Local aromaticities in large polyacene molecules.

There has been controversy on the relative aromaticities of individual rings in a large polyacene molecule. Nucleus-independent chemical shift (NICS) values suggest that the highly reactive inner rings might be more aromatic than the outer ones and even more aromatic than benzene. We evaluated the bond resonance energies (BREs) and hypothetical geometry-independent pi-electron currents for a series of linear polyacenes and noticed that for large polyacene molecules the inner rings are never more aromatic than the outer ones. Global HOMA (harmonic oscillator model of aromaticity) values are highly correlative with percentage topological resonance energies (% TREs) but not with average NICS values. Magnetic properties, such as NICS and ring-current intensity, are highly dependent on molecular geometry and so must be carefully related to aromaticity.