Ring currents in the porphyrins: pi shielding, delocalisation pathways and the central cation.
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.
Steiner
Erich
E
School of Biosciences, University of Exeter, Exeter, UKEX4 4QD.
Soncini
Alessandro
A
Fowler
Patrick W
PW
D002412:Cations / Q000737:chemistry; D011166:Porphyrins / Q000737:chemistry*
D002412:Cations; D011166:Porphyrins