Partial correctness of exits from concurrent structures

A rudimentary exit-mechanism from the parallel command of the language fragment CSP is introduced. A method for embedding invariants in a standard partial correctness system with pre-and postconditions is presented. Proof rules for exits from concurrent systems are introduced, and a simple data flow system is verified.     

Varför Tunneldioder?

In the last two years the electronic industry has shown a great interest in tunnel diodes. This article gives a short explanation of the basic effect and summarizes the more important features of the device. The general fabrication technique is outlined, and some associated problems are discussed.     

Untersuchungen an Chitin. 2. Mitt. Einflu? der Deacetylierung auf die L?slichkeit

Ohne Zusammenfassung     

Fast Oxidative Cyclooligomerization towards Low‐ and High‐Symmetry Thiophene Macrocycles

Macrocycles with quaterthiophene subunits were obtained by cyclooligomerization by direct oxidative coupling of unsubstituted dithiophene moieties. The rings were closed with high selectivity by an α,β′‐connection of the thiophenes as proven by NMR spectroscopy. The reaction of the precursor with terthiophene moieties yielded the symmetric α,α′‐linked macrocycle in low yield together with various differently connected isomers. Blocking of the β‐position of the half‐rings yielded selectively the α,α′‐linked macrocycle. Selected cyclothiophenes were investigated by scanning tunneling microscopy, which displayed the formation of highly ordered 2D crystalline monolayers.     

Symmetry of three-center,four-electron bonds

Three-center, four-electron bonds provide unusually strong interactions; however, their nature remains ununderstood. Investigations of the strength, symmetry and the covalent versus electrostatic character of three-center hydrogen bonds have vastly contributed to the understanding of chemical bonding, whereas the assessments of the analogous three-center halogen, chalcogen, tetrel and metallic -type long bonding are still lagging behind. Herein, we disclose the X-ray crystallographic, NMR spectroscopic and computational investigation of three-center, four-electron [D–X–D]⁺ bonding for a variety of cations (X⁺ = H⁺, Li⁺, Na⁺, F⁺, Cl⁺, Br⁺, I⁺, Ag⁺ and Au⁺) using a benchmark bidentate model system. Formation of a three-center bond, [D–X–D]⁺ is accompanied by an at least 30% shortening of the D–X bonds. We introduce a numerical index that correlates symmetry to the ionic size and the electron affinity of the central cation, X⁺. Providing an improved understanding of the fundamental factors determining bond symmetry on a comprehensive level is expected to facilitate future developments and applications of secondary bonding and hypervalent chemistry.

The factors determining the symmetry and the fundamental nature of the three-center, four-electron bonds are assessed.     

Advances in the search for superheavy elements at GSI

The nuclear shell model predicts that the next doubly magic shell-closure beyond ²⁰⁸Pb is at a proton number between Z=114 and 126 and at a neutron number N=184. The outstanding aim of experimental investigations is the exploration of this region of spherical ‘SuperHeavy Elements’ (SHE’s). This article describes the experiments that were performed at GSI, Darmstadt. They resulted in an unambiguous identification of elements 107 to 112. They were negative so far in searching for elements 113, 116 and 118. Plans are presented for an upgrade of the experimental set-up aiming at an improved sensitivity for further exploration of SHE’s.