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

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Physical Chemistry

Background:

• Cyclacenes exhibit an even-odd pattern in magnetic properties, suggesting aromaticity in even-membered systems.
• Previous studies indicated potential aromatic stabilization in cyclacenes based on magnetic criteria.

Purpose of the Study:

• To disentangle the effects of structural strain and aromatic stabilization on cyclacene energy.
• To investigate the thermodynamic stability of cyclacenes using direct probes.

Main Methods:

• Employed strain-corrected heats of hydrogenation as a thermodynamic probe.
• Utilized thermally-assisted-occupation density functional theory (TAO-DFT) to capture strong static correlation.
• Analyzed magnetic properties including diamagnetic susceptibility exaltation, nucleus-independent chemical shifts, and anisotropy of induced current density (ACID).

Main Results:

• Cyclacene stability is primarily dictated by strain energy, not aromatic contributions.
• Aromaticity predicted by magnetic properties does not correlate with thermodynamic stabilization.
• Strain energy plays a dominant role, while aromatic contributions are negligible.

Conclusions:

• Resolves the long-standing question on the impact of aromaticity on cyclacene stability.
• Clarifies that structural strain, not aromaticity, is the key factor governing cyclacene energy and behavior.
• Provides fundamental insights into the reactivity and electronic properties of cyclacenes.