Cyclacene Stability: The Interplay of Strain, Aromaticity and Force Coupling

The energy of cyclacenes arises from a subtle interplay between structural strain and aromatic stabilization. To disentangle these effects, we employ strain-corrected heats of hydrogenation as a direct thermodynamic probe, supported by thermally-assisted-occupation density functional theory that is capable of capturing strong static correlation. Previous analyses of magnetic properties demonstrated a pronounced even-odd pattern: cyclacenes with an even number of fused rings fulfill magnetic criteria of aromaticity, whereas odd-membered analogs do not according to diamagnetic susceptibility exaltation, nucleus-independent chemical shifts, and anisotropy of induced current density (ACID). Our results demonstrate, however, that this predicted aromaticity does not translate into discernible thermodynamic stabilization. Instead, cyclacene stability is dictated primarily by strain energy, with aromatic contributions playing only a negligible role. These findings resolve a long-standing question regarding the impact of aromaticity on cyclacene stability and clarify the fundamental factors that govern their reactivity and electronic behavior.