Mössbauer effect and NMR study of amorphous Ni80−x Fe x B16Si4

The magnetic phase diagram of amorphous Ni₈₀? _x Fe _x B₁₆Si₄ was studied by de-magnetization measurement, by⁵⁷Fe-Mössbauer spectroscopy and pulsed spin echo NMR.     

Felkin–Anh is not enough

The understanding of the electronic effects of the diastereoselective addition of a nucleophile to a polar substituted aldehyde or ketone is not complete, with several theories competing to explain the data. For numerous hydride reductions of 3‐X‐2‐butanones (X = F, Cl, Br), the selectivity for the major syn isomer is significantly and consistently higher for X = Br than for X = F. This result is rationalized as a shift in mechanism from Cornforth (X = F) to Felkin–Anh (X = Br). The experimental data is well modeled by ab initio calculations for the addition to these ketones by BH₃, but not by other nucleophiles such as LiH or LiAlH₄. The energetic ordering of the BH₃ transition states largely follows the trends for the ground state ketones. Here, consistent with electrostatic arguments, the anti orientation of the C―X and C?O bonds is always lower in energy than the syn arrangement. The gauche conformer is intermediate between these two, becoming gradually lower in energy as X increases in size. The hyperconjugative interaction invoked by the Felkin–Anh model provides only a modest stabilization of the relevant transition states as judged by NBO analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Influence of excited state aromaticity in the lowest excited singlet states of fulvene derivatives

The absorption spectra and excited state dipole moments of four differently substituted fulvenes have been investigated both experimentally and computationally. The results reveal that the excited state dipole moment of fulvenes reverses in the first excited singlet state when compared to the ground state. The oppositely polarized electron density distributions, which dominate the ground state and the first excited singlet state of fulvenes, respectively, reflect the reversed π-electron counting rules for aromaticity in the two states (4n + 2 vs. 4n, respectively). The results show that substituents indeed influence the polarity of fulvenes in the two states, however, cooperative interactions between the substituents and the fulvene moiety are most pronounced in the ground state.     

Effect of colloidal particle size on adsorbed monodisperse and bidisperse monolayers

Coating hydrogel films or microspheres by an adsorbed colloidal shell is one synthesis method for forming colloidosomes. The colloidal shell allows control of the release rate of encapsulated materials, as well as selective transport. Previous studies found that the packing density of self-assembled, adsorbed colloidal monolayers is independent of the colloidal particle size. In this paper we develop an equilibrium model that correlates the packing density of charged colloidal particles in an adsorbed shell to the particle dimensions in monodisperse and bidisperse systems. In systems where the molar concentration in solution is fixed, the increase in adsorption energy with increasing particle size leads to a monotonic increase in the monolayer packing density with particle radius. However, in systems where the mass fraction of the particles in the adsorbing solutions is fixed, increasing particle size also reduces the molar concentration of particles in solution, thereby reducing the probability of adsorption. The result is a nonmonotonic dependence of the packing density in the adsorbed layer on the particle radius. In bidisperse monolayers composed of two particle sizes, the packing density in the layer increases significantly with size asymmetry. These results may be utilized to design the properties of colloidal shells and coatings to achieve specific properties such as transport rate and selectivity.     

Instrumental neutron activation analysis of rock samples from Ugep area of Cross River State,Nigeria

The theory and analytical procedures in Instrumental Neutron Activation Analysis (INAA) are discussed. Application of the method to the determination of sixteen trace elements: Na, K, Mn, Sm, La, Fe, Th, Sc, Co, Ba, Rb, Br, Sb, Cs and Ta in some rock samples from Ugep in the Lower Benue Region of Nigeria, is described. The high radioactivity observed in some of the samples analyzed has been associated with the higher-than-normal concentrations of Th and K in the samples. Also, Ba and Fe concentrations in some of the samples were found to be anomalous when compared with average expected values in normal sedimentary rocks.     

Assessment of organolead species in the Austrian Danube-basin using GC-MIP-AED

In order to evaluate the effectiveness of recent restrictions of Austrian government for the consumption of leaded gasoline, ionic organolead species were determined in 13 sampling sites of the Austrian and Slovakian Danube-basin in 4 bimonthyl sampling campaigns. Speciation analysis was performed using a rapid and sensitive method based on Grignard derivatization of the ionic organolead species and a GC-MIP-AED coupling for separation and detection. The operational variables were optimized for chromatographic resolution and detection limits. 100 ml of sample were used for the analyses and the detection limits for trimethyllead were 0.5 ng Pb l^–1 and for triethyllead 0.85 ng Pb l^–1. In general, absence of organolead induced pollution has been observed for most of the sampling locations and campaigns. Only the part of the river Danube between Vienna (Austria) and Bratislava (Slovakia), which is loaded by various intensive anthropogenic sources, showed in two campaigns the occurrence of very low trimethyl- and triethyllead concentrations, ranging between 1.2–12.0 ng Pb l^–1 and 1.6–5.8 ng Pb l^–1, respectively. Received: 21 March 1996 / Revised: 6 May 1996 / Accepted: 9 May 1996