Electron delocalization and aromaticity measures within the Hückel molecular orbital method

In this paper, we review the electronic aromaticity measures from the perspective of the Hückel molecular orbital (HMO) theory. The analysis of FLU, PDI, Iring and SCI in the framework of the HMO theory provides an interesting scenario for the interpretation of these indices. Within the Hückel theory the formulas for the Coulson bond orders are easily obtained in a closed form for annulenes, which enables the production of analytical expressions for some of the aromaticity measures. These analytical functions are used to study the ring size dependence of current aromaticity indices. Besides, HMO calculations of polycyclic benzenoids complete the analysis of the electronic aromaticity indices reviewed in this paper, by showing how HMO theory explains the changes in aromaticity due to annulation. All these results help grasping the meaning and the behavior of the electronic aromaticity indices.