Description of Aromaticity in Porphyrinoids

Like the larger nonplanar Mobius rings, porphyrinoid aromaticity is not due primarily to the macrocyclic pi conjugation of the corresponding annulene perimeters. The block-localized wave function (BLW)-derived aromatic stabilization energies (ASE) of several porphyrinoids reveal that, on a per atom basis, the appended 6 pi electron heterocycles of porphyrinoids confer aromaticity much more effectively than the macrocyclic 4n+2 pi electron conjugations. There is no direct relationship between thermochemical stability of porphyrinoids and their macrocyclic 4n or 4n+2 pi electron counts. Porphyrinoids having an antiaromatic macrocyclic 4n+2 pi electron conjugation pathway (e.g., 4) as well as those having no macrocyclic conjugation (e.g., 9) can be stabilized by aromaticity. Computed nucleus independent chemical shifts (NICS) and the anisotropy of the induced current density (ACID) disclose the intricate local versus macrocyclic circulation interplay for several porphyrinoids.