Aromaticity and Antiaromaticity in Five‐Membered C₄H₄X Ring Systems: “Classical” and “Magnetic” Concepts May Not Be “Orthogonal”

Energetic, geometric, and magnetic criteria yield quantitatively the same order of aromaticity (antiaromaticity) for the five-membered ring systems 1: the aromatic 6π electron systems are stabilized, their bond lengths are equalized, and the magnetic susceptibility exaltations are diamagnetic. In contrast, the antiaromatic 4π electron systems are destabilized, double bonds are localized, and susceptibility exaltations are paramagnetic.