The aromatic pathways of porphins, chlorins and bacteriochlorins

Abstract

The aromatic pathways of free-base porphins, chlorins and bacteriochlorins have been studied by calculating the nuclear magnetic shieldings at selected points outside the molecules. The strengths of the induced ring currents for a given magnetic field have been obtained by using the aromatic ring current shieldings (ARCS) method. We found that pyrrolic rings with an inner hydrogen have a local ring current of the same magnitude as the ring current for the pyrrole molecule. The local ring current for pyrrolic rings without an inner hydrogen is between half and one fourth of the pyrrole value depending on the porphyrin. The C₂H₂ units of the pyrrolic rings in free-base porphin have recently been suggested to function as exocyclic bridges, but the present study indicates that this is not the case. The suggested 18π-[16]annulene inner cross aromatic pathway does not exist until all pyrrolic rings are saturated in the β-positions. Free-base trans-porphin was found to have the largest ring-current susceptibility among the molecules studied. Porphyrins for which the aromatic pathway has to pass the nitrogen of a pyrrolic unit with an inner hydrogen have significantly smaller ring currents than free-base trans-porphin. We also show that the ¹H NMR shielding of the inner hydrogens is a sensitive measure of the strength of the ring current for the studied molecules.