title: Ring currents in the porphyrins: pi shielding, delocalisation pathways and the central cation.
abstract: It is shown that the ipsocentric orbital-based model explains how the charge of the central cation drives the delocalisation pathway in metalloporphyrins. A positive charge +Ze at the centre of the porphin ring gives rise to a two-way radial transfer of charge within the pi structure of the porphin macrocycle. This manifests itself in a change of pathway of the global pi current, as Z increases from Z = 0, from an inner- through a bifurcated- to an outer-pathway. Changes of pathway can be interpreted in terms of a specific pi shielding effect whereby electrons in high-lying pi orbitals are screened from the central charge by the electrons in lower-lying orbitals of the same symmetry. These changes in pi structure are essentially independent of accompanying changes in the sigma structure.
authors.affiliation: School of Biosciences, University of Exeter, Exeter, UKEX4 4QD.
authors.forename: Erich
authors.initials: E
authors.lastname: Steiner
authors.forename: Alessandro
authors.initials: A
authors.lastname: Soncini
authors.forename: Patrick W
authors.initials: PW
authors.lastname: Fowler
mesh_terms: D002412:Cations / Q000737:chemistry; D011166:Porphyrins / Q000737:chemistry*
chemical_list: D002412:Cations; D011166:Porphyrins