THE PARA DELOCALIZATION INDEX (PDI): A NEW ELECTRONIC CRITERION OF AROMATICITY

An overview of recent advances on the study of aromaticity through the use of delocalization indices (DI) obtained from double integration over atomic basins of the exchange-correlation density in the framework of the Atoms in Molecules (AIM) theory is presented. To quantify aromaticity in a given six-membered ring, we propose to use an average of all delocalization indices between para-related carbons. This new electronic criterion of aromaticity is clearly connected to the idea of electron delocalization so often found in textbook definitions of aromaticity. We first demonstrate the validity of this new aromaticity index by applying it to a series of planar polycyclic aromatic hydrocarbons (PAH). Then, this index is used to quantify local aromaticity in bowl-shaped PAHs and fullerenes. Finally, we emphasize the multidimensional character of aromaticity by showing the difficulty that some of the most common indices of aromaticity have to correctly quantif