Cooperative effects in restricted rotations of aryl groups in 2,4,6,8-tetraary-3,7-diazabicyclo[3.3.1]nonanes and related systems

The free energies, enthalpies and entropies of activation for the rotation of the aryl groups in the N,N′-dimethyl and N,N′-dichloro derivatives of rel-(2R,4S,6S,8R)-2,4,6,8-tetrakisaryl-3,7-diazabicyclo[3.3.1]nonanes have been determined and compared with related tricyclic systems which cannot undergo chair-to-boat conformational changes. It is concluded on the basis of the large, negative value of ΔS³ for rotation of the 2(4) aryl group that this rotational process is correlated with rotation of the 8(6) aryl group. The 8(6) aryl group can however rotate independently of the 2(4)-aryl group. The usefulness of the entropies of activation as a criterion for cooperative rotation of groups is briefly discussed.     

Hochauflösende29Si-Festkörper-NMR-Spektroskopie: Einfluß des Si-O-Si-Bindungswinkels auf die isotrope chemische Verschiebung von t- und h-Y2Si2O7

Starting from high-resolution solid-state²⁹Si NMR spectra of two modifications of Y₂Si₂O₇ with different Si-O-Si bond angles it was verified that increasing bond angles cause high-field isotropic²⁹Si chemical shifts.

     

Thermal degradation of metal novolak precursors for high temperature superconductors: thermogravimetric and mass spectroscopy studies

High temperature superconductor (HTS) materials were prepared from novolak metal precursors in a special annealing process. In the present paper the thermal behaviour of pure cresol novolaks and novolak metal salt (nitrate) precursors was investigated by means of simultaneous thermogravimetric analysis and mass spectroscopy in air. The results show significant differences in thermal stability between the pure novolaks and the novolak metal precursors, with respect to the process as well as the products of decomposition. The main volatile products for the precursors are CO₂, H₂O and NO, thus indicating that the thermal degradation results in the formation of metal oxides at a temperature of about 300 °C.     

On (ɛ,k)-normal words in connecting dynamical systems

The theorem of Copeland and Erdös on normal numbers relative to an integer base is generalised to infinite sequences composed of words from a certain class of languages. In particular, this proves the Copeland—Erdös result relative to a Pisot number as base.     

Darstellung,Eigenschaften und Molekülstruktur eines Titan-tris(diazadien)-Komplexes

Preparation, Properties, and Molecular Structure of a Titanium-tris(diazadiene) Complex (R = i-C₃H₇) 1 was obtained by reaction of TiCl₄ · 2 THF with Li₂(NR? CH = CH? NR) in the presence of the ligand molecules. The compound was characterized by ¹H-, ¹³C-n.m.r., and mass spectroscopy. The molecular structure was determined by X-ray diffraction. The bonding distances in the identically coordinated diazadiene ligands are similar to those of radical.     

Calibration of laser-ablation ICP-MS. Can we use synthetic standards with pneumatic nebulization?

An approach for the determination of trace element concentrations in high purity metals, using an inductively coupled plasma mass spectrometer (ICP-MS) with a laser-ablation system for direct solid sample introduction after calibration with nebulized liquid standards was made. Due to the inherent differences in the rate of sample introduction with laser-ablation and pneumatic nebulization, a matrix element must be used as an internal standard. This is problematical for elements that have no isotope with a relative abundance of less than 0.1 %, since the ion signals would be too high for direct measurement, and reduction of the ablation rate would compromise the sensitivity for trace elements. Due to the high stability of ICP-unit and mass filter of the instrument used, it was found that the tail of a mass-peak of the matrix element could be used as an internal standard. Therefore, a position at –0.5 amu from the matrix-isotope (e.g. ^62.5Cu in copper samples) was used for internal standardization. The standard deviation of this signal in a period of 2.5 h was 3.6% RSD with no notable drift when the laser ablation was used for sample introduction. The calibration of the matrix-element by nebulizing liquid standards showed that the ion signal measured on the peak-tail is directly proportional to the element concentration in the ICP. This indicates that the peak shape is not only stable, but also independent of the peak height. The advantages of this method lie in the easy preparation of calibration standards for quantitative measurements with a laser-ablation system and access to homogeneous standards for materials, that are difficult to homogenize in the solid state. The calibration of the traces is performed relatively to a fixed concentration of the matrix element. Calibrations were carried out for trace concentrations in high purity copper and good recoveries were obtained for high-purity reference standards. Received: 23 February 1998 / Revised: 20 July 1998 / Accepted: 25 July 1998     

Aminocarbonylthioformiate – Darstellung und Reaktionen

Aminocarbonylthioformates — Preparation and Reactivity Sodium aminocarbonylthioformate NaSCOCONH₂ (II) is obtained by reaction of sodium cyanodithioformate with acetone in presence of a secondary amine and water. II permits to produce the acid HSCOCONH₂ · H₂O (IX) and salts M^II(SCOCONH₂)₂ · 2H₂O. In basic aqueous solutions hydrolysis of II to oxaminate and oxalate, respectively, takes place, an excess of aqueous ammonia leads to amidino formic acid. In organic solvents the reaction of IX with N-bases yields stable ammonium salts. The results of i.r. spectroscopic and thermogravimetric measurements are discussed.