Description of heteroaromaticity on the basis of π-electron density anisotropy

Abstract

It is demonstrated that there is a direct connection between aromaticity and the anisotropy of the π-electron density on planes parallel to the molecular ring. The electron density anisotropy on the plane is measured through the ratio of the in-plane Hessian eigenvalues associated with the eigenvectors lying in the plane. Computations on a wide-ranging set of well-characterized monocyclic systems containing heteroatoms validate the correlation between this one-electron density-based descriptor and aromaticity; in aromatic compounds, the in-plane Hessian eigenvalues are degenerate (or near degenerate) and the anisotropy of the π-electron density is undirected, whereas the results for antiaromatic rings are reversed and the degeneracy of the eigenvalues completely disappears. This finding is in line with our very recent study on [n]annulenes and provides further evidence that the anisotropy of the π-electron density should be considered as a new manifestation of aromaticity.

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

“…In the case of the six‐membered ring of 16 , the lost aromatic character is smaller than the corresponding rings in 14 and 15 . Also, the aromaticity of seven‐membered ring in 16 is very similar to that of heptafulvene and, therefore, the seven‐membered ring in 16 has the antiaromatic character …”

“…Several attempts have been made in recent years to use the topological properties of the electron density distribution to describe the aromatic character, but there is some doubt about the existence of a general straightforward link between the local properties of the electron density and the aromatic character, and also about the general applicability of them beyond simple compounds . These arguments stem from the fact that the electron density distribution and its topological properties at a point in molecular space highly depends on the distance from nuclei as well as the type of nuclei and, therefore, it seems reasonable to doubt that the local properties of the electron density are not suitable descriptors to measure the aromaticity in complex molecules such as heterocyclic compounds.…”

“…Recently, we have demonstrated that the anisotropy of the π‐electron density ( ρ π ) on planes parallel to the molecular ring can be used to assess the aromaticity of a large number of annulenes (C n H n ), azines (nitrogen‐containing six‐membred rings), substituted penta‐ and hepta‐fulvenes, and hetero‐monocyclic compounds with different charges, multiplicities and ring sizes ,. This topological property of ρ π can be simply determined through the ratio of two in‐plane positive eigenvalues of the π‐electron density Hessian matrix at the ring critical point (RCP) and all scanned points along the axis perpendicular to the molecular ring plane.…”

“…Recently, we have demonstrated that the anisotropy of the p-electron density (1 p ) on planes parallel to the molecular ring can be used to assess the aromaticity of a large number of annulenes (C n H n ), azines (nitrogen-containing six-membred rings), substituted penta-and hepta-fulvenes, and heteromonocyclic compounds with different charges, multiplicities and ring sizes. [36,37] This topological property of 1 p can be simply determined through the ratio of two in-plane positive eigenvalues of the p-electron density Hessian matrix at the ring critical point (RCP) and all scanned points along the axis perpendicular to the molecular ring plane. For the aromatic compounds, the two positive eigenvalues are degenerate and the pattern of the p-electron density distribution on the parallel planes is isotropical and undirected, whereas the degeneracy of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 the eigenvalues for antiaromatic rings completely disappears and the shape of the electron density distribution on the planes is anisotropical and directed.…”

Electron Density Analysis of Hyperconjugation

“…However, in a recent work by the present author, electron density analysis is generalized to π‐type electron density topology, which enables the detection of a hyperconjugative interaction. This approach is also used for the characterization of several concepts and interactions including aromaticity, antiaromaticity, heteroaromaticity, metal–π interaction, and van der Waals complexes …”