Thermosensitive hairy hybrid nanoparticles synthesized by surface-initiated atom transfer radical polymerization

This article reports on the synthesis of thermosensitive polymer brushes on silica nanoparticles by atom transfer radical polymerization (ATRP) and the study of thermo-induced phase transitions in water. Silica nanoparticles were prepared by the St?ber process and the surface was functionalized by an ATRP initiator. Surface-initiated ATRPs of methoxydi(ethylene glycol) methacrylate (DEGMMA) and methoxytri(ethylene glycol) methacrylate (TEGMMA) were carried out in THF at 40 degrees C in the presence of a free initiator, benzyl 2-bromoisobutyrate. The polymerizations were monitored by 1H NMR spectroscopy and gel permeation chromatography. The hairy hybrid nanoparticles were characterized by thermogravimetric analysis and scanning electron microscopy, and the thermoresponsive properties were investigated by variable temperature 1H NMR spectroscopy and dynamic light scattering. The cloud points of free poly(DEGMMA) and poly(TEGMMA) in water were around 25 and 48 degrees C, respectively. The thermo-induced phase transitions of polymer brushes on silica nanoparticles began at a lower temperature and continued over a broader range (4-10 degrees C) than those of free polymers in water (< 2 degrees C).     

The limitations of Slater's element-dependent exchange functional from analytic density-functional theory

Our recent formulation of the analytic and variational Slater-Roothaan (SR) method, which uses Gaussian basis sets to variationally express the molecular orbitals, electron density, and the one-body effective potential of density-functional theory, is reviewed. Variational fitting can be extended to the resolution of identity method, where variationality then refers to the error in each two-electron integral and not to the total energy. However, a Taylor-series analysis shows that all analytic ab initio energies calculated with variational fits to two-electron integrals are stationary. It is proposed that the appropriate fitting functions be charge neutral and that all ab initio energies be evaluated using two-center fits of the two-electron integrals. The SR method has its root in Slater's Xalpha method and permits an arbitrary scaling of the Slater-Gàspàr-Kohn-Sham exchange-correlation potential around each atom in the system. The scaling factors are Slater's exchange parameters alpha. Of several ways of choosing these parameters, two most obvious are the Hartree-Fock (HF) alpha(HF) values and the exact atomic alpha(EA) values. The former are obtained by equating the self-consistent Xalpha energy and the HF energies, while the latter set reproduces exact atomic energies. In this work, we examine the performance of the SR method for predicting atomization energies, bond distances, and ionization potentials using the two sets of alpha parameters. The atomization energies are calculated for the extended G2 set of 148 molecules for different basis-set combinations. The mean error (ME) and mean absolute error (MAE) in atomization energies are about 25 and 33 kcal/mol, respectively, for the exact atomic alpha(EA) values. The HF values of exchange parameters alpha(HF) give somewhat better performance for the atomization energies with ME and MAE being about 15 and 26 kcal/mol, respectively. While both sets give performance better than the local-density approximation or the HF theory, the errors in atomization energy are larger than the target chemical accuracy. To further improve the performance of the SR method for atomization energies, a new set of alpha values is determined by minimizing the MAE in atomization energies of 148 molecules. This new set gives atomization energies half as large (MAE approximately 14.5 kcal/mol) and that are slightly better than those obtained by one of the most widely used generalized-gradient approximations. Further improvements in atomization energies require going beyond Slater's functional form for exchange employed in this work to allow exchange-correlation interactions between electrons of different spins. The MAE in ionization potentials of 49 atoms and molecules is about 0.5 eV and that in bond distances of 27 molecules is about 0.02 A. The overall good performance of the computationally efficient SR method using any reasonable set of alpha values makes it a promising method for study of large systems.     

Nanoscale near-field infrared spectroscopic imaging of silica-shell/gold-core and pure silica nanoparticles

Spectroscopic near-field imaging of single silica-shell/Au-core and pure silica nanoparticles deposited on a silicon substrate is performed in the infrared wavelength range (λ = 9–11 μm) using scattering-type scanning near-field optical microscopy (s-SNOM). By tuning the wavelength of the incident light, we have acquired information on the spectral phonon–polariton resonant near-field interactions of the silica-shell/Au-core and pure silica nanoparticles with the probing tip. We made use of the enhanced near-field coupling between the high index Au-core and the probing tip to achieve spectral near-field contrast of the thin silica coating (thickness < 10 nm). Our results show that spectroscopic imaging of thin coating layers and complex core–shell nanoparticles can be directly performed by s-SNOM.     

Oxidation of enones to α′-acetoxyenones using manganese triacetate

The oxidation of α,β-unsaturated ketones with manganese (III) triacetate in benzene furnishes α'-acetoxyenones in good yields.     

A Mössbauer effect study of structural ordering in rapidly quenched Fe–Ga alloys

Samples of Fe_100−xGa_x (x=8.3, 17.9, 20.5 and 23.3) were prepared by rapid solidification from the melt using a single Cu roller. X-ray diffraction studies of all samples showed them to be single phase with the disordered BCC structure. No evidence of superlattice reflections from D0₃ ordering was observed for any of the samples. Room-temperature ⁵⁷Fe Mössbauer effect spectra indicated that all samples were ferromagnetically ordered. Spectra were fit to distributions of hyperfine fields. The x=8.3 sample showed a hyperfine field distribution that was single peaked and indicated a reasonably random distribution of local Fe environments. The x=17.9 and 20.5 samples showed hyperfine field distributions that were bimodal and indicated two distinct local Fe environments. The x=23.3 sample showed three distinct field components. It is suggested that the x=8.3, 17.9 and 20.5 alloys are primarily a disordered BCC phase. The x=8.3 alloy shows a small amount of short-range Ga–Ga pairing, while this short-range pairing is significantly greater in the x=17.9 and 20.5 alloys. The three field components in the x=23.3 alloy correspond well to the two sites associated with the D0₃ phase and a third component corresponding to a remaining L1₂ phase suggesting the presence of at least short-range D0₃ clustering in this alloy.     

Hyperfine magnetic field on Cd-111 in heusler alloys Co2MnZ (Z=Si,Ga, Ge,Sn)

The time differential perturbed angular correlation method has been used to measure, as a function of temperature, the hyperfine magnetic field at Cd sites in the Heusler alloys Co₂MnZ (Z=Si, Ga, Ge, Sn). The hyperfine fields, normalized to the total magnetic moment per formula unit, show an approximately linear trend toward more positive values with increasing lattice parameter.     

An investigation of Fe oxidation states and site distributions in a Tibetan tektite

Room temperature ⁵⁷Fe Mössbauer effect measurements have been used to investigate the local Fe environment and the Fe³⁺/Fe²⁺ ratio in a Tibetan tektite. The spectrum has been analyzed in terms of two quadrupole splitting distributions; one corresponding to Fe²⁺ and one corresponding to Fe³⁺. The Fe²⁺ sites show a mean isomer shift of +1.011 mm/s relative to \alpha-Fe and a mean quadrupole splitting of 1.845 mm/s. The Fe²⁺ quadrupole splitting distribution showed a large peak for positive values, which has been identified with Fe²⁺ with local octahedral coordination, and a smaller peak for negative values, which has been identified with Fe²⁺ with local tetrahedral coordination. The Fe³⁺ sites showed an average isomer shift of +0.20 mm/s and a single peaked quadrupole splitting distribution with a mean near zero. The Fe³⁺/Fe²⁺ ratio, based on the relative areas of the spectra components was found to be 0.015. These results are discussed in terms of those obtained using fitting methods based on discrete sites and other distributions of parameters. Hyperfine parameters for the Tibetan tektite are compared with those obtained for tektites from other regions of the Australasian strewn field. All tektites studied show similar quadrupole distributions and similar mean hyperfine parameters. Fe³⁺/Fe²⁺ ratios ranged from 0.005 to 0.036. The relationship of the microstructure of the Tibetan tektite with that of tektites from various regions of the Australasian strewn field is discussed.