Beitrag zur direkten Sauerstoffbestimmung in organischen Verbindungen

Zusammenfassung Es wird über zehnjährige Erfahrungen mit der maßanalytischen Mikrobestimmung des Sauerstoffes nachUnterzaucher berichtet. Silikon-schlauchverbindungen dürfen nicht verwendet werden, da sie gasdurchlässig sind. Die Gesamtanalysenzeit kann durch Verwendung eines Klappbrenners auf 15 Minuten verkürzt werden. Die Möglichkeit, das vonEmich im Prinzip vorgeschlagene CO₂-Multiplikationsverfahren anzuwenden, wurde überprüft. Zur Endbestimmung wird für Routine-analysen dem jodometrischen Verfahren wegen seiner Schnelligkeit und Empfindlichkeit der Vorzug gegeben.

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Summary A report is given of 10 years of experiences with the volumetric micro determination of oxygen by theUnterzaucher procedure. Silicone tube connections must not be used since they are permeable to gases. The total analysis time can be shortened to 15 minutes by using a shutter burner. A study was made of the possibility of using the multiplication procedure that in principle was recommended byEmich. For routine analyses, it was found preferable to employ the iodometric method for the final measurement because of its rapidity and sensitivity.

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Résumé On donne un compte rendu sur des expériences effectuées depuis une dizaine d'années sur le microdosage volumétrique de l'oxygène d'aprèsUnterzaucher. Il ne faut pas employer pour ces expériences de tubes siliconés car ils sont perméables aux gaz. La durée totale d'une analyse peut être ramenée à 15 minutes en employant un brûleur à charnière. On a examiné la possibilité d'appliquer le procédé de multiplication de CO₂ préconisé dans le principe parEmich. Pour la détermination du point équivalent dans les analyses en série, on donne la préférence au procédé iodométrique en raison de sa rapidité et de sa sensibilité.

     

Solventless synthesis of monodisperse Cu2S nanorods, nanodisks, and nanoplatelets

Cu(2)S nanocrystals with disklike morphologies were synthesized by the solventless thermolysis of a copper alkylthiolate molecular precursor. The nanodisks ranged from circular to hexagonal prisms from 3 to 150 nm in diameter and 3 to 12 nm in thickness depending on the growth conditions. High resolution transmission electron microscopy (HRTEM) revealed the high chalcocite (hexagonal) crystal structure oriented with the c-axis ([001] direction) orthogonal to the favored growth direction. This disk morphology is thermodynamically favored as it allows the extension of the higher energy [100] and [110] surfaces with respect to the [001] planes. The hexagonal prism morphology also appears to relate to increased C-S bond cleavage of adsorbed dodecanethiol along the more energetic [100] facets relative to [001] facets. Monodisperse Cu(2)S nanodisks self-assemble into ribbons of stacked platelets. This solventless approach provides a new technique to synthesize anisotropic metal chalcogenide nanostructures with shapes that depend on both the face-sensitive thermodynamic surface energy and the surface reactivity.     

An Efficient Strategy to Orthogonally Protected (R)- and (S)-alpha-Methyl(alkyl)serine-Containing Peptides via a Novel Azlactone/Oxazoline Interconversion Reaction

A novel strategy for the synthesis of (R)- and (S)-alpha-methyl(alkyl)serine-containing peptides is presented. Using (S)-phenylalanine cyclohexylamide 6 as chiral auxiliary, the optically pure azlactones (R)- and (S)-2 were synthesized via a novel azlactone/oxazoline interconversion reaction (Figures 3 and 6). These azlactones constitute fully protected and activated synthetic equivalents of (R)- and (S)-alpha-methylserine and can be directly incorporated into peptides without further protective group manipulations. Like other alpha,alpha-dialkylated glycines, optically pure alpha-alkylserines can be used to stabilize beta-turn and alpha-helical conformations in short peptides.     

Reduced local energy as a criterion for the accuracy of approximate H2 wave-functions

The goodness of the local fit of an approximate wave-function, \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \psi $\end{document}, to the exact function, ψ₀, is \documentclass{article}\pagestyle{empty}\begin{document}$ |\tilde \psi - \psi _0 | $\end{document}. From this quantity the global accuracy of \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \psi $\end{document} is defined and a “working supposition” is presented, which quantitatively relates the global accuracy to the accuracy of expectation values. Two criteria based on the accuracy of the reduced local energy and the density respectively, are presented as alternatives to \documentclass{article}\pagestyle{empty}\begin{document}$ |\tilde \psi - \psi _0 | $\end{document}. The relative global accuracies of eight wave-functions for H₂ are determined using the two criteria. The ‘working supposition’ is applied and predictions are made concerning the relative accuracies of the expectation values of the following operators: z², r², x² + y², 3z² ?; r², ξ, r, r, and E_L (the reduced local energy). The success rate is high (>90%) except for those operators which are sensitive to interelectron coordinates or derivatives of the wave-function.     

Ergodic theorems for coupled random walks and other systems with locally interacting components

Summary In [5] the second author introduced a variety of new infinite systems with locally interacting components. On the basis of computations for the finite analogues of these systems, he made conjectures ragarding their limiting behavior as t. This paper is devoted to the construction of these processes and to the proofs of these conjectures. We restrict ourselves primarily to spatially homogeneous situations; interesting problems remain unsolved in inhomogeneous cases. Two features distinguish these processes from most other infinite particle systems which have been studied. One is that the state spaces of these systems are noncompact; the other that even though the invariant measures are not generally of product form, one can nevertheless compute explicitly the first and second moments of the number of particles per site in equilibrium. The second moment computations are of inherent interest of course, and they play an important role in the proofs of the ergodic theorems as well.Research supported in part by NSF Grant MCS 77-02121Research supported in part by NSF Grant MCS 77-03543.