From localized to delocalized annulenes: how ring strain enhances delocalization in higher annulenes

Strategies to stabilize delocalized structures in higher annulenes have been explored using density functional calculations at the B3LYP/6-31G(d) level. Symmetrically placed methylene bridges in [30]- and [42]annulenes favor delocalized structures (D6h) over localized (D3h) ones whereas [54]- and [66]annulenes still prefer localized structures after modification. Aromatic stabilization energy in delocalized structures reaches a constant value of 1.5 kcal/mol per π-electron pair from [54]annulene. Strain is higher in smaller annulenes (D6h) and decreases as the annulenes become larger. Localized annulenes (D3h) exhibit much smaller ring strain energy, which steeply falls down for higher annulenes.