A PROTON NMR-STUDY OF THE REACTIONS WITH ACID OF MESO-TETRAPHENYLPORPHYRINS WITH VARIOUS NUMBERS OF 4-DIMETHYLAMINO GROUPS

Proton NMR spectroscopy has been used to study the protonation of meso-tetraphenylporphyrins with zero, one, or four para dimethylamino substituents. Changes in the spectra are similar for corresponding protons through this series of compounds. We give the following interpretations. The internal pyrrolenine nitrogen atoms always add two protons before any peripheral dimethylamino groups react. There is no evidence for a monoprotonated intermediate between the free base and diprotonated porphyrin in the chloroform-trifluoroacetic acid solvent system used. The chemical shifts of the NMR signals of the internal N-H protons are strongly acid-dependent and suggest a loss of aromatic ring current at the porphyrin core at intermediate acid concentrations. Changes in chemical shifts of protons on the periphery of these aromatic systems are much smaller and in the opposite sense. The signals of the pyrrole H(beta) protons move discontinuously upfield upon diprotonation and then drift continuously downfield as acid concentration is increased beyond that required for diprotonation. These changes are also in the directions to be expected if aromatic ring currents of the porphyrin decrease to a minimum at the acid concentration at which the internal nitrogen atoms are fully protonated. We suggest that this is due to the known distortion from planarity of the diprotonated tetraarylporphyrin aromatic system. Signals of protons attached to the meso-substituted aromatic rings move unidirectionally downfield as acid concentration increases.