Aromaticity on the edge of chaos: an Ab initio CCSD(T) study of the bimodal balance between aromatic and non-aromatic structures for 10-π-diheterocins and heteronins

CCSD(T) coupled cluster ab initio SCF-MO calculations for 10-π-heteroannulenes reveal a range of potential surface characteristics, ranging from single-minimum aromatic planar species to triple-minimum systems involving both planar-aromatic and the two enantiomers of a C₂ symmetric non-planar non-aromatic species. For the specific case of 1,4-dioxocine, the existence of barriers separating the three minima is attributed to an anti-aromatic Möbius-like transition state connecting the two equilibrium forms.