Extrusion of pastes with a piston extruder for the determination of the local solid and fluid concentration, the local porosity and saturation and displacement profiles by means of NMR imaging

 Nuclear Magnetic Resonance (NMR) was used to investigate the extrusion behaviour of PTFE pastes in a ram extruder. By means of ¹H-NMR imaging (MRI) it is possible to determine the local proton density and therefore, the local fluid concentration. The ¹⁹F-MRI provides the local solid concentration. Thus the local saturation and the local porosity can be calculated with the information of the local fluid and solid concentration. Furthermore displacement profiles can be derived from NMR images by means of correlation techniques without any preparation or marking of the pastes. Received: 8 May 2000   Accepted: 1 May 2001     

SPECTRAL CHARACTERISTICS AND MECHANISM OF CHEMILUMINESCENCE FROM TRYPTOPHAN SOLUTIONS INDUCED BY UV-IRRADIATION

The spectral distribution of the chemiluminescence, fluorescence and phosphorescence of tryptophan aqueous solutions irradiated with high and low pressure mercury lamps has been measured. The blue emission bands in the region of 380–520 nm observed both in the chemi- and photoluminescence, as well as an absorbance increase at 230 and 330 nm, indicate oxidative degradation of tryptophan leading to the formation of derivatives of N-formylkynurenine, xanthurenic and anthranilic acids. Red emission bands at 630 and 705 nm in the spectrum of the chemiluminescence, an enhancement of light intensity by D₂O and its decrease by NaN₃ and DABCO suggest a partial contribution of O₂(¹Δ_g) to the photooxidation and chemiluminescence of tryptophan. The enthalpy of the exergonic reactions, leading to the formation of luminescing products, was calculated to average -270 kJ-mol.     

Simple organocatalysts in multi-step reactions: An efficient one-pot Morita-Baylis-Hillman-type α-hydroxymethylation of vinyl ketones followed by the convenient,temperature-controlled one-pot etherification using alcohols

1,4-Diazabicyclo[2.2.2]octane (DABCO) was utilized as versatile catalyst in a one-pot synthesis: First, for the preparation of alcoholic formaldehyde solutions from para-formaldehyde catalysed by using low loadings of inexpensive DABCO. Second, for the fast α-hydroxymethylation of alkylic and aromatic vinyl ketones in high yields. In a third step, the same catalyst can be used for an optional, temperature controlled in situ etherification of the Morita-Baylis-Hillman product with various alcohols on a multi-gram scale in moderate to good overall yields and high purities. Furthermore, an application of the ether in the enantioselective synthesis of a common building block for total synthesis is shown, thus providing a gram-scale access from inexpensive bulk chemicals.     

Structural and electronic effects involving pyridine rings in 4-methylpyridine Cu4OX6L4 complexes. II. Correlations based on molecular structure of the Cu4OCl6(4-Mepy)4 complex

Correlations involving bond lengths and bond angles in the molecular structure of the Cu₄OCl₆(4-Mepy)₄ complex (4-Mepy = 4-methylpyridine) with four symmetrically independent molecules present in the unit cell showed that the donor-acceptor behavior involving the π-back donation into the pyridine rings of the 4-Mepy ligands is most effectively stimulated by a suitable orientation of the pyridine rings in the trigonal bipyramidal geometry. The pyridine ring planes are almost in parallel orientation with one of the three Cu-Cl bonds. The bond lengths of these Cu-Cl bonds are in a significant linear correlation with the Cu-N bond lengths and the bonds lengths of the pyridine rings. The pyridine rings orientation is affected by distortion of the trigonal bipyramidal geometry to tetragonal pyramidal coordination, by out-of plane pyridine rings deviation and in-plane pyridine rings tilting, by puckering of the pyridine rings and by the effects of the methyl groups. The pyridine rings in at least seven of the sixteen trigonal bipyramidal coordinations exhibit an orientation supporting the π-back bonding between the Cu(II) atoms and the pyridine rings.     

Crystal and molecular structure of diastereomeric 2-diphenylthiophosphinoyl-cis-(4,6-dimethyl)-1,3-dithianes

The crystal and molecular structures of a pair of diastereomeric 2-diphenylthiophosphinoyl-cis-(4,6-dimethyl)-1,3-dithianes 2 have been determined by the X-ray method. The differences in corresponding bond distances in cis-2 and trans-2 are discussed in context of the anomeric effect operating in this heterocyclic system. The comparison of selected structural parameters between diastereomeric cis- and trans-dithianes 2 and their 5-t-butyl analogs 1 is also discussed. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:537–541, 1998     

Charge carrier transport in heterogeneous conducting polymer materials

The similarities of conductivity mechanisms of composites and nanocomposites and doped conjugated polymers that are also characteristic of specific heterogeneity are discussed. It is shown that the formulae developed to account for internal heterogeneity of conductive polymers can be applied also for polymer composites in spite of low overall content of the conductive phase. For fully organic nanocomposites (reticulate doped polymers) showing metal-like conductivity and a crossover temperature effective contribution of metallic phase is estimated. Examples of different properties of nanoparticles forming conducting networks as compared with the bulk crystals are discussed.