The aromatic pathways of porphins, chlorins and bacteriochlorins

The aromatic pathways of free-base porphins, chlorins and bacteriochlorins have been studied by calculating the nuclear magnetic shieldings at selecte d points outside the molecules. The strengths of the induced ring currents for a given magnetic field have been obtained by using the aromatic ring cu rrent shieldings (ARCS) method. We found that pyrrolic rings with an inner hydrogen have a local ring current of the same magnitude as the ring curren t for the pyrrole molecule. The local ring current for pyrrolic rings witho ut an inner hydrogen is between half and one fourth of the pyrrole value de pending 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 18 pi -[16]annulene inner cross aromatic pathway does not exist until all pyrroli c rings are saturated in the beta-positions. Free-base trans-porphin was fo und to have the largest ring-current susceptibility among the molecules stu died. Porphyrins for which the aromatic pathway has to pass the nitrogen of a pyrrolic unit with an inner hydrogen have significantly smaller ring cur rents than free-base trans-porphin. We also show that the H-1 NMR shielding of the inner hydrogens is a sensitive measure of the strength of the ring current for the studied molecules.