Connecting and combining rules of aromaticity: towards a unified theory of aromaticity

Abstract

Most of the archetypal aromatic compounds present high symmetry and have degenerate highest-occupied molecular orbitals. These orbitals can be fully occupied resulting in a closed-shell structure or can be same-spin half-filled. This closed-shell or same-spin half-filled electronic structure provides an extra stabilisation and it is the origin of several rules of aromaticity such as the Hückel 4N+2 rule, the lowest-lying triplet excited state 4N Baird rule, the 4N rule in Möbius-type conformation, the 4N+2 Wade-Mingos rule in closo boranes or the 2(N+1)2 Hirsch rule in spherical aromatic species. Combinations of some of these rules of aromaticity can be found in some particular species. Examples of these combinations will be discussed and the validity of some of these rules will be assessed. Moreover, it is possible to establish connections with some of these rules of (anti)aromaticity and, therefore, they can be partially generalised, which represents a step forward to a future unified theory of aromaticity