄ on the three different sizes of SiNTs including (5, 5), (7, 7), and (9, 9) SiNTs at ambient temperatures and pressures from 1 up to 10 MPa. As a comparison, the adsorption and separation of H₂ and CH₄ on the (8, 8) CNTs which are isodiameter with (5, 5) SiNTs are also simulated. Results are indicative of remarkable enhancement of H₂ and CH₄ adsorption capacity on the SiNTs compared to the CNTs, which arise from stronger van der Waals (VDW) attractions. In the case of methane adsorption on SiNTs, the stored volumetric energy exceeds the goal of the US Freedom CAR Partnership by 2010, which can not be achieved by methane compression at such low pressures. Moreover, simulation results indicate that SiNTs preferentially adsorb methane relative to hydrogen in their equimolar mixture, which results in efficient separation of these gases from each other at 293 K.     

A new characteristic approach for incompressible thermo‐flow in Cartesian and non‐Cartesian grids

A virtual‐characteristic approach is developed for thermo‐flow with finite‐volume methodology in which a multidimensional characteristic (MC) scheme is applied along with artificial compressibility. To obtain compatibility equations and pseudo‐characteristics, energy equation is taken into account in the MC scheme. With this inherent upwinding of convective fluxes, no artificial viscosity is required even at high Reynolds numbers. Another remarkable advantage of the MC scheme lies in its faster convergence rate with respect to the averaging scheme that is found to exhibit substantial delays in convergence. As benchmarks, forced and mixed convections in a cavity and in flow over cylinder and between parallel plates are examined for a wide range of Reynolds, Grashof, and Prandtl numbers. The MC and averaging schemes are applied for simulation purposes. Results show the better performance of the MC scheme in forced and mixed convections. Results confirm the robustness of the MC scheme in terms of accuracy and convergence. Copyright © 2015 John Wiley & Sons, Ltd.     

High Stability of a Phosphenium Ion Allows Umpolung of P--H Bonds

The high intrinsic stability of 1,3,2-diazaphospholenium cations enhances ionic polarization of covalent P--X bonds in P -halogeno- and P -hydrido-diazaphospholenes. The physical properties of the latter suggest a hydridic nature of the P--H bond, and their reactivities display an "Umpolung" as compared to known reaction patterns of phosphines.     

Metal-insulator transition and superconductivity in SnAr films

We studied the conductivity and superconducting transition temperature T_c of SnAr films. The films were prepared by condensing the SnAr mixture on a sapphire substrate held at 5 K. A plot of the conductivity as a function of Sn concentration shows a metal-insulator transition which agrees with a percolation model consisting of Sn clusters embedded in solid Ar. A drop of T_c is observed below the percolation threshold.     

Light-induced dehydrogenation of 3,4-dihydropyrimidin-2(1H)-ones

Abstract  

UV light irradiation of Biginelli 3,4-dihydropyrimidin-2(1H)-ones in chloroform in an argon atmosphere leads to dehydrogenation of these compounds to their corresponding pyrimidin-2(1H)-ones in excellent yields. Irradiation in the same solvent under an oxygen atmosphere generates, in addition, various hitherto unidentified products. A light-induced electron transfer from the substrate to the solvent is proposed as the initial event, supported by the detection of dichloromethane and hydrogen chloride in the photolysate.     

Unexpected regio- and stereoselective [4 + 3] cycloaddition reaction of azomethine ylides with benzylidene thiazolidinediones: synthesis of pharmacologically active spiroindoline oxazepine derivatives and theoretical study

Molecular Diversity - An unexpected regio- and stereoselective [4?+?3] cycloaddition reaction of azomethine ylides with 5-benzylidenethiazolidine-2,4-diones has been successfully...