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The aromatic pathways of free-base porphins, chlorins and bacteriochlorins have been studied by calculatingthe nuclear magnetic shieldings at selected points outside the molecules. The strengths of the induced ringcurrents for a given magnetic Ðeld 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 magnitudeas the ring current for the pyrrole molecule. The local ring current for pyrrolic rings without an innerhydrogen is between half and one fourth of the pyrrole value depending on the porphyrin. The C H units of2 2the pyrrolic rings in free-base porphin have recently been suggested to function as exocyclic bridges, but thepresent study indicates that this is not the case. The suggested 18p-[16]annulene inner cross aromatic pathwaydoes not exist until all pyrrolic rings are saturated in the b-positions. Free-base trans-porphin was found tohave the largest ring-current susceptibility among the molecules studied. Porphyrins for which the aromaticpathway has to pass the nitrogen of a pyrrolic unit with an inner hydrogen have signiÐcantly smaller ringcurrents than free-base trans-porphin. We also show that the 1H NMR shielding of the inner hydrogens is asensitive measure of the strength of the ring current for the studied molecules

The aromatic pathways of porphins, chlorins and bacteriochlorins