Self-assembly mechanism of PEG-b-PCL and PEG-b-PBO-b-PCL amphiphilic copolymer micelles in aqueous solution from coarse grain modeling

We followed the self-assembly of high-molecular weight MePEG- b -PCL (poly(methyl ethylene glycol)-block-poly(ε-caprolactone)) diblock and MePEG- b -PBO- b -PCL (poly(methyl ethylene glycol)-block-poly(1,2-butylene oxide)-block-poly(ε-caprolactone)) into micelles using molecular dynamics simulation with a coarse grain (CG) force field based on quantum mechanics (CGq FF). The triblock polymer included a short poly(1,2-butylene oxide) (PBO) at the hydrophilic-hydrophobic interface of these systems. Keeping the hydrophilic length fixed (MePEG₄₅), we considered 250 chains in which the hydrophobic length changed from PCL₄₄ or PBO₆- b -PCL₄₃ to PCL₆₂ or PBO₉- b -PCL₆₁. The polymers were solvated in explicit water for 2 μs of simulations at 310.15 K. We found that the longer diblock system undergoes a morphological transition from an intermediate rod-like micelle to a prolate-sphere, while the micelle formed from the longer triblock system is a stable rod-like micelle. The two shorter diblock and triblock systems show similar self-assembly processes, both resulting in slightly prolate-spheres. The dynamics of the self-assembly is quantified in terms of chain radius of gyration, shape anisotropy, and hydration of the micelle cores. The final micelle structures are analyzed in terms of the local density components. We conclude that the CG model accurately describes the molecular mechanisms of self-assembly and the equilibrium micellar structures of hydrophilic and hydrophobic chains, including the quantity of solvent trapped inside the micellar core.     

An investigation of calibration materials for the measurement of metals in ambient particulate matter on filters by LA-ICP-MS

Three different types of simple and low-cost calibration material for the measurement of the metals content of ambient particulate matter (PM) on filters using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) have been compared: cellulose ester filters spiked with multi-element calibration solutions, pellets of compressed ambient particulate matter certified reference material (CRM), and powdered ambient particulate matter CRM adhered to a surface. Elements determined were As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn, each at approximate levels of 1000?ng per filter. Blank filters spiked with multi-elemental standards were found to be significantly more reproducible and repeatable than materials based on powdered reference materials. However, a comparison of these spiked filters with real samples of ambient PM showed that the analytical sensitivities obtained per mass of analyte were significantly different. It is concluded that the spiked filters could act as very effective quality control standards correcting, to within 1%, drifts in LA-ICP-MS measurements of up to 10%, or as indirect calibration materials supported by additional measurements using traditional wet chemical techniques.     

On the Solvent‐free Structure of a Jäger‐type Cobalt(II) N2O2‐Chelate Complex

A combined synchrotron X‐ray and density functional theory (DFT) study on the structure of a Jäger‐type N₂O₂ chelate complex was carried out. The ethoxy‐substituted bis(3‐oxo‐enaminato)cobalt(II) complex ( 1 ) was an original sample from the laboratory of the late Professor Ernst‐G. Jäger (University of Jena, Germany). Single‐crystal X‐ray analysis revealed essentially flat molecules of 1 , which are unsolvated and coordinatively unsaturated. The DFT calculations on the isolated molecule predict a planar structure for the non‐hydrogen atoms, which is a local minimum on the energy surface. The crystal packing is achieved through off‐set stacking (staircase arrangement), resulting in a herringbone pattern in the space group P2₁2₁2₁. The structure of 1 is compared to known structures of related bis(3‐oxo‐enaminato)cobalt(II) complexes ( 2 – 4 ). Original bulk material of 1 was investigated by scanning electron microscopy (SEM), powder X‐ray diffraction (PXRD), melting point determination, and infrared (IR) spectroscopy.     

Threshold crack speed controls dynamical fracture of silicon single crystals

Fracture experiments of single silicon crystals reveal that after the critical fracture load is reached, the crack speed jumps from zero to approximately 2 km/sec, indicating that crack motion at lower speeds is forbidden. This contradicts classical continuum fracture theories predicting a continuously increasing crack speed with increasing load. Here we show that this threshold crack speed may be due to a localized phase transformation of the silicon lattice from 6-membered rings to a 5-7 double ring at the crack tip.     

Gold Carbenoids: Lessons Learnt from a Transmetalation Approach

Carbophilic catalysts that are based on Au^I allow a host of different nucleophiles to be added across various π systems. 1 – 3 Although many of these reactions are thought to proceed via gold carbenoids, the challenge to observe and characterize these putative intermediates has basically been unmet. 4 The current mechanistic interpretation therefore largely relies on indirect evidence and computational data, some of which are subject to debate. 5 In an attempt to fill this gap, we pursued a potential route to gold carbenoids by formal transmetalation of chromium or tungsten Fischer carbene complexes with [LAu]⁺. Whereas this transformation proceeds with exceptional ease as long as a stabilizing heteroelement is present on the carbene center, it stops half‐way in its absence. Rather unusual bimetallic arrays are formed, which allow the charge density to delocalize over several positions. The obvious difficulty of releasing an “unstabilized” gold carbenoid has potential mechanistic implications for the understanding of π‐acid catalysis in general.     

Nanocomposites of Tantalum‐Based Pyrochlore and Indium Hydroxide Showing High and Stable Photocatalytic Activities for Overall Water Splitting and Carbon Dioxide Reduction

Nanocomposites of tantalum‐based pyrochlore nanoparticles and indium hydroxide were prepared by a hydrothermal process for UV‐driven photocatalytic reactions including overall water splitting, hydrogen production from photoreforming of methanol, and CO₂ reduction with water to produce CO. The best catalyst was more than 20 times more active than sodium tantalate in overall water splitting and 3 times more active than Degussa P25 TiO₂ in CO₂ reduction. Moreover, the catalyst was very stable while generating stoichiometric products of H₂ (or CO) and O₂ throughout long‐term photocatalytic reactions. After the removal of In(OH)₃, the pyrochlore nanoparticles remained highly active for H₂ production from pure water and aqueous methanol solution. Both experimental studies and density functional theory calculations suggest that the pyrochlore nanoparticles catalyzed the water reduction to produce H₂, whereas In(OH)₃ was the major active component for water oxidation to produce O₂.     

Asymmetric anti-resonant reflecting optical waveguides (arrow) as chemical sensors

Anti-resonant reflecting optical waveguides (ARROW) are described which trap light in a low index layer between a lower, high-index confining layer and an upper total internal reflection boundary. In this configuration, most of the light (greater than 80%) travels in the low index porous polymer layer, the refractive index of which is monitored by examining the angle at which light is coupled out of the waveguide. It is shown that asymmetric ARROW sensors can be constructed using conventional chemical vapour deposition and spin-coating techniques and their sensitivity is as predicted by theoretical modelling.     

Oxazolines as chiral building blocks for imidazolium salts and N-heterocyclic carbene ligands

Enantiomerically pure imidazolium triflates can be readily prepared from bioxazolines and oxazolineimines; deprotonation of imidazolium triflate 2 gives a chiral N-heterocyclic carbene that can act as a ligand in a catalytically active palladium complex.     

Regioselective silylation of sugars through palladium nanoparticle-catalyzed silane alcoholysis

Palladium(0)-catalyzed silane alcoholysis was applied to sugars for the first time using tert-butyldimethylsilane (TBDMS-H) and Ph(3)SiH as the silanes. The catalyst is a colloidal solution of Pd(0) generated in situ from PdX(2) (X = Cl(-), OAc(-)) and TBDMS-H in N,N-dimethylacetamide. The colloid has been characterized by dynamic light scattering and transmission electron microscopy and consists of catalytically highly active nanoparticles of approximately 2 nm diameter. The silane alcoholysis reaction is an effective method for the regioselective silylation of methyl and phenyl glycosides and generates hydrogen gas as the only side product. For many of the sugar substrates investigated, the distribution of regioisomers obtained is complementary to that of the traditional R(3)SiCl/base (base = pyridine, imidazole) methodology and gives convenient access to the 3,6- rather than the 2,6-silylated pyranosides, obtained as the main product by the silyl chloride method. The method also allows a selective axial silylation of levoglucosan and 1,3,5-O-methylidene-myo-inositol. In an attempt to rationalize the observed regioselectivities, ab initio predictions (HF/3-21G) have been made on the relative energies of some of the silylated products. They suggest that the observed regioselectivities do not reflect a kinetic vs thermodynamic product distribution but are induced by the silylation agent employed. Models for the possible origin of the observed regioselectivity in both silylation methods (silane- and silyl chloride-based) are discussed.     

Very low-pressure pyrolysis of cyclobutyl cyanide. The cyano stabilization energy

A very low-pressure pyrolysis (VLPP) apparatus has been constructed and shown to yield kinetic data consistent with other VLPP systems. The technique has been applied to the pyrolysis of cyclobutyl cyanide over the temperature range of 833–1203°K. The reaction was found to proceed via a single pathway to yield ethylene and vinyl cyanide. If A_∞ is based on previous high-pressure data for this reaction and for cyclobutane pyrolysis, then RRKM theory calculations show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by where θ=2.303 RT in kcal/mole. If A_∞ is adjusted relative to the more recent parameters for cyclobutane pyrolysis suggested by VLPP studies, then the Arrhenius expression becomes The cyano group reduces the activation energy for cyclobutane pyrolysis by 6±1 kcal/mole, and on the basis of a biradical mechanism this value may be attributed to the cyano stabilization energy.