title: The aromatic character of magnesium porphyrins
abstract: The aromaticity of magnesium porphyrins have been studied by calculating the nuclear magnetic shieldings in selected points outside the molecules. The strength of the induced ring currents for a given magnetic field have been obtained by using the aromatic-ring-current-shielding (ARCS) method. Nucleus-independent chemical shift (NICS) calculations provide additional information about the current routes in the multiple-ring systems. The total aromatic pathway of magnesium porphyrins must be considered as a superposition of several (4n + 2) pi-electron Huckel pathways. We found that all beta-unsaturated pyrrolic rings have local ring currents the strength of which is 70-90% of the current strength for the pyrrole molecule. The present study also shows that the 18pi-[16]annulene aromatic pathway does not exist in magnesium porphyrins until all four pyrrolic units are saturated in the beta-position.
authors.affiliation: Department of Chemistry, University of Helsinki, Finland. jonas@iki.fi
authors.initials: J
authors.lastname: Juselius
authors.initials: D
authors.lastname: Sundholm