The Limit of the Current Aromaticity Concept

Abstract

We have examined aromatic properties of a series of fifteen cyclic 2π-electron charged and neutral systems composed of second-, third-, and fourth-row elements. Three different aromaticity criteria were employed: magnetic (including three versions of NICSπ,zz methods and current density analysis), electron density-based (using the MCI and MCIπ methods), and energetic (via the BLW method, the simplest form of valence bond theory). The results reveal that the aromaticity rankings derived from these three approaches often diverge and there is no correlation between the three methods. It seems that what is known about aromaticity in compounds composed of second-row elements does apply beyond the second row. This inconsistency suggests that the concept of aromaticity should be applied to systems containing heavier elements only after careful evaluation using multiple, independent criteria. Even then, the advantage of labeling such systems as “aromatic” remains debatable. Also noticeably, our findings indicate that NICS values should not be used as a “black box” measure of aromaticity.