title: Validity and limitations of the bridged annulene model for porphyrins.
abstract: According to the bridged annulene model, macrocyclic aromaticity of a porphyrinoid species can be attributed to the annulene-like main macrocyclic conjugation pathway (MMCP). Macrocyclic aromaticity, however, is given theoretically as a sum of contributions from all macrocyclic circuits. We found that the aromaticity due to each macrocyclic circuit is determined formally but broadly by Hückel's [4n + 2] rule of aromaticity. Nitrogen atoms in the pyrrolic rings effectively suppress the variation in the number of π electrons staying along each macrocyclic circuit. As a result, all or most macrocyclic circuits in oligopyrrolic macrocycles are made aromatic (or antiaromaitc) in phase with the MMCP. Thus, the MMCP is not a determinant of macrocyclic aromaticity but can be regarded as a good indicator of this quantity. This is why the bridged annulene model appears to hold for many porphyrins.
authors.affiliation: Department of Chemistry, Faculty of Science, Shizuoka University, Oya, Shizuoka 422-8529, Japan. scjaiha@yahoo.co.jp
authors.forename: Jun-ichi
authors.initials: J
authors.lastname: Aihara
authors.forename: Yuto
authors.initials: Y
authors.lastname: Nakagami
authors.forename: Rika
authors.initials: R
authors.lastname: Sekine
authors.forename: Masakazu
authors.initials: M
authors.lastname: Makino