Spirocyclic Nitroxide Biradicals: Synthesis and Evaluation as Dynamic Nuclear Polarizing Agents

A method for the synthesis of rigid nitroxide biradicals with various spatial orientations between the radical centers is reported. Diketones were employed as substrates for tin amine protocol (SnAP) reagents to provide the parent spirocyclic diamines. Oxidation by peroxyacids provided the corresponding nitroxide biradicals. A set of four different biradicals with various interelectron distances and torsion angles between the radical planes was synthesized using this method. The exact geometries were determined by X-ray crystallography and the biradicals were investigated by EPR spectroscopy and evaluated for their dynamic nuclear polarization (DNP) performance. ¹H-DNP enhancements in the range of 1.2–2.1 at 14.1 Tesla (600 MHz spectrometer) were achieved. This synthetic methodology opens a promising alternative to access nitroxide biradicals with various torsional angles and inter radical distances.     

Independence for knights on hexagon and triangle boards

For three kinds of chess-like knights, one on triangle and two on hexagon boards, the independence numbers for their knight graphs are considered. We determine these numbers completely for two kinds of these knights, and for one residue class modulo 4 in the case of the third kind.     

Fabrication of atomic gratings based on self-organization of adsorbates with repulsive interaction

Received: 5 May 1997/Accepted: 6 May 1997     

Experiences with infrared microsampling for thin-layer and high-performance liquid chromatography

Summary IR-spectroscopy can assist in the reliable identification of chromatographic analytes. With the direct IR-measurement of TLC and HPLC fractions, the substrate or the eluent, respectively, can render difficulties in the detection of microsamples. After isolation of the analyte, the microsampling for the different separation techniques is the same. Experiences are reported using diffuse reflectance and transmittance measurements from micropellets and enriched solutions in microcells. The performances of the microsampling techniques are compared. For often varying samples, the sample handling presented is more flexible than the different existing interfaces for the coupling of chromatography with IR-spectroscopy.

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Erfahrungen mit der IR-Spektroskopie im Mikrobereich für die Analyse von DC- und HPLC-Fraktionen

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Dedicated to Prof. Dr. W. Fresenius on the occasion of his 75th birthday     

Modern performance control of gamma-ray spectrometers for INAA

An approach is presented for the performance control of -ray spectrometers allowing for the identification of sources of malfunctioning. The traditional check consists of inspection of the FWHM of the peaks of the control source, their peak positions and areas followed by an energy calibration. The resolution and shape of a peak in the spectrum originating from an applied pulse generator are more responsive to the electronic noise than the -ray peaks. At the laboratory for INAA in Delft, the traditional energy calibration has been replaced by a spectrometer performance control which complements the traditional method by (1) an analysis of the shape of the pulser peak, (2) an inspection of the low energy and high energy tails of the peaks, (3) specification of quantified criteria for approval of the performance, (4) generation of control charts for visual inspection of the parameters determined and (5) extensive documentation of all operational characteristics for eventual correlations. This paper demonstrates how this approach to SPC enables to identify sources which cause malfunctioning of the spectrometer.     

Kristallstruktur des Bis[lithium-tris(trimethylsilyl)-hydrazids] und Reaktionen mit Fluorboranen, -silanen und -phosphanen

Crystal Structure of Bis[lithium-tris(trimethylsilyl)hydrazide] and Reactions with Fluoroboranes, -silanes, and -phospanes Tris(trimethylsilyl)hydrazine reacts with n-butyllithium in n-hexane to give the lithium-derivative 1 . The reaction of 1 with SiF₄, PhSiF₃, BF₃ · OEt₂, F₂BN(SiMe₃)₂ and PF₃ leads to the substitution products 2–6 . The 1,2-diaza-3-bora-5-silacyclopentane 7 is formed by heating (Me₃Si)₂N? N(SiMe₃)(BFNSiMe₃)₂ ( 5 ) at 250°C. In the reaction of (Me₃Si)₂N? N(SiMe₃)PF₂ ( 6 ) with lithiated tert.-butyl(trimethylsilyl)amine the hydrazino-iminophosphene (Me₃Si)₂N? N = P? N(SiMe₃)(CMe₃) ( 8 ) is obtained. In the molar ratio 2:1 1 reacts with SiF₄ and BF₃ · OEt₂ to give bis[tris(trimethylsilyl)hydrazino]silane 9 and -borane 10 .     

Large-sample neutron activation analysis: Present status and prospects

Neutron activation analysis is attractive for trace-element determinations in large samples. Facilities for reactor irradiation and -ray spectrometry of kilogram-size cylindrical samples are described. The thermal neutron flux is ca. 5·10¹²m^–2·s^–1 with a _th/ _epi>10⁴, so neutron self-thermalization can be neglected. The correction for the neutron attenuation within the sample is derived from measurement of the neutron flux depression just outside the sample. Correction for -attenuation in the sample is performed via linear attenuation coefficients derived via transmission measurements. Also the natural radioactivity in the sample is taken into account. Examples are given of materials to which large sample INAA has been applied successfully, and further lines of development and exploration are indicated.     

Biosynthesis of iso-fatty acids in myxobacteria

The fatty acid (FA) profiles of the myxobacteria Stigmatella aurantiaca and Myxococcus xanthus were investigated by acidic methanolysis of total cell extracts and GC or GC-MS analysis. The main components were 13-methyltetradecanoic acid (iso-15:0) and (Z)-hexadec-11-enoic acid (16:1, omega-5 cis). The biosynthesis of iso-FAs was investigated in several feeding experiments. Feeding of isovaleric acid (IVA) to a mutant impaired in the degradation of leucine to isovaleryl-CoA (IV-CoA)(bkd mutant) of M. xanthus only increased the amount of iso-odd FAs, whereas feeding of isobutyric acid (IBA) gave increased amounts only of iso-even FAs. In contrast, a bkd mutant of S. aurantiaca gave increased amounts of iso-odd and iso-even fatty acids in both experiments. We assumed that in S. aurantiacaalpha-oxidation takes place. [D(7)]-15-Methylhexadecanoic acid was synthesised and fed to S. aurantiaca as well as [D(10)]leucine and [D(8)]valine to elucidate this pathway in more detail. The iso-fatty acid was degraded by alpha- and beta-oxidation steps. [D(10)]Leucine was strongly incorporated into iso-odd and iso-even fatty acids, whereas the incorporation rates for [D(8)]valine into both types of fatty acids were low. Thus alpha-oxidation plays an important role in the biosynthesis of iso-fatty acids in S. aurantiaca. The incorporation rates observed after feeding of [D(10)]leucine and [D(8)]valine are the highest for iso-17:0 compared to the other acids. This indicates the central role of iso-17:0 in the biosynthesis of iso-FAs. The shorter homologues seem to be formed mainly by alpha-oxidation and beta-oxidation of this acid. After feeding of traces of unsaturated counterparts of this labelled FA occurred in the extracts indicating that desaturases are active in the biosynthesis of unsaturated fatty acids in S. aurantiaca.