A neutron powder diffraction study of the helimagnetic structure of TlCo2Se2−xSx

The magnetic layer structure of TlCo₂Se_2−xS_x has been thoroughly re-investigated with neutron powder diffraction. The cobalt magnetic moments are ferromagnetically arranged within the layers, but the interlayer coupling differs profoundly with varying composition (x): the spins in TlCo₂Se₂ form a helix along the c-axis with a turning-angle of ∼119° at 1.4 K. This kind of helical structure prevails for 0≤x≤1.5 with a gradual decrease of the angle with increasing sulphur content, down to 34°, showing an almost linear relationship with the interlayer distance of Co-Co. For x≥1.75 the interlayer coupling changes to ferromagnetic. Unexpectedly, two helices were found to coexist at x=0.5 and x=1.0. The interaction between adjacent cobalt layers is there characterized by an incommensurate angle (106°, resp., 73°) together with a commensurate angle of 90°. The magnetic structures have been refined as two magnetic phases, each having a characteristic wave vector. A tentative model where the symmetry of the structure and the interlayer distance compete is considered for explaining the simultaneous occurrence of the two kinds of diffraction profile satellites.     

Automated and rapid online determination of 15N abundance and concentration of ammonium, nitrite, or nitrate in aqueous samples by the SPINMAS technique

On the basis of the principle of reaction continuous-flow quadrupole mass spectrometry, an automated sample preparation unit for inorganic nitrogen (SPIN) species was developed and coupled to a quadrupole Mass Spectrometer (MAS). The SPINMAS technique was designed for an automated, sensitive, and rapid determination of 15N abundance and concentration of a wide variety of N-species involved in nitrogen cycling (e.g. NH4+, NO3-, NH2OH etc.). In this paper, the SPINMAS technique is evaluated with regard to the determination of 15N abundance and concentration of the most fundamental inorganic nitrogen compounds in ecosystems such as NH4+, NO2-, and NO3-. The presented paper describes the newly developed system in detail and demonstrates the general applicability of the system. For a precise determination of 15N abundance and concentration, a minimum total N-amount of 10 microg NH4+ - N, 0.03 microg NO2- - N, or 0.3 microg NO3- - N has to be supplied. Currently, the SPINMAS technique represents the most rapid and only fully automated all-round method for a simultaneous determination of 15N abundance and total N-amount of NH4+, NO2-, or NO3- in aqueous samples.     

Solid-supported reagents and catalysts for the preparation of large ring compounds

Parallel combinatorial synthesis in solution using immobilized reagents, catalysts, and scavengers has emerged as a powerful technique for the preparation of diverse libraries of compounds. This technique has only recently been applied to the synthesis of large-ring compounds. In this comprehensive review several strategies are presented and discussed, including Pd-catalyzed allylic alkylation, Stille-coupling, macrolactonization and macrolactamization using solid supported reagents and catalysts. In several cases site isolation has allowed operation of these macrocyclization reactions in concentrated solution (pseudo-dilution effect).     

Vibrational Spectra of Ethylenediamine Salts. I. Tentative Assignments of Infrared and Far Infrared Spectra

Abstract

We wish to report on some hitherto partly unnoticed and unexplained features in the infrared spectra of hydrogen halides of ethylenediamine. We have prepared normal, C-, N-, and perdeuterated ethylenediammonium chlorides and bromides and measured their infrared spectra in the range 4000–10 cm^?1.     

Synthetic and Biological Studies on New Urea and Triazole Containing Cystobactamid Derivatives

Cystobactamids belong to the group of arene-based oligoamides that effectively inhibit bacterial type IIa topoisomerases. Cystobactamid 861-2 is the most active member of these antibiotics. Most amide bonds present in the cystobactamids link benzoic acids with anilines and it was found that some of these amide bonds undergo chemical and enzymatic hydrolysis, especially the one linking ring C with ring D. This work reports on the chemical synthesis and biological evaluation of thirteen new cystobactamids that still contain the methoxyaspartate hinge. However, we exchanged selected amide bonds either by the urea or the triazole groups and modified ring A in the latter case. While hydrolytic stability could be improved with these structural substitutes, the high antibacterial potency of cystobactamid 861-2 could only be preserved in selected cases. This includes derivatives, in which the urea group is positioned between rings A and B and where the triazole is found between rings C and D.     

Polymer membranes in medicine

Mass transfer within the human body primarily occurs across biological membranes. Medicine mimics nature and uses synthetic membranes for therapeutic purposes. The state of the art of polymer membrane applications in medicine is reviewed. The target for membrane material developments is mainly the improvement of the patients' safety and convenience. To provide optimal blood compatibility a new biocompatible polymer for hemodialysis membranes has been developed in our group: a polycarbonate with polyether blocks. The copolymer is suitable for production of membranes with excellent performance. In addition to therapeutic purposes polymer membranes can provide diagnostic functions. Recently a blood glucose test strip based on a synthetic membrane has been developed. The membrane separates off the erythrocytes and contains enzymes and reagents. It enables the diabetic a precise and convenient measurement.     

Polymerforschung und -technologie im Iran

Im Laufe der letzten Jahre wurden - von den meisten westlichen Fachkollegen unbemerkt - an mehreren iranischen Universitäten und Forschungsinstituten Abteilungen eingerichtet, die sich in Forschung und Lehre mit Chemie, Physik, Anwendungs- und Verfahrenstechnik der Polymere beschäftigen. Das 1986 gegründete Polymer Research Center of Iran organisierte im Mai dieses Jahres das 1. Internationale Seminar.     

Simulation and Experiment in X-ray Topography and Diffractometry

X-ray double crystal topography and high resolution diffractometry reveal very small deformations of the crystal lattice. However, this information can be directly obtained from the topographs or diffractometer curves only if certain conditions are fulfilled. Generally a deformation model has to be used, that is compared to the measurements by simulation calculations based on the dynamical diffraction theory. Trial and error allow to adapt the parameters of the model. An example illustrates that with a few parameters an automatic fit is possible.