Zinc octaethylporphin radical cation is shown to form a diamagnetic dimer in methanol-chloroform, 10:1, at low temperatures. At 293°, the AH of dimerization is -17.5 ± 0.4 kcal/mol-1 and AS is -38 ± 1 cal deg-1 mol-1. The rate constant of dimerization, obtained by the temperature-jump method under these conditions, is 1.3 ± 0.2 X 10s M-1 sec-1, and that of dissociation is 5.0 ± 0.7 X 101 23 sec-1. The stability of the dimer is attributed to a novel -' bond formed by overlap of the half-filled orbitals in the face-to-face dimer of porphyrin rings. The nmr and optical spectra agree with this assignment. The oxidation of zinc octaethylporphin in the solid state and the preparation of the solid dimer containing approximately 10 % of the monomeric radical are also described. Bulk magnetic susceptibility measurements, determination of the elementary cell by X-ray spectroscopy, optical, infrared, and mass spectroscopy, as well as elemental analysis are all consistent with the formulation of this solid as (ZnOEP-Br2)2.
320 RELVETICA CHIMICA ACTA ganglich ; die nicht auszuschliessenden Moglichkeiten einer statistischen Orientierungs-Unordnung oder gar einer Rotation der Pyridin-Molekeln sollen durch die Interpretation von Infrarot-Spektren (ev. auch von aus deuteriertem Pyridin umkristallisierter Substanz) geklart werden. Die Verfeinerung beider Strukturmodelle nach der Methode der kleinsten Quadrate ist zur Zeit noch im Gange; bisher hat sie mit isotropen Temperaturfaktoren zu sog. R-Werten von 17,7y0 fur die Verbindung I, bzw. von 21,3y0 fur die Verbindung I1 gefuhrt . Nach ihrem Abschluss sollen weitere Einzelheiten, insbesondere Bindungslangen und -winkel, in der 2. Krist. veroffentlicht werden. Die Autoren sind Hcrrn Dr. G. EIGENMANN fur die Herstellung der Kristallproben, Herrn M. FEHLMANN fur Mithilfe bei Intensitatsmessungen und Berechnungen, und der CIRA AKTIEN-GESELLSCHAFT, Basel, fur die Unterstiitzung dieser Arbeit zu Dank verpflichtet.
SUMMARYThe crystal structure determinations of two pyridinium salts of 1 : 2-chromium complexes formed by 0, 0'-dihydroxy-trans-azo molecules show that the structure of the anions corresponds to the DREW-PFITZNER arrangement [2], as postulated by G. SCHETTY [l] for cases where the ligands form five-and six-membered rings with the metal ion. The correct structure formulae are given (cf. formulae I and 11) ; as the refinement of both structures is still going on, further details (such as bond lengths and bond angles) will be published in 2. Krist.
Formylbiliverdin‐Derivate werden durch Photooxigenierung von Metallporphyrinen niedrigen Oxidationspotentials über ihre Metallkomplexe erstmals dargestellt und spektroskopisch sowie als 2,4‐Dinitrophenylhydrazone charakterisiert.
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