Rotational constants of malonaldehyde and isotopic species derived from ab initio results

A quantum mechanical treatment based on ab initio results for the 21-dimensional potential energy surface of malonaldehyde is extended to yield the rotational constants of the lowest two states of the hydrogen transfer motion. The approximate separation of rotation from internal motion by an Eckart type transformation allows one to use a restricted basis set involving not more than one quantum of excitation for the 20 small vibrations. The results agree with experiment on the apparent structural effects of tunnelling excitation and their dependence on isotopic substitution. In the case of asymmetrically substituted species, the extent of localization confirms qualitative estimates from observation. The second moments derived from the rotational constants indicate that the molecular extension is underestimated in the O···O direction and overestimated perpendicular to this direction within the molecular plane. The agreement with experiment is shown to improve significantly by correcting the equilibrium geometry but only marginally by varying further properties of the minimum energy path.     

NIEDERKOORDINIERTE PHOSPHORVERBINDUNGEN, 701 1-METALLA-2,5-DIPHOSPHA PENTA-1,3-DIENE DURCH 1,4-ADDITION VON METALLCARBONYLCYCLOPENTADIENYLHYDRIDEN AN 1,4-DIPHOSPHABUTA-1,3-DIEN

Abstract

Reactions of carbonyl cyclopentadienyl hydrides from molybdenum and tungsten with 1,4-diphosphabuta-1,3-diene yield metalla-diphospha-pentadiene with a metal-phosphorus-double bond.

Metalla-diphosphapentadiene mit einer Metall-Phosphor-Doppelbindung werden durch Umsetzung von Carbonylcyclopentadienylhydriden des Molybdäns und Wolframs mit einem 1,4-Diphospha-buta-1,3-dien erhalten.     

NIEDERKOORDINIERTE PHOSPHORVERBINDUNGEN, 631. IMINO(BISMETHYLEN)PHOSPHATE

Abstract

Two imino(bismethylene)phosphat-ions can be synthesized via reaction of phenylamino- and α-naphthylaminobismethylenephosphorane with n-butyllithium in tetrahydrofuran. Anions containing [sgrave]³λ⁶-bounded phosphorus are stabilized in the crystal with THF-solvated lithium-kations.

Zwei Imino(bismethylen)phosphat-ionen werden durch Umsetzung des Phenylamino- und des α-Naphthylaminobismethylenphosphorans mit n-Butyllithium in Tetrahydrofuran erhalten. Die Anionen mit [sgrave]³λ⁶-gebundenem Phosphor werden im Kristall durch THF-solvatisierte Lithium-Kationen stabilisiert.     

Total and Semi‐Syntheses of Antimicrobial Thuggacin Derivatives

The total and semi‐synthesis of 13 new macrolactones derived from thuggacin, which is a secondary metabolite from the myxobacterium Sorangium cellulosum, are reported. The thuggacins have attracted much attention due to their strong antibacterial activity, particularly towards Mycobacterium tuberculosis. This study focuses on 1) thuggacin derivatives that cannot equilibrate by transacylation between the three natural thuggacins A–C, 2) the roles of the thiazole ring, and 3) the hexyl side chain at C2. Semi‐synthetic O‐methylation at C17 suppressed the transacylations without a substantial loss of antibacterial activity. Exchanging the C17–C25 side chain for simplified hydrophobic chains led to complete loss of antibacterial activity. Exchange of the thiazole by an oxazole ring or removal of the hexyl side chain at C2 had no substantial effect on the biological properties.     

Degradation of a polyester‐urethane coating: Physical properties

In this article we studied the evolution of thermomechanical properties of a polyester‐urethane coating during degradation under different degradation conditions, i.e., aerobic and anaerobic conditions with and without dry/wet cycling during degradation. Dynamic mechanical and thermal analyses show that under aerobic conditions the coatings become stiffer and more brittle in the glassy state. This stiffening is probably due to the increase in the amount of hydrogen bonding and the formation of oxidized groups which increase the polarity of the material and enhance the interactions of the polymer segments. However, oxidation reactions result in a considerable decrease in cross‐link density and stiffness in the rubbery state. Both changes, in the glassy and rubbery states, give rise to development of internal stresses. These stresses increase as the degradation process proceeds. Nevertheless, for samples exposed to anaerobic conditions, the stiffness remains constant in the glassy state and the cross‐link density slightly increases as a result of degradation. This reconfirms the dominance of the effect of oxidation reactions on the mechanical failure of the coatings. Oxygen permeation measurements show a more‐or‐less time‐independent diffusion coefficient and a gradual decrease in solubility of oxygen as a function of exposure time. This results in a slight decrease in oxygen permeation (mainly in the early stage of the degradation) as degradation proceeds. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 659–671     

Microswimmers – From Single Particle Motion to Collective Behavior

Locomotion of autonomous microswimmers is a fascinating field at the cutting edge of science. It combines the biophysics of self-propulsion via motor proteins, artificial propulsion mechanisms, swimming strategies at low Reynolds numbers, the hydrodynamic interaction of swimmers, and the collective motion and synchronisation of large numbers of agents. The articles of this Special Issue are based on the lecture notes of an international summer school, which was organized by the DFG Priority Programme 1726 “Microswimmers – From Single Particle Motion to Collective Behaviour” in the fall of 2015. The minireviews provide a broad overview of the field, covering both elementary and advanced material, as well as selected areas from current research.     

DYNAMIC STEREOCHEMISTRY OF SYMMETRICALLY BUT DIFFERENTLY SUBSTITUTED DIPHOSPHANES

Abstract

The main interest of our work in Bonn during the last years was in the field of phosphorus-carbon compounds with multiple bonds. Beside the synthesis of new classes of compounds we were particularly interested in the amazing analogy between the PC- and the CC-double bond, which could be proved by evidence of E/Z isomers and a clear indication of pericyclic reactions.