Assessment of Clar's aromatic π-sextet rule by means of PDI, NICS and HOMA indicators of local aromaticity

The local aromaticity of a series of benzenoid systems was determined through the use of structurally (HOMA) and magnetically (NICS) based measures, and also by using a new electronically based indicator of aromaticity, the para-delocalization index (PDI). The results were compared with the predictions of Clar's aromatic pi-sextet rule. It is found that, for all analyzed benzenoid hydrocarbons having a single Clar structure, local aromaticity orderings of the different six-membered rings given by all descriptors of aromaticity tested are identical and in agreement with Clar's aromatic pi-sextet rule. For benzenoid species that are described by a superposition of Clar structures, HOMA and PDI indices provide local aromaticity values that are totally consistent with Clar's rule, whereas NICS results deviate somewhat from the estimations based on this model. Copyright (c) 2005 John Wiley & Sons, Ltd.