Aromaticity indices revisited: refinement and application to certain five-membered ring heterocycles

Three simple improvements are introduced into the conventional Bird and Pozharskii systems of the structural aromaticity indices (AIs) to eliminate discrepancies between them. The Gordy bond length/bond order relationship is re-calibrated from the experimental bond lengths. Cyclopentadiene molecule with the geometry from MW has been chosen as a unified reference structure with no aromaticity. The scheme of Pozharskii is augmented with normalization by the mean bond order. Using the refined AIs, several regularities in the aromaticity of the azoles as a function of both chemical structure and environment have been specified.