Möbius aromatic forms of 8-π electron heteropines

Abstract

Ab initio calculations at the B3LYP//6-31G(d) level predict that Möbius-like conformations of O, NF, S and PF-substituted 7-membered ring 8-π electron perfluoro-annulenes with an axis of symmetry exist, but they are of higher energy than isomers with a plane of symmetry. Chiral inversion of the Möbius perfluoroazepine system via a planar structure is shown to be an orbital symmetry forbidden process for a closed shell singlet state, resulting from the nodal characteristics of the highest occupied Möbius molecular orbital. The orbital origins of an unusual electron-correlation dependent lengthening predicted for the N–F bond in the Möbius conformation of the azepine but largely absent in the analogous phosphorus system are discussed. Structural variations based on incorporating a biphenyl motif are explored, but in no case was the Möbius form lower in energy than the achiral non-aromatic geometries retaining a plane of symmetry.