NMR-SPECTRA OF THE PORPHYRINS .39. PARAMAGNETIC SHIFTS IN COBALT(II) PORPHYRINS

A model of the paramagnetism in cobalt(II) prophyrin 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-degrees to the porphyrin plane, whereas in the ortho-substituted derivatives the phenyl rings are essentially orthogonal. Similarly, the geometry for the beta-ethyl groups of octaethylporphyrin and etioporphyrin I was found to be 50-degrees 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-degrees. Finally, the novel compound meso-tetra(p-phenoxyphenyl)porphyrin, an analogue of diphenyl either, was prepared to determine the twist angles between the two phenyl rings. These were found to be 60-degrees by correlating observed paramagnetic shifts with those calculated. This agrees well with MNDO studies, which gave a value of 59-degrees.