Mapping the Global Ring Currents in Porphyrins and Chlorins

Abstract

Visualization and mapping of induced current density using the ab initio ipsocentric distributed-origin method reveals the global ring currents in anionic, free-base and metallo- forms of porphin, chlorin and bacterio chlorin. All exhibit an essentially monocyclic circulation, but with characteristic additional bifurcations through unsaturated pyrrole units. The orbital model specific to the ipsocentric approach rationalizes the diatropic ring currents in all these systems in terms of magnetic activity of just a few electrons. The greater part of the magnetic response is ascribed to virtual transitions involving the two highest occupied pπ and two lowest unoccupied p⋆ orbitals, the same set invoked in the longstanding four-orbital model of the electronic spectra of these systems. A model, based on charge screening, accounts for the different degrees of bifurcation observed in the delocalisation pathways.