The defining feature of aromatic hydrocarbon compounds is a cyclic molecular structure stabilized by the delocalization of pi electrons that, according to the Hückel rule, need to total 4n + 2 (n = 1,2, em leader ); cyclic compounds with 4n pi electrons are antiaromatic and unstable. But in 1964, Heilbronner predicted on purely theoretical grounds that cyclic molecules with the topology of a Möbius band--a ring constructed by joining the ends of a rectangular strip after having given one end half a twist--should be aromatic if they contain 4n, rather than 4n + 2, pi electrons. The prediction stimulated attempts to synthesize Möbius aromatic hydrocarbons, but twisted cyclic molecules are destabilized by large ring strains, with the twist also suppressing overlap of the p orbitals involved in electron delocalization and stabilization. In larger cyclic molecules, ring strain is less pronounced but the structures are very flexible and flip back to the less-strained Hückel topology. Although transition-state species, an unstable intermediate and a non-conjugated cyclic molecule, all with a Möbius topology, have been documented, a stable aromatic Möbius system has not yet been realized. Here we report that combining a 'normal' aromatic structure (with p orbitals orthogonal to the ring plane) and a 'belt-like' aromatic structure (with p orbitals within the ring plane) yields a Möbius compound stabilized by its extended pi system.

In twisted single-mode optical fibers the polarization of light is affected by an elastooptically induced optical activity and by a modification of any linear birefringence present. These effects are discussed theoretically and demonstrated experimentally. The activity/twist ratio is alpha/tau approximately 0.13 ... 0.16 universally in weakly guiding silica fibers. Twisted fibers may be used as polarization rotators. A fiber with a +/-68 degrees double twist operates as a fast/slow mode interchanger, suitable for delay equalization.

The microscopic theory of superconductivity worked out by Bardeen, Cooper, and Schrieffer led to a fundamental understanding of the phenomenon. However, this theory does not provide a materials aspect. Starting from the special situation of chemical bonding in carbides and carbide halides of the rare earth metals a general view of the origin cability to high-temperature superconof superconductivity is developed, based ductors discovered during the last on a tendency for (pairwise) localization decade is discussed. of conduction electrons. This approach is tested in terms of special features in Keywords: band structure * carbides * the electronic band structure and com-cuprates * lanthanoids -superconducpared with existing physical models of tivity electron pairing in real space. Its appli-

Heilbronner in 1964 predicted that annulenes with ".. a planar perimeter of N=4r AO's, which would yield an open shell configuration when occupied by 4r electrons, can be twisted into a closed shell Möbius strip perimeter without loss in pi electron energy". We have been able to synthesize the first [4n]annulene with such a Möbius topology and now present further Möbius isomers and the details of their preparation as stable compounds. To address the question whether the twist in the pi system has an effect on the properties we systematically investigate energy, geometry and magnetic parameters of a large number isomers of [16]annulenes. The Möbius twisted annulenes are consistently more aromatic than the non-twisted isomers. This is true for the parent as well as our benzoannelated systems. Our results are in contrast to those published recently by C. Castro, W. L. Karney, P. von R. Schleyer et al.