ELF Analysis of Out-of-Plane Aromaticity and In-Plane Homoaromaticity in Carbo[N]annulenes and [N]Pericyclynes

The aromaticity and homoaromaticity in ring carbomers of [ N ]annulenes and [ N ]cycloalkanes (i.e., [ N ]pericyclynes) are investigated using the topological analysis of the electron localization function (ELF). In a qualitative viewpoint, the ELF picture of aromaticity in carbo[ N ]annulenes is systematically “expanded” with respect to those of the parent [ N ]annulenes. The ELF analysis allows us to evaluate the weight of the various resonance forms of carbo[ N ]annulenes corresponding to (i) out-of-plane cyclic π electron delocalization and (ii) in-plane cyclic π electron homodelocalization. The latter is also quantified for the parent [ N ]pericyclynes. The chemical relevance of this evaluation is discussed by comparison with Bader's atoms-in-molecules topological analysis of the electron density and Hückel analysis. New criteria of homoaromaticity based on ELF analysis are proposed and further illustrated on the archetypes of homoaromaticity such as the homotropenylium cation and the cyclopropylcarbinyl cation.