Determination of the low ω cusp in the velocity autocorrelation spectrum of liquid sodium

Recently performed high resolution incoherent neutron scattering experiments on liquid sodium at smallA and low make the precise evaluation of the frequency distribution of the liquid feasible. An appropriate extension of the well known Egelstaff formula for the evaluation of the frequency distribution in a liquid is deduced. The new analysis is shown to be exact in the smallQ low region, removing the singularity of the above formula at 0 andQ0. The easy applicability of the evaluation is demonstrated on neutron scattering data of liquid sodium, where the long discussed low frequency cusp ofz() could be revealed.     

The crystallization kinetics and morphology of nitric acid trihydrate

Crystallization kinetics of stable and metastable nitric acid trihydrate (NAT) were investigated by time dependent X-ray powder diffraction (XRD) measurements. Kinetic conversion curves were evaluated adopting the Avrami model. The growth and morphology of the respective crystallites were monitored in situ on the cryo-stage of an environmental scanning electron microscope (ESEM) under a partial pressure of nitrogen gas (0.3 Torr, 40 Pa). The results show a close relationship between the presence of ice in the sample and the crystallization mechanism of NAT, which results in different shapes and sizes of NAT crystal particles.     

Insights into the mechanism of the site-selective sequential palladium-catalyzed cross-coupling reactions of dibromothiophenes/dibromothiazoles and arylboronic acids. Synthesis of PPARbeta/delta agonists

A reactivity study, aided by NMR spectroscopy, allowed a mechanistic rationale to be postulated for the palladium-catalyzed regioselective coupling of arylboronic acid (and arylstannane where feasible) at the position next to the sulfur atom in functionalized dibromothiophenes and dibromothiazoles. The analysis of the NMR spectra (using 19F from the boronic acid CF3 group and 31P from the phosphine of the catalyst as probes) of the entire reaction starting from the dibromoheterocycles allowed the qualitative proposal that the transmetalation is the rate-limiting step for both sequential substitution processes. The extremely facile oxidative addition at the C-Br bond next to the sulfur atom of the heterocycle instead determines the positional selectivity. An additional Stille reaction then replaced the second halogen, providing the trisubstituted heterocyclic scaffolds of PPAR ligands, which displayed PPARbeta/delta agonist activity, as revealed by reporter assays in living cells.     

Photochemistry of planar E-hexatrienes: an AM1 study

The photochemical formation of an allylcyclopropene derivative from E-hexatriene is studied with the AM1 semiempirical method. Two possible reaction mechanisms, with a differing sequence of [1,2]-hydrogen migration and [1,3]-bond formation, are compared. In this study a new phenomenon emerged: when three bonds of the triene are twisted by 90°, a conical intersection between ground and excited state occurs, through which efficient radiationless decay and product formation can take place. The reaction mechanism in which [1,3]-bond formation is the primary step is the more efficient of the two mechanisms studied, but decay via the conical intersection is somewhat more facile.     

Dimerization of metallated pigments as primary step in the construction of membrane gaps on gold,polymeric wires and isolable micelles

β‐Octasubstituted porphyrins form face‐to‐face dimers in solution, which are stabilized by π‐π′ interactions in case of π‐radical formation. Polymerization of viologen substituted zinc porphyrins leads depending on stereochemistry to micelles or wire pairs. Anionic porphyrins were also adsorbed to gold electrodes or colloids and entrapped in membrane gaps. Heterodimerization of these porphyrins adsorbed to solid phase surfaces and entrapped within a 2 nm membrane gap was detectable by fluorescence quenching. Ruthenium complexes with two bipyridyl and one dioctadecyl dipyridyl methylene malonate ligand form head‐to‐head dimers in aqueous solution. The octadecyl substituents of the dimer constituents point to opposite directions and large multilayer micelles are formed upon aggregatetion in water. They survive drying. Dimerization processes are discussed as common basis for the formation of four uncommon supramolecular assemblies.     

Improved MR imaging for patients with metallic implants

Pediatric oncology patients with large metallic prostheses were imaged with one of two MR imaging techniques: 1) the "tilted view-angle" technique, 2) or a higher readout bandwidth technique. The tilted view-angle method uses an additional gradient in the slice selection direction during readout. The high bandwidth technique increases the readout bandwidth and shortens the echo time (TE). High bandwidth and short echo times were implemented in both T(1)-weighted (T(1)W) turbo spin echo and turbo short tau inversion recovery (STIR) sequences. Both imaging techniques reduced the size of metal-induced image artifacts. The tilted view-angle method reduced the artifact to a greater degree but had inherent shortcomings. The reformatted images were blurred and shifted. The area of interest was often moved outside of the field of view, unless parameters were adjusted on the basis of a pre-scan calculation. The high readout bandwidth, short echo technique required no special preparation and reduced metal artifacts without image blurring. The combination of high-bandwidth, shorter echo turbo STIR and T(1)W turbo spin echo sequences with subtraction of pre- from post-contrast images allowed effective fat suppression without local field inhomogeneity affects. This greatly improved our ability to evaluate suspected disease near metallic implants in pediatric cancer patients.