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Issue 10, 2000
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The aromatic pathways of porphins, chlorins and bacteriochlorins

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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.

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Publication details

The article was received on 12 Jan 2000, accepted on 08 Mar 2000 and first published on 13 Apr 2000

Article type: Paper
DOI: 10.1039/B000260G
Citation: Phys. Chem. Chem. Phys., 2000,2, 2145-2151
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    The aromatic pathways of porphins, chlorins and bacteriochlorins

    J. Jusélius and D. Sundholm, Phys. Chem. Chem. Phys., 2000, 2, 2145
    DOI: 10.1039/B000260G

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