Aromaticity and antiaromaticity: what role do ionic configurations play in delocalization and induction of magnetic properties?

Abstract

AbstractCmHm (m= 4, 6, 8) species are analyzed in Dmh and D(1/2)mh geometries by means of valence bond (VB) calculations. The fundamental factors that distinguish aromatic and antiaromatic modes of electron delocalization are elucidated by analysis of the mixing between the covalent‐state and ionic structures that distribute the electrons in all possible modes available in the cycle. The major difference found is that, by contrast to the aromatic species where all the ionic structures mix into the covalent state, in the antiaromatic species the set of diagonal‐ionic structures is excluded from mixing with the covalent‐state, owing to its fundamental symmetry features. This exclusion of covalent–ionic mixing is expressed at the most fundamental building blocks of the wavefunction; the spin‐alternant state. The spin‐alternant state is a resonance hybrid of the two spin‐alternant determinants. This resonance hybrid will support a collective motion of the pπ‐electrons around the perimeter of the ring only if ionic structures can mix to mediate the electronic flow. It is shown that in aromatic species all the ionic structures mix and sustain a continuous electronic flow around the ring perimeter. By contrast, owing to the exclusion of the diagonal‐ionic structures in antiaromatic compounds, the electronic flow in antiaromatic species is interrupted. Symmetry and angular momentum analyses of the ground state in the presence of an external magnetic field show that the properties of the spin‐alternant state can qualitatively describe the magnetic properties of the two classes. The continuous flow of π‐electrons mediated by the ionic structures of aromatic species is responsible for the enhanced diamagnetism of these species. By contrast, paramagnetic π‐ring current in antiaromatic species becomes possible only in the presence of the magnetic field that allows the mixing of the otherwise excluded ionic structures. Copyright © 2003 John Wiley & Sons, Ltd.