Current density, chemical shifts and aromaticity

The intimate link between chemical shifts and magnetic criteria for aromaticity prompts a search for detailed understanding of patterns of current density induced in pi systems by external magnetic fields. Conceptual and practical advantages of calculation of current densities with a specific method of distribution of origin of vector potential, the ipsocentric choice, where the induced current density at each point is calculated with that point as origin, are outlined. This choice leads uniquely to canonical molecular orbital contributions that are free of unphysical occupied-occupied mixing. Characteristic magnetic response of delocalized pi systems is then effectively restricted to the activity of a small number of frontier electrons, governed by simple symmetry and node-counting rules, and readily visualized in current density maps. Localized orbitals for sigma systems can also be used, again eliminating occupied-occupied mixing. For integrated properties (magnetizability and nuclear shieldings), the ipsocentric method gives, in a well-defined sense, the orbital contributions that are best for purposes of interpretation. The general theory is illustrated by maps for a set of annelated pentalenes; the known benzopentalene (1) and 1,2:4,5-dibenzopentalene (2), the still unknown isomer of 2, 1,2:5,6-dibenzopentalene (3), and cyclopent[b,c]acenaphthylene (4), an unknown isomer of pyracylene, all of which consist of fusions of formally aromatic and anti-aromatic pi-conjugated systems.