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

## TL;DR

Cyclacene stability is mainly due to structural strain rather than aromaticity, resolving a long-standing debate.

## Contribution

Demonstrates that ring strain, not aromaticity, primarily governs cyclacene thermodynamic stability.

## Key findings

- Strain-corrected heats of hydrogenation show strain dominates cyclacene stability.
- Even–odd aromaticity patterns do not translate into thermodynamic stabilization differences.
- Aromatic contributions to cyclacene stability are negligible compared to strain effects.

## Abstract

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.

Cyclacene stability arises from a balance between structural strain and aromaticity. Strain‐corrected heats of hydrogenation reveal that cyclacene thermodynamic stability is governed primarily by ring strain rather than aromatic stabilization. Pronounced even–odd aromaticity effects predicted by magnetic criteria do not result in significant differences in thermodynamic stability for cyclacenes.