When Delocalization Does Not Imply Aromaticity: Rethinking Aromaticity Indices through Reactions of Anti-Aromatic Molecules

Computational data underlying the publication of the same name. Abstract: Aromaticity is a widely used concept associated with four different aspects: energetic stabilization, electron delocalization, bond length equalization, and diatropic ring currents. Conversely, the opposite case of anti-aromaticity is characterized by energetic destabilization, electron localization, bond length alternation, and paratropic ring currents. To explore these concepts, we investigate automerizations and inversions via planarization of anti-aromatic molecules. The increase in energy from ground to transition state indicates destabilization – an increase of their anti-aromatic character. Using quantitative quantum-chemical indices of the four previously mentioned aspects of aromaticity, we investigate whether they truly measure anti-aromaticity. We show that this is not the case for all aromaticity indices: While magnetic indices predict an increase in anti-aromaticity, other indices contradict each other. This is particularly evident for geometric and electronic indices, which show a numerical increase corresponding merely to an increase in electron delocalization but not to a decrease of anti-aromaticity. As a result, such indices may be appropriate for equilibrium structures but not for transition states. This work emphasizes the importance of employing multiple aromaticity indices combined with conceptual reasoning. We suggest including automerizations in test sets for evaluating newly proposed aromaticity indices – delocalization in anti-aromatic molecules should lead to an increase in anti-aromaticity.