Aromaticity – a theoretical notion

In 2023, a themed collection on “Emerging frontiers in aromaticity” was published in Chemical Science. The collection included a Perspective Essay entitled “Aromaticity – Quo Vadis”, which presents a wealth of viewpoints on the multiple definitions of aromaticity. The present Prefatory Review revives the viewpoint that aromaticity is a theoretical notion and as such, its meaning is theory dependent. Therefore, when aromaticity is made to correspond to two or more ‘experimental’ ideas, e.g., energetic, structural, electronic, magnetic, it would be absurd to maintain that aromaticity is explicitly defined by each of these ideas in turn. The Review emphasizes that the descriptor ‘theoretical’ in general, including in the context of aromaticity does not mean ‘computational’. The theoretical notion of aromaticity is illustrated by Craig’s rules of aromaticity and by Craig’s second type of aromaticity based on symmetry and delocalization in pπ-dπ bonds, recently highlighted as ‘Craig-Type Möbius aromaticity’.