Two-electron homoaromatics with heteroatom bridges

Zwitterionic mono- and bis-homoaromatics 2a–f and 5a comprised of positively charged NMe₂, P(C₆H₅)₂ or As(C₆H₅)₂ bridges and anionic three-center two-electron (3c2e) delocalized boron heterocyclic units, were prepared and characterized by NMR as well as by X-ray structure analyses. The boron chemical shifts of the trishomoaromatic dianion 12a with an oxygen bridge compare well with those computed ab initio for model 12b. Analysis of the electronic structure of the bishomoaromatic 5u and its anionic analog 11u gives insight into the origin of the trend of increasing effectiveness of BC₂, B₂C and B₃ 3c2e bonds: higher electronegativity of carbon vs boron prevents symmetric delocalization in rings with B₂C and especially BC₂ centers.     

Synthese et etude spectrale de F-alkyl-4 coumarines

In this paper, we report synthesis and spectral characteristics of 24 new F-alkyl-4 coumarins and their synthetic intermediates, the corresponding F-alkyl β-keto esters. The reaction yields depend on the nature of the starting materials and the cyclisation agents. ¹H and ¹⁹F N.M.R. chemical shifts and coupling constants are assigned in every compound and a new kind of long distance H-F coupling constant ⁵J is observed and interpreted by means of geometric, steric and electronic effects as coupling through space.     

Metabolic products of microorganisms. 98. Phosphinothricin and phosphinothricyl-alanyl-analine

From cultures of Streptomyces viridochromogenes a new antibiotic, phosphinothricylalanyl-alanine, has been isolated. The new amino acid phosphinothricin is 2-amino-4-methyl-phosphino-butyric acid, its structure is confirmed by synthesis. The tripeptide is highly active against Gram-positive and Gram-negative bacteria and against the fungus Botrytis cinerea. Phosphinothricin is an active glutamine synthetase inhibitor.     

Dimorphie von SrTa4O11 - ein Schritt von der tetragonalen Bronzestruktur zur Struktur von CaTa4O11

Dimorphism of SrTa₄O₁₁–a Step from the Tetragonal Structure of Bronzes to the Structure of CaTa₄O₁₁ Hexagonal SrTa₄O₁₁ is a new modification and isostructural with CaTa₄O₁₁. It was obtained by heating the already known SrTa₄O₁₁ in a chlorine atmosphere at 1000–1100°C. The Guinier powder pattern could be indexed with the following hexagonal unit cell: a = 6.25 Å; c = 12.33 Å. In air at 1180°C SrTa₄O₁₁(hex.) changes into the well-known TTB-modification (corresponding to the tetragonal tungsten bronzes). The transition of SrTa₄O₁₁(TTB) to SrTa₄O₁₁(hex.) was only observed in the presence of a transporting agent (Cl₂) or a mineralizer (melt of B₂O₃) at temperatures below 1100°C. This transition could not be achieved by means of a solid state reaction. In a (Ca, Sr) Ta₄O₁₁ solid solution with at least 78 At.-% Ca the hexagonal form could be stabilized even at temperatures where otherwise the TTB-modification occurred.     

Laser micro mass analysis of bulk polymeric samples

Laser micro mass spectrometry (LAMMS) was used for organic micro-analysis of bulk polymeric samples with a lateral resolution of about 3 m. The experiments were performed with a LAMMA 1000 instrument which was modified by the addition of a transmission illumination system for the observation of thin sections and a heating/cooling sample stage with sample transfer lock. It was found that crystallites or agglomerates of low-molecular-weight compounds could be identified by both molecular ions and characteristic fragments, and cyclic oligomers in bulkpolyamide 6 could be detected as cationized species if cations were present in the sample. On the other hand, the identification of high-molecular-weight polymers, e.g., in inclusions, was based on structurally significant fragments of the repeat unit. For samples with a low absorbance at the laser wavelength used, this fragment ion yield could be maximized by evaporating a thin absorbing layer onto the sample surface. If thin sections could be prepared from a sample a micro-FTIR analysis was possible. Such experiments gave results which were complementary to those of the corresponding LAMMS analysis.     

Hyperbolic uniformization of Fermat curves

The ground-breaking research on the uniformization of curves was conducted at the beginning of the last century. Nevertheless, there are few examples in the literature of algebraic curves for which an explicit uniformization is known. In this article we obtain an explicit uniformization of the Fermat curves F _N , for each . The results presented here are based in part on an earlier study of the second author [6] in which each Riemann surface F _N (ℂ) was described as a quotient of the complex disk by a Fuchsian group Γ. 2000 Mathematics Subject Classification Primary—11F03, 11F06; Secondary—11F30 This work was partially supported by MCYT BFM2000-0627 and BMF2003-01898.     

Discriminantal Arrangements, Fiber Polytopes and Formality

Manin and Schechtman defined the discriminantal arrangement of a generic hyperplane arrangement as a generalization of the braid arrangement. This paper shows their construction is dual to the fiber zonotope construction of Billera and Sturmfels, and thus makes sense even when the base arrangement is not generic. The hyperplanes, face lattices and intersection lattices of discriminantal arrangements are studied. The discriminantal arrangement over a generic arrangement is shown to be formal (and in some cases 3–formal), though it is in general not free. An example of a free discriminantal arrangement over a generic arrangement is given.