Chemical Physics Letters, Vol.349, No.3-4, 307-312, 2001
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 (D-6h) over localized (D-3h) 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 pi -electron pair from [54]annulene. Strain is higher in smaller annulenes (D-6h) and decreases as the annulenes become larger. Localized annulenes (D-3h) exhibit much smaller ring strain energy, which steeply falls down for higher annulenes.