Dynamic purity determinations. I. Location of the solidus by nuclear magnetic resonance

The error in purity determinations by differential scanning calorimetry has been studied on two different kinds of systems, metallic and organic, the latter with different types of impurities added. The organic systems have been corrected for solid-solubility error by a nuclear magnetic resonance technique.The impurities chosen for the organic systems were either quite dissimilar to the matrix or similar enough to render solid solution probable. This solid solution leads to deviations from the equation used for dynamic purity determinations. The nuclear magnetic resonance determination of the solidus establishes the proper starting temperature for area summation on a DSC curve.

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Zusammenfassung Der Fehler in Reinheitsbestimmungen mittels Differential-Scanning-kalorimetrie wurde an zwei verschiedenartigen Systemen untersucht, metallisch und organisch, wobei zu letzteren verschiedentliche Verunreinigungen zugefügt wurden. Der durch Feststoff-Feststofflöslichkeit verursachte Fehler in den organischen Systemen wurde mittels einer kernmagnetischen Resonanzmethode korrigiert.Die für die organischen Systeme gewählten Verunreinigungen waren entweder sehr verschieden von der Matrix, oder aber genügend ähnlich, um feste Lösungen wahrscheinlich zu machen. Diese festen Lösungen führen zu Abweichungen von der für dynamische Reinheitsbestimmungen angewendeten Gleichung. Die Bestimmung der kernmagnetischen Resonanz des Feststoffs ermöglicht die Bestimmung der richtigen Anfangstemperatur für die Flächen-summierung auf der DSC-Kurve.

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On leave from The Institute for Physical Chemistry, Polish Academy of Science, Warsaw, Poland.

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The authors are grateful to the National Science Foundation for its support through Research Grant CHE 76-08765, and to S. E. Walter for assistance with some calculations.     

Electron correlation as the driving force for charge transfer: charge migration following ionization in N-methyl acetamide

A hole charge created in a molecule, for instance, by ionization, can migrate through the system solely driven by electron correlation. The migration of a hole charge following ionization in N-methyl acetamide (a molecular system containing a peptide bond) is investigated. The initial hole charge is localized at one specific site of the molecule. Ab initio calculations show that nearly 90% of the hole migrates to a remote site of the molecule in 4.2 fs. This migration of charge is highly efficient and ultrafast. The underlying mechanism for this migration of a hole charge is identified and compared with a simple model.     

Temperature dependence of the reaction C2H (C2D) + O2 between 295 and 700 K

The rate coefficients for the reactions of C₂H and C₂D with O₂ have been measured in the temperature range 295 K T 700 K. Both reactions show a slightly negative temperature dependence in this temperature range, with k_C2H+O2 = (3.15 ± 0.04) × 10⁻¹¹ (T/295 K)^{−(0.16 ± 0.02)} cm³ molecule⁻¹ s⁻¹. The kinetic isotope effect is k_C2H/k_C2D = 1.04 ± 0.03 and is constant with temperature to within experimental error. The temperature dependence and the C₂H + O₂ kinetic isotope effect are consistent with a capture-limited metathesis reaction, and suggest that formation of the initial HCCOO adduct is rate-limiting.     

N‐Heterocyclic Silylenes in Boron Chemistry: Facile Formation of Silylboranes and Silaborinines

Reaction of a N‐heterocyclic silylene (NHSi) with PhBX₂ (X=Cl, Br) readily afforded six‐membered silaborinines through an insertion/ring expansion sequence. Increasing the sterics of the borane from phenyl to duryl enabled the selective generation and isolation of the highly colored silylborane intermediates. Theoretical studies on the mechanism and energetics of the silaborinine formation were fully consistent with the experimental observations.