Relative hardness as a measure of aromaticity

Aromatic stabilisation energies using the recently introduced isomerisation method of Schleyer and Puhlhofer (Org. Lett. 2002, 4, 2873-2876) are computed for a series of five-membered C4H4X compounds ranging from anti-aromatic to highly aromatic. These energies are correlated with the classical estimate from a homodesmotic reaction. Moreover, this isomerisation method is also used to compute relative magnetic susceptibilities and hardnesses. The latter is shown to be linearly related to other aromaticity estimates, such as the aromatic stabilisation energy and Nucleus Independent Chemical Shifts. Finally, this methodology is applied to a study of the aromaticity of a recently synthesised platinabenzene compound.