The NMR spectra of the porphyrins 16—zinc(II) <i>meso</i>‐tetraphenylporphyrin (Zn TPP) as a diamagnetic shift reagent. A quantitative ring current model

Abstract

Abstract The facile preparation of zinc(II) meso ‐tetraphenylporphyrin (Zn TPP) and derivatives from substituted benzaldehydes and pyrrole, combined with the calculation of ring current shifts in these molecules, provides a useful series of selective diamagnetic shift reagents. The porphyrin‐ligand equilibrium is examined for some nitrogenous bases and the complexation shifts (δ P values) are obtained in precisely the same manner as LIS. The parameterization of the double‐dipole model of the porphyrin ring current is given, with the inclusion of the phenyl ring currents and free rotation about the substrate‐Zn bond. Precise agreement with the porphyrin proton chemical shifts and the complexation shifts of the geometrically rigid substrates of pyridine, 4‐picoline and quinuclidine is obtained. In contrast, the 13 C complexation shifts in these molecules are subject to additional effects other than the ring current shift.