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, annulene are specifically discussed. The delocalized D(6h) structure is stable in benzene and annulene, but not in annulene. The D(6h) structure of 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 annulene is confirmed to be a transition state that leads to the equivalent D(3h) global minimum in both the forward and reverse directions.
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