Ring currents in the porphyrins: π 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 π structure of the porphin macrocycle. This manifests itself in a change of pathway of the global π 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 π shielding effect whereby electrons in high-lying π orbitals are screened from the central charge by the electrons in lower-lying orbitals of the same symmetry. These changes in π structure are essentially independent of accompanying changes in the a structure. © The Royal Society of Chemistry 2005.