Cyclic Aromaticity within Huckel and Quasi-Correlated Huckel-like Models

Abstract

The paper deals with quantifying aromaticity in π-electron networks by unsophisticated MO techniques. The focus is placed on local aromaticity measures associated with individual benzenoid rings. We revised the ring aromaticity index due to Cioslowski et al (2007) by including explicitly net charges and electron unpairing effects. Our previously introduced quasi-correlated tight-binding (QCTB) approximation serves here as an easily available tool for taking account of π-electron correlations. The latter crucially influence the behavior of large and even small conjugated π-structures with a nontrivial topology. Numerical applications of Hückel and QCTB models to measuring local aromaticity are reported for various structural classes (polycyclic aromatic hydrocarbons (PAHs), graphene nanoflakes, and others). We analytically investigate the aromaticity in conjugated monocycles CNHN (neutral and charged ones). Furthermore, in the same manner several PAH structures (oligocenes, pyrene, perylene, etc.) are considered in their charged states, and the results are compared with those of related quinoid-type systems, such as p-diphenoquinodimethane. It is shown that, unlike usual PAHs, quinodimethane structures tend to increase their aromaticity in dicationic (dianionic) form. In our studies of nanographene aromaticity we find a decrease of the local aromaticity as we move to a center of graphene structures, that is in a sharp contrast to the predictions of NICS (nucleus independent chemical shift), a rather criticized approach. A particular emphasis is being put on measuring local aromaticity in highly correlated π-systems. Typical non-Kekule hydrocarbons (e.g., triangulene radical and polyradicals), are also studied within QCTB by which characteristic difficulties caused by the occurrence of many non-bonding π-MOs, are simply obviated.