NMR identification of C11 to C40 branched hydrocarbons derived from the decomposition of polyisobutylene

The composition of nonvolatile fluids obtained from thermally cracking and hydrogenating polyisobutylene was determined by using a combination of gas chromatography and nuclear magnetic resonance spectroscopy (NMR). This work involved the separation and characterization of a homologous series, C₁₁–C₄₀, of sixteen branched hydrocarbon species consisting of repeating isobutylene structures. As a result of this investigation, useful correlations between NMR spectra and molecular structure for highly branched hydrocarbons were developed. The data demonstrate that these hydrocarbons are unique species characterized by “crowded” and sterically hindered geminal methyl and isolated methylene groups. NMR solvent shift studies in benzene solutions indicate that it is possible to differentiate between maximally crowded geminal methyl groups and between maximally crowded methylene groups in these structures. Results of the benzene-induced solvent effects are discussed with respect to the stereochemistry of these molecules and related to existing solvent shift data. These results suggest that these hydrocarbons are polar or nearly polar materials. Successive losses of isobutylene units from stabilized tertiary radicals can account for the formation of the major species identified in these fluids. Higher carbon numbered species have lower refractive indices and densities and higher molal volumes than predicted by calculations.     

Ein neuer Photoionisationsdetektor für die Gas-Chromatographie

Zusammenfassung In einem Gas-Chromatographen wird der FID durch einen neuen Photoionisationsdetektor (PID) ersetzt. Der Photoionisationsdetektor wird kurz beschrieben und auf seine operativen Vorteile (einfache Anwendung, Fehlen von Betriebsmitteln wie Brenngase usw.) hingewiesen. Es werden einige Messungen mit dem modifizierten Gerät vorgenommen und die Ergebnisse mit denen des FID verglichen. Bedingt durch systemspezifische Vorteile des Photoionisationsdetektors—verglichen mit dem FID ist der Rauschpegel stark vermindert und wegen des größeren Wirkungsgrades der Photoionisation steht ein höheres Signal zur Verfügung—sowie durch das Fehlen von O₂ und seinen Problemen ist eine erhebliche Empfindlichkeitssteigerung des Gesamtsystems festzustellen. Außerdem ergibt sich, wiederum verglichen mit dem FID, ein weiterer Dynamikbereich. Weiter werden mit Hilfe des Photoionisationsdetektors gewonnene Meßergebnisse mit publizierten Ergebnissen anderer Detektoren verglichen und die Vorteile des PID aufgezeigt.

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A new photoionization detector for gas chromatography

Summary In a commercially available GC the FID was replaced by a newly designed photoionization detector (PID). The PID and its operational advantages (ease of use, no neccessity for combustion gas and the related facilities) are described briefly. Some measurements are made with the such modified GC and the results are compared with those obtained with the FID. Specific advantages of the PID — compared to a FID — the noise level caused by the system is lower and (due to the higher ionization efficiency of the PID) the signal is larger — and due to the lack of oxygen and its altendant problems the overall system sensitivity is strikingly better. In addition, the dynamic range — again compared with the FID — is wider. Further, performance data obtained with the PID are compared with published data of other detectors and the advantages of the PID are shown clearly.

     

Trapped-particle modes and asymmetry-induced transport in single-species plasmas

A new asymmetry-induced transport mechanism in pure electron plasmas is shown to be proportional to the damping rate of the corresponding trapped-particle mode, with simple scalings for all other parameters. This transport occurs when axial particle trapping exists due to variations in the electric or magnetic confinement fields. This new transport is strong for even weak unintentional trapping (deltaB/B approximately 10(-3)), and may be prevalent in transport experiments with magnetic or electrostatic theta asymmetries.     

Synthesis of a cyclic peptide containing norlanthionine: effect of the thioether bridge on peptide conformation

Two diastereomeric analogues of ring C of nisin incorporating a novel norlanthionine residue have been synthesized via a triply orthogonal protecting group strategy. A full structural study was carried out by NMR, which elucidated the conformational properties of the two peptides and enabled the identity of each diastereoisomer to be proposed.     

Exclusion limits on the WIMP-nucleon cross section from the cryogenic dark matter search

The Cryogenic Dark Matter Search (CDMS) employs Ge and Si detectors to search for weakly interacting massive particles (WIMPs) via their elastic-scattering interactions with nuclei while discriminating against interactions of background particles. CDMS data, accounting for the neutron background, give limits on the spin-independent WIMP-nucleon elastic-scattering cross section that exclude unexplored parameter space above 10 GeV/c2 WIMP mass and, at >75% C.L., the entire 3sigma allowed region for the WIMP signal reported by the DAMA experiment.     

Visualization of flameholding mechanisms in a supersonic combustor using PLIF

Laser-induced fluorescence of OH and CH₂O was imaged to investigate the flame stabilization mechanism in a flameholder with a Mach 2.4 free stream. Ethylene was burned in a rectangular cavity with two points of injection: the aft wall and the cavity floor. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. In conditions near lean blowout, significant changes in the flameholding mechanisms were observed. Improved CH₂O fluorescence signal was obtained by taking advantage of the long fluorescence lifetime at low pressures and delaying the camera gate to reduce the background signal.