The B1u and B2u Vibrational Modes Causing a D6h to D3h Transition in Benzene and in [18]- and [30]Annulenes

Abstract

Abstract Alternation of long and short carbon–carbon bonds in large cyclic polyenes are investigated from the point of view of vibrational analyses. [6(2n + 1)]Annulenes, where n is 0, 1, 2,···, are cyclic polyenes that can have planar bond-delocalized D6h or bond-localized D3h structures without severe nonbonded interactions. The B1u and B2u modes of vibration that potentially cause a D6h to D3h transition in benzene, [18]annulene, and [30]annulene are specifically discussed. The delocalized D6h structure is stable in benzene and [18]annulene, but not in [30]annulene. The D6h structure of [30]annulene has an imaginary B2u mode of 416i cm−1 that has relevance to the so-called Kekulé B2u mode of benzene. The D6h structure of [30]annulene is confirmed to be a transition state that leads to the equivalent D3h global minimum in both the forward and reverse directions.