Beitrag zur Bestimmung der Spurenverunreinigungen in Rheniummetall

Zusammenfassung Spuren von Zn, Cu, Ni, Co, Fe, Mn und Mo in Rheniummetall werden aus einer Einwaage ermittelt. Mo wird photometrisch als ternäres Mo(VI)Hydroxylamin-PAR-Chelat bestimmt. Die übrigen Elemente werden als PAN-Chelate gefällt und im abgetrennten Niederschlag mittels Röntgenfluoreszenzanalyse bestimmt.

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A contribution to the determination of trace contaminants in rhenium metal

Summary Traces of Zn, Cu, Ni, Co, Fe, Mn, and Mo in rhenium metal were determined in a single weighed sample. Molydenum was determined photometrically as ternary Mo(VI)-hydroxylamine-PAR-chelate. The other elements were precipitated as PAN-chelates and then determined in the isolated precipitate by means of roentgen fluorescence analysis.

     

Zur Polymorphie von SrTeO3

Beside the transition into a ferroelectric phase at 583 K, monoclinic SrTeO₃(I) at ?1260 K transforms slowly into a high-temperature form (SrTeO₃(II)) which after quenching undergoes rapid phase transitions at 648 and 733 K. X-Ray data of triclinic α-SrTeO₃(II) (room temperature), monoclinic β-SrTeO₃(II) (660 K), and monoclinic γ-SrTeO₃(II) (770 K) are presented.     

High resolution field desorption mass spectrometry III

Summary For the examples shown in this study in all cases FD-MS allowed the determination of the molecular weight and the elemental composition. Some of the characteristic features of sulphonic acids and sulphonates in field desorption are described. The method is obviously very well suited for the analysis of non-volatile technical products (e.g. dyestuffs, detergents, drugs). Information about the molecular weight, the structure, organic and inorganic by-products and impurities is obtained and thus the applicability of FD-MS to the analysis of actual analytical problems is clearly demonstrated.

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Hochauflösungs-Felddesorptions-MassenspektrometrieIII. Sulfonsäuren und Sulfonate

Zusammenfassung Für die untersuchten Verbindungen ergab FD-MS in allen Fällen das Molekulargewicht und die Bruttoformel. Einige der charakteristischen Merkmale von Sulfonsäuren und Sulfonaten bei der Felddesorption werden beschrieben. Die Methode ist für die Analyse nichtflüchtiger, technischer Produkte (z. B. Farbstoffe, Detergentien, Pharmazeutica) sehr gut geeignet. Sie liefert Informationen über das Molekulargewicht, die Struktur, organische und anorganische Nebenprodukte und Verunreinigungen.

Part II.: Schulten, H.-R., Games, D. E.: Glycosides. Biomed. Mass Spectrom. 1, 120 (1974).     

Tilt of atomic force microscope cantilevers: effect on spring constant and adhesion measurements

In atomic force microscopy, the cantilevers are mounted under a certain tilt angle alpha with respect to the sample surface. In this paper, we show that this increases the effective spring constant by typically 10-20%. The effective spring constant of a rectangular cantilever of length L can be obtained by dividing the measured spring constant by cos2 alpha(1 - 2D tan alpha/L). Here, alpha is the tilt angle and D is the size of the tip. In colloidal probe experiments, D has to be replaced by the radius of the attached particle. To determine the effect of tilt experimentally, the adhesion force between spherical borosilicate particles and planar silicon oxide surfaces was measured at tilt angles between 0 degrees and 35 degrees. The experiments revealed a significant decrease of the mean apparent adhesion force with a tilt of typically 20-30% at alpha = 20 degrees. In addition, they demonstrate that the adhesion depends drastically on the precise position of contact on the particle surface.     

Use of molecular dynamics in the design and structure determination of a photoinducible beta-hairpin

The study presented here consists of three parts. In the first, the ability of a set of differently substituted diazobenzene-based linkers to act as photoswitchable beta-turn building blocks was assessed. A 12-residue peptide known to form beta-hairpins was taken as the basis for the modeling process. The central (beta-turn) residue pair was successively replaced by six symmetrically ((o,o), (m,m), or (p,p)) substituted (aminomethyl/carboxymethyl or aminoethyl/carboxyethyl) diazobenzene derivatives leading to a set of peptides with a photoswitchable backbone conformation. The folding behavior of each peptide was then investigated by performing molecular dynamics simulations in water (4 ns) and in methanol (10 ns) at room temperature. The simulations suggest that (o,o)- and (m,m)-substituted linkers with a single methylene spacer are significantly better suited to act as photoswitchable beta-turn building blocks than the other linkers examined in this study. The peptide containing the (m,m)-substituted linker was synthesized and characterized by NMR in its cis configuration. In the second part of this study, the structure of this peptide was refined using explicit-solvent simulations and NOE distance restraints, employing a variety of refinement protocols (instantaneous and time-averaged restraining as well as unrestrained simulations). We show that for this type of systems, even short simulations provide a significant improvement in our understanding of their structure if physically meaningful force fields are employed. In the third part, unrestrained explicit-solvent simulations starting from either the NMR model structure (75 ns) or a fully extended structure (25 ns) are shown to converge to a stable beta-hairpin. The resulting ensemble is in good agreement with experimental data, indicating successful structure prediction of the investigated hairpin by classical explicit-solvent molecular dynamics simulations.     

Molecular structure of an alkyl-side-chain polymer-water interface: origins of contact angle hysteresis

A new and direct approach to verify surface heterogeneity as the microscopic origin of contact-angle hysteresis is demonstrated. IR-visible sum-frequency-generation spectroscopy (SFG) was used to selectively probe the molecules at the interface of an alkyl-side-chain polymer [poly(vinyl n-octadecyl carbamate-co-vinyl acetate)] with water. The spectra indicate that in contact with water, the polymer surface is heterogeneous (having areas of differing surface energies). This evidence of surface heterogeneity supports the hysteresis observed in the advancing and receding contact angles of the polymer surface with water. The same measurements made for the chemically and structurally similar surface of an octadecyltrichlorosilane self-assembled monolayer indicates a homogeneous surface at the water interface. In this case, contact-angle hysteresis measurements implicate surface roughness as the cause of hysteresis. Atomic force microscopy measurements of roughness for these surfaces further support our conclusions. The polymer-water interface was probed using SFG at above-ambient temperatures, and an order-to-disorder transition (ODT) of alkyl side chains at the interface was observed, which closely follows the melting of crystalline side chains in the bulk. This transition explains the increased wettability of the polymer, by water, when the temperature is raised above the bulk melting temperature. Furthermore, the irreversibility of this ODT suggests that the disordered polymer-water interface is the thermodynamic equilibrium state, whereas the before-heating structure of this interface is a kinetically hindered metastable state.     

Preparation and redox properties of N,N,N-1,3,5-trialkylated flavin derivatives and their activity as redox catalysts

Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H2O2 oxidation of methyl p-tolyl sulfide. Introduction of electron-withdrawing groups increased the stability of the reduced catalyst precursor.     

Elektronenstoss-induzierter zerfall von benzol-tricarbonyl-chrom- und -wolfram-derivaten; ein vergleich

The Electron impact mass spectra of (CO)₃ MC₆H₅—X complexes (M = Cr, W; X = OCH₃, OC₄H₉, CO₂CH₃, CO₂C₄H₉) were recorded. From metastable transitions and by high-resolution measurements complete fragmentation diagrams were obtained; in some cases comparative structure determinations of fragment ions were carried out by collisional activation. The fragmentation of the tungsten complexes considerably differs from that of the chromium compounds. The differences may be attributed to the stronger-electrophilic character as well as to the more pronounced tendency of tungsten to attain higher oxidation states.     

Structure of starch-bentonite gels

Mixed gels of starch and bentonite are investigated in the interval 0.056–0.089 of total solids/water ratio and 0-100% starch in the solids. The bentonite was a sodium calcium bentonite with a Na/Ca ratio of 1.76. In water it forms gels consisting of a network of band-type structures. Starch forms gels through hydrogen bonds between granules and/or amylose and amylopectin present on the external surfaces of granules and/or in fully stretched form. Mixed gels of bentonite and starch were obtained by adding corn starch granules to the already formed bentonite gels and heating the mixture above the Kofler gelatinization temperature. Amylose and amylopectin were adsorbed on strands of band-type structures of mont-morillonite lamellae. Starch gellation, e.g. diffusion of amylose out of the granule, was facilitated in the presence of bentonite. On the other hand, the presence of starch favored delamination of the montmorillonite particle into thinner lamellae. Maximum gelatinization and polymer adsorption were observed for gels with 20% starch and 80% bentonite. Montmorillonite networks formed the continuous phase for 0-80% starch. At higher starch concentrations, montmorillonite flakes were dispersed within the polymer network. Increase in the water content of the gels caused segregation of the bentonite and starch.