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NMR spectra of the porphyrins. Part 39. Paramagnetic shifts in cobalt(II) porphyrins

✍ Scribed by Raymond J. Abraham; Ian Marsden; Lucy Xiujing

Publisher
John Wiley and Sons
Year
1990
Tongue
English
Weight
666 KB
Volume
28
Category
Article
ISSN
0749-1581

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✦ Synopsis

Abstract

A model of the paramagnetism in cobalt(II) porphyrin complexes has been developed which successfully calculates paramagnetic shifts for protons not affected by the contact term. The model utilizes a single large dipole positioned on the cobalt(II) atom, and calculates the pseudo‐contact shift using the dipole approximation.

With this model the angle of twist of the peripheral meso phenyl rings in tetraphenylporphyrin (TPP) derivatives was determined. In TPP and the meta‐ and para‐substituted derivatives studied the phenyl rings are at ca. 65° to the porphyrin plane, whereas in the ortho‐substituted derivatives the phenyl rings are essentially orthogonal. Similarly, the geometry for the β‐ethyl groups of octaethylporphyrin and etioporphyrin I was found to be 50° out of the plane of the porphyrin inclined to the meso proton, and that of the comparable propionate groups of coproporphyrin I to be 55°. Finally, the novel compound meso‐tetra(p‐phenoxyphenyl)porphyrin, an analogue of diphenyl ether, was prepared to determine the twist angles between the two phenyl rings. These were found to be 60° by correlating observed paramagnetic shifts with those calculated. This agrees well with MNDO studies, which gave a value of 59°.

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