The magnetic shielding function of molecules and pi-electron delocalization

Magnetic shielding is reviewed as the shielding function. The approaches that are used in the literature to discuss electron delocalization and aromaticity are interpreted in terms of magnetic shieldings and current densities. First, the basic theory for the computation of the tensorial magnetic shielding function of a molecule is outlined. Second, the special magnetic properties of aromatic molecules are discussed, both in terms of the ring current model and in terms of electron delocalization. Third, the mapping of the current density in several approaches, the topological analysis of the current density, the anisotropy of the current density (ACID) approach, and the aromatic ring current shielding (ARCS) method are discussed. Methods that are directly related to the computation of the magnetic shielding function in and around molecules are also covered. Finally, four illustrative molecules are considered including, the aromatic standard, benzene; the antiaromatic standard, D₂h cyclobutadiene; and the Al₄^2- and Al₄^4- clusters.