Characterization of Local Aromaticity in Polycyclic Conjugated Hydrocarbons Based on Anisotropy of π-Electron Density

In recent contributions, it was demonstrated that the anisotropy of pi-electron density could be considered as a reliable descriptor to probe the aromaticity in a wide range of annulene and hetero-monocyclic compounds. The electron density anisotropy on a plane can be simply measured by the ratio of two in-plane Hessian eigenvalues associated with the eigenvectors lying in the plane. This descriptor is undirected and has a circular symmetry for aromatic rings, whereas it is directed and has an elliptical shape in antiaromatic cycles. This research takes a step forward towards the extension of this approach to polycyclic conjugated hydrocarbons. The obtained results show that the anisotropy of the pi-electron density works nicely for the assessment of local aromaticity in polycyclic compounds. This finding provides clear evidence to have a universally successful aromaticity descriptor based on the topological properties of one-electron density distribution.