Mössbauer study of hydrogenated Fe-Zr amorphous alloys

The effect of electrochemical hydrogenation was investigated in Fe₉₀Zr₁₀ and Fe₈₉Zr₁₁ amorphous alloys by means of⁵⁷Fe Mössbauer spectroscopy. Significant changes in the Mössbauer spectra as well as in the hyperfine field distribution of hydrogenated samples were found with increasing hydrogen concentration. It was established that the dependence of Curie temperature on hydrogen content had a maximum, and the hydrogen had two mean localization sites. By comparing the Mössbauer spectra of hydrogenated samples as-quenched and annealed before hydrogenation it was shown that low-temperature relaxation processes were going on at aging temperature as low as 150°C in this amorphous alloy and the low-temperature relaxation processes modify the localization of hydrogen. The combination of the hydrogenation and Mössbauer techniques gives a very sensitive method for detecting structural changes.     

Kinetic studies on the decomposition of cyano complexes. I. the thermal decomposition of cyanoferrate(II) acids

The formation of HCN from solid iron(II) cyano complex acids is studied by a non-isothermal kinetic method. A derivatograph is used for the measurements and the kinetic parameters are calculated by different methods using the Horowitz—Metzger, Coats—Redfern and Zsako equations. The results are discussed and the kinetic parameters (energy and entropy of activation) are compared with IR and Mössbauer spectroscopic data.     

The behavior of two thiosugar thioglycosides towards oxidation

Oxidation of 4-cyanophenyl 2,5-anhydro-1,6-dithio-- -glucoseptanoside with magnesium monoperoxyphthalate (MMPP) resulted in a mixture containing two endo- and two exo-monosulfoxides as well as two endo-exo bis-sulfoxides differing in the chirality of the sulfoxide groups. Besides the aforementioned six products a further endo–exo bis-sulfoxide isomer as well as an exo-sulfone was obtained via oxidation with NaIO₄. Oxidation of 4-nitrophenyl 1,5-dithio-β- -arabinopyranoside with MMPP yielded only two endo-sulfoxides, while oxidation with NaIO₄ in turn led to cleavage of the carbohydrate ring. The formed dialdehyde was stabilized by cyclisation to a hemiacetal, which on further treatment with MMPP afforded an exo-monosulfoxide. The position of oxidation as well as the chirality of the respective sulfoxide groups was established by NMR spectroscopy and X-ray crystallography.     

Nachbargruppeneffekte in der Gas-Phase: Intramolekulare Deacetalisierung von Hydroxyacetalen unter den Bedingungen der chemischen Ionisation

Neighboring-Group Participation in the Gas Phase: Intramolecular Deacetalization of Hydroxyacetals under the Conditions of Chemical Ionization

1 Herrn Prof. Dr. Christoph Tamm zum 70. Geburtstag gewidmet

Under the conditions of chemical ionization (NH₃, isobutane, ND₃), hydroxyacetals do not show the expected [M + H⁺] signal, but the peak corresponding to the loss of the alcohol component of the acetal. This reaction is dependent on the presence of the OH group in the vicinity of the acetal and on the ring size of the heterocycle being formed. M^+· is detected to a small extent as a consequence of charge exchange during chemical ionization.     

One-dimensional reaction coordinates for diffusive activated rate processes in many dimensions

For multidimensional activated rate processes controlled by diffusive crossing of a saddle point region, we show that a one-dimensional reaction coordinate can be constructed even when the diffusion anisotropy is arbitrary. The rate constant, found using the potential of mean force along this coordinate, is identical to that predicted by the multidimensional Kramers-Langer theory. This reaction coordinate minimizes the one-dimensional rate constant obtained using a trial reaction coordinate and is orthogonal to the stochastic separatrix, the transition state that separates reactants from products.     

Analysis of submicroliter samples using micro thermospray flame furnace atomic absorption spectrometry

A recently described thermospray flame furnace atomic absorption spectrometric (TS-FF-AAS) system has been modified in order to extend the applicability of the method for the determination of elemental traces in very small sample volumes (microliter or submicroliter). As an easily available, effective thermospray vaporizer, a fused silica capillary was used and the liquid sample was transported by 1 MPa (10 bar) gas pressure delivered by a standard gas cylinder. A 0.3 microL sample volume can be analyzed with a higher power of detection than using 3 orders of magnitude larger sample volumes with conventional flame atomic absorption spectrometry. The relative standard deviations (N=12) for 0.3 microL volumes and 5 microg/mL Pb samples amount to 3.1% and 3.8% in signal height and signal area, respectively. The detection limit was found to be 69 ng/mL. Initial experiments with other elements (Cd, Hg, Tl, Zn) led to similar results.     

Formation and multinuclear magnetic resonance investigation of silylammonium tetracarbonylcobaltate contact ion pairs

Silylcobalt tetracarbonyls were reacted with various amines (B) in non-polar solvents to form silylammonium tetracarbonylcobaltate contact ion pairs formulated as [BSiR ₃ ⁺ , ⁻Co(CO)₄]. The compounds were characterized by IR and multinuclear magnetic resonance spectroscopy both in solution and in solid state. Their properties are analogous to the known ion pairs [BH⁺, ⁻Co(CO)₄] and to amine adducts of halosilanes as well.     

Length dependence of excitation energies in linear polyenes: Localized and delocalized descriptions

The length dependence of the lowest allowed transition energy of linear polyenes is studied using delocalized SCF and localized excitonic approaches. Within the PPP SCF approximations the calculated transition energies converge to a finite values as N^?1 as the number of double bonds (N) becomes large, when the excited state contains all singly excited configurations. On the other hand, the fully localized excitonic method at the level of single excitations, although it predicts a gap in the excitation spectrum of an infinite polyene, gives results which converge to this value as N^?2. The inclusion of double and triple excitations into the excitonic method by means of perturbation theory does not appear to change this behavior. The reasons for the discrepancy between the two approaches is analyzed. Experimentally, the transition energies in solution converge to a finite value as N^?1 to a good degree of approximation. If it is assumed that the solvent shift is constant for long polyenes, the available experimental results favour the delocalized approach as the starting point in describing the length dependence of the excitation energy of long polyenes.     

Identification of nitric oxide donors by biomimetic HTS application

Metalloporphyrin catalyzed biomimetic oxidation was used for the identification of nitric oxide (NO) donors with diverse chemical structure. Methodology was validated by testing known NO donors. Efficient automation of the test allowed us to investigate a subset of our corporate library. Several hits identified in this campaign were validated in both the chemical and also microsomal model that revealed all hits to be active in the biological system, as well. One of the hits showed comparable activity to V-PYRRO/NO, the prototypic liver selective NO donor.     

Iterative, orthogonal strategy for oligosaccharide synthesis based on the regioselective glycosylation of polyol acceptors with partially unprotected n-pentenyl-orthoesters: further evidence for reciprocal donor acceptor selectivity (RDAS)

An efficient iterative, orthogonal protocol based on the regioselective glycosyl coupling of D-mannose polyols with, partially unprotected, n-pentenyl orthoester donors permits the synthesis of linear and branched oligosaccharides with minimal protecting group tampering.