On the Hebb–Wagner polarisation of SrTiO3 doped with redox-active ions

Hebb–Wagner polarisation is analysed for the case that internal redox-reactions complicate the situation. In the general case current–voltage characteristics deviate significantly from the Hebb–Wagner equation. Analytical results are derived and relevant approximations given. Theoretical and experimental current–voltage relations obtained for a 2.15×10¹⁸ cm⁻³ Fe-doped SrTiO₃ single crystal at P_O2=10⁵ Pa, agree reasonably well, and the results of the partial conductivities match findings obtained from other experiments. In addition, the fact is emphasised that a Hebb–Wagner-type current–voltage relation only requires the fulfilment of a power law for the non-blocked carrier concentrations with respect to the component partial pressure and not a field-free situation. In this case the Faraday constant appearing in the Hebb–Wagner equation has to be replaced by F, where is the product of the number of electrons necessary to ionise the gaseous component (n) times the absolute characteristic exponent (|N|). The condition of zero field, i.e. negligible chemical potential gradient with respect to the non-blocked species is identical to |N|=1/n. If |N|=const≠1/n, the internal field is non-zero but is still proportional to the gradient of the component potential.     

Perfusion imaging of the human lung using flow-sensitive alternating inversion recovery with an extra radiofrequency pulse (FAIRER)

Pulmonary perfusion is an important parameter in the evaluation of lung diseases such as pulmonary embolism. A noninvasive MR perfusion imaging technique of the lung is presented in which magnetically labeled blood water is used as an endogenous, freely diffusible tracer. The perfusion imaging technique is an arterial spin tagging method called Flow sensitive Alternating Inversion Recovery with an Extra Radiofrequency pulse (FAIRER). Seven healthy human volunteers were studied. High-resolution perfusion-weighted images with negligible artifacts were acquired within a single breathhold. Different patterns of signal enhancement were observed between the pulmonary vessels and parenchyma, which persists up to TI = 1400 ms. The T1s of blood and lung parenchyma were determined to be 1.46s and 1.35 s, respectively.     

New strategies for synthesis of partially fluorinated heterocycles as precursors for trifluoromethyl-substituted polymers

The building-block strategy is applied to the synthesis of new compounds with two five-membered heteroaromatic units. The heteroaromatic rings contain a fluorine atom which is activated towards nucleophilic displacement reactions by an adjacent trifluoromethyl group. The difunctional compounds can be converted into poly(arylene ether)s by base-catalyzed polycondensation with bisphenols or bisthiophenols. Trifluoromethylated poly(arylether oxazole)s and poly(arylthioether oxazole)s are obtained by this route. All representatives of these two new classes of polymers are soluble in common organic solvents. Reaction conditions for the polycondensation as well as thermal stability, glass transition temperatures, and molar masses of the polymers are described.     

Transformation der kubischen Thetafunktion

Ohne Zusammenfassung     

Aus dem Gebiet der Differenzengleichungen

Ohne ZusammenfassungW. Wirtinger zum 70. Geburtstage.     

On-Surface Synthesis of Porous Carbon Nanoribbons on Silver: Reaction Kinetics and the Influence of the Surface Structure

We report on the influence of the surface structure and the reaction kinetics in the bottom-up fabrication of porous nanoribbons on silver surfaces using low-temperature scanning tunneling microscopy. The porous carbon nanoribbons are fabricated by the polymerization of 1,3,5-tris(3-bromophenyl)benzene directly on the Ag surface using an Ullmann-type reaction in combination with dehydrogenative coupling reactions. We demonstrate the successful on-surface synthesis of porous nanoribbons on Ag(111) and Ag(100) even though the self-assemblies of the intermediate organometallic structures and covalently-linked polymer chains are different on both surfaces. Furthermore, we present the formation of isolated porous nanoribbons by kinetic control. Our results give valuable insights into the role of substrate-induced templating effects and the reaction kinetics in the on-surface synthesis of conformationally flexible molecules.     

In Situ Mass Spectrometric and Kinetic Investigations of Soai's Asymmetric Autocatalysis

Chemical reactions that lead to a spontaneous symmetry breaking or amplification of the enantiomeric excess are of fundamental interest in explaining the formation of a homochiral world. An outstanding example is Soai's asymmetric autocatalysis, in which small enantiomeric excesses of the added product alcohol are amplified in the reaction of diisopropylzinc and pyrimidine-5-carbaldehydes. The exact mechanism is still in dispute due to complex reaction equilibria and elusive intermediates. In situ high-resolution mass spectrometric measurements, detailed kinetic analyses and doping with in situ reacting reaction mixtures show the transient formation of hemiacetal complexes, which can establish an autocatalytic cycle. We propose a mechanism that explains the autocatalytic amplification involving these hemiacetal complexes. Comprehensive kinetic experiments and modelling of the hemiacetal formation and the Soai reaction allow the precise prediction of the reaction progress, the enantiomeric excess as well as the enantiomeric excess dependent time shift in the induction period. Experimental structural data give insights into the privileged properties of the pyrimidyl units and the formation of diastereomeric structures leading to an efficient amplification of even minimal enantiomeric excesses, respectively.