New Synthetic Routes to β-Olefinic Trialkoxyphosphonium Salts and Phosphonates: Organometallic Variants of the Michaelis-Arbuzov Reaction

The low-temperature addition of tertiary phosphites to [(allyl)Fe (CO)₄]⁺X^? complexes proceeds regio- and stereospecifically and produces metal-coordinated β-olefinic trialkoxyphosphonium ions. These can be converted by various routes into the uncomplexed phosphonium salts or phosphonates. Similar reactions of acyclic [(dienyl)Fe(CO)₃]⁺X^? compounds give metal-coordinated (2,4-dien-1-yl)trialkoxyphosphonium salts or dialkyl (2,4-dien-1-yl)phosphonates. The mechanisms and their relationship to the classical Michaelis-Arbuzov reaction are discussed. The new compounds are characterized, if possible, by ¹H-, ¹³C-, and ³¹P-NMR spectra. The new phosphonium salts and phosphonates, potentially useful for Wittig-Horner reactions, are difficult to obtain by conventional routes.     

The lifetime of the 266 keV level in 206Tl

The lifetime of the ²⁰⁶Tl 2^? state at 266 keV has been measured by the delayed coincidence technique. The state was populated via the γ-decay of high-lying states excited in the ²⁰⁵Tl (d, p)²⁰⁶Tl reaction at E_d = 9.0 MeV. Delayed coincidences between a Si surface-barrier detector and a Ge(Li) detector resulted in a mean life for the 266 keV level of τ = 3.3(2) ns. The deduced B(E2) value of 171 (11) e² · fm⁴ indicates strong configuration mixing of two-hole states in this level.     

Berechnung des bezüglich dem Wärmeübergang durch Filmkondensation optimalen Wellenprofiles mit Hilfe der Variationsrechnung

Zusammenfassung Seit R. Gregorig ist bekannt, daß eine feine Wellung der Oberfläche auf deutlich verbesserte Kondensationswerte an vertikalen Wänden führt. Ungeklärt blieb aber bislang die Frage, ob es eine optimale Auslegung der Wellenformen gibt. Alle Rechenverfahren zu diesem Thema beruhen entweder auf starken Restriktionen in der Gesamtheit der Wellenprofile, welche im Lösungsweg erfaßt werden, oder sie beinhalten starke vereinfachende Annahmen über den Zusammenhang zwischen der Filmhautkontur und der Wellung der Rohrwand. Das hier angezeigte Lösungsverfahren benötigt keine derartigen einschränkenden Annahmen. Sein Ergebnis zeigt die bezüglich dem Wärmeübergang durch Filmkondensation optimale Wellenform an.

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Summary The significance of surface tension forces for film condensation on tubes with finely fluted of finned surfaces was formulated for the first time by Gregorig. Gregorig's solution and attempts to extend his solution have been limited to special shapes or involved additional simplifying assumptions.This study presents a convenient method for optimizing the design of the tube surface. The solution procedure involves all theoretical tube wall profiles and is not limited by additional simplifying assumptions. The optimization was carried out by using the Euler-Lagrange differentional equations. The optimal profile shape with respect to condensation heat transfer is about half as thick as the shapes proposed in previous publications.

     

NMR relaxation study of thermal degradation in cured PMR polyimides

We have studied cross-linking and thermal degradation of high-performance first-and second-generation PMR-15 polyimides, both thermoset and thermoplastic versions, by performing nonspectroscopic NMR solid echo T^*₂ relaxation measurements at temperatures up to 430°C using probes built for this purpose. We employ signal averaging and automated decomposition of the relaxation decays into two Gaussian components, the slower of which gradually appears above 300°C. Tracking the molecular mobility spectrum in terms of the relative intensity of the components and their relaxation times as temperature is cycled, we detect essentially no irreversible effects below the glass transition, measure permanent mobility reductions attributable to completion of cure, and find that exposure to temperatures above 380°C on the order of 1 h is required for substantial thermal degradation to occur. These results are closely supported by thermal and mechanical measurements on parallel specimens. Second-generation PMR resins appear to have higher microscopic rigidity and reduced viscous fraction at high temperatures. ©1995 John Wiley & Sons, Inc.