The B(1u) and B(2u) vibrational modes causing a D(6h) to D(3h) transition in benzene and in [18]- and [30]annulenes

Alteration of long and short carbon-carbon bonds in large cyclic poleyenes are investigated from the point of view of vibrational analyses. [6(2n+1)]Annulenes, where n is 0, 2, ···, are cyclic polyenes that can have planar bond-delocalized D(6h) or bond-localized D(3h) structures without severe nonbonded interactions. The B(1u) and B(2u) modes of vibration that potentially cause a D(6h) to D(3h) transition in benzene, [18]annulene are specifically discussed. The delocalized D(6h) structure is stable in benzene and [18]annulene, but not in [30]annulene. The D(6h) structure of [30]annulene has an imaginary B(2u) mode of 416i cm^-1 that has relevance to the so-called Kekule B(2u) mode of benzene. The D(6h) structure of [30]annulene is confirmed to be a transition state that leads to the equivalent D(3h) global minimum in both the forward and reverse directions.