Possible D6h structures in neutral and negatively-charged cyclic polyenes

The geometrical and electronic structures of large cyclic polyenes, annulenes, and their tetraanions are studied using the MNDOC method coupled with a conventional perturbation treatment. It is clarified that in neutral [18]annulene the D6h structure is 6.4 kcal/mol more favorable, but in neutral [30]annulene the D6h structure is less stable by 4.6 kcal/mol. Moreover, in neutral [42], [54], and [66]annulenes the D3h structures are clearly more stable than the corresponding D6h ones, and in the tetraanion of [18]annulene the D6h structure is more favorable.