Self-assembly of oligomeric porphyrin rings

A cobalt porphyrin equipped with two different but geometrically complementary pyridine ligands self-assembles to form an unusually stable complex with approximately 12 porphyrin monomers arranged in a macrocyclic array.     

Structure elucidation with lanthanide-induced shifts. 4—Bound shifts vs relative shifts

The use of lanthanide shift reagents (LSR's) to obtain additional structural information from nuclear magnetic resonance studies has gained widespread acceptance. However, there has not been general agreement with regard to the most appropriate methodology for analysis of the shifted NMR spectra. We present arguments that only the bound shifts (Δ₁) corresponding to the LS complex should be used for correlation of lanthanide-induced shifts with molecular structure by means of the pseudocontact equation. Several examples are discussed of compounds for which the relative induced shifts are dependent on the concentration of LSR. For such cases it is not possible for both Δ₁ and Δ₂ (the bound shift corresponding to the LS₂ complex) to correlate successfully with the correct structure. Alternative methods of obtaining bound shifts are critically evaluated.     

Polarisation parameters in neutral pion photoproduction

The polarisation parameters Σ, P and T have been measured for the process γp→π⁰p in the photon energy range 1300–2100 MeV and c.m. angles between 30° and 110°, in an experiment with a polarised beam and polarised target. The results are compared with a recent theoretical analysis which fits data from threshold to 16 GeV. The new data are in general agreement with the analysis, but with some significant discrepancies in detail.     

Microstructure of ferroelectric domains in BaTiO3 observed via x-ray microdiffraction

X-ray microdiffraction utilizing Fresnel zone plate focusing optics has been used to study microstructural properties of individual 90 degree ferroelectric domains in BaTiO3. Diffraction measurements at a microfocused spot resolution of 0.3 microm over domain widths of approximately 10 microm unambiguously reveal features of lattice buckling, rotation, and strain near domain boundaries. Our results may be understood within the context of bound residual strain due to lattice mismatch and elastic interactions between neighboring domains.     

Tris(thioacetals) from benzene hexathiol: towards covalent self-assembly

Mixtures of isomers are available from the reaction of benzene hexathiol with three equivalents of p-tolualdehyde and kinetic traps avoided under the reported catalytic conditions, establishing tris(thioacetals) as potential building blocks for covalently self-assembled complex structures.     

Monte Carlo simulation of growth of porous SiOx by vapor deposition

A random network model containing defects has been developed and applied to the deposition of amorphous SiOx films on a flat substrate. A new Monte Carlo procedure enables dangling bonds to migrate and annihilate. The degree of porosity in the films is found to increase with oxygen content. As the oxygen content increases a larger fraction of pore surfaces is covered with oxygen, and the density of dangling bonds on pore surfaces decreases. Oxygen plays the role of a surfactant, lowering the energies of pore surfaces and enhancing the porosity of amorphous SiO2 compared to amorphous Si.     

Current-induced embrittlement of atomic wires

Recent experiments suggest that gold single-atom contacts and atomic chains break at applied voltages of 1 to 2 V. In order to understand why current flow affects these defect-free conductors, we have calculated the current-induced forces on atoms in a Au chain between two Au electrodes. These forces are not by themselves sufficient to rupture the chain. However, the current reduces the work to break the chain, which results in a dramatic increase in the probability of thermally activated spontaneous fracture of the chain. This current-induced embrittlement poses a fundamental limit to the current-carrying capacity of atomic wires.     

The total synthesis of pantocin B

[reaction: see text] Pantocin B, an unusual antibiotic produced by Erwinia herbicola, effectively controls E. amylovora, the pathogen causing the plant disease fire blight. A total synthesis of pantocin B from L-alanine, glycine, and L-malic acid is reported.