Quantification of Aromaticity of Heterocyclic Systems Using Interaction Coordinates.

Recently, we proposed an aromaticity index based on interaction coordinates (AIBIC) ( J. Phys. Chem. A 2016 , 120 , 2894 - 2901 ). This index works well for the aromatic hydrocarbons. However, in the case of heterocyclic systems, the AIBIC overestimates the aromaticity indicating many of them to be more aromatic than benzene, which seems unlikely. Because of the differences in the electronegativity of the carbon and the other heteroatoms, the electron density is partially localized near the more electronegative atom(s) of the aromatic fragment. This localized electron density does not contribute to the aromaticity that is due to the delocalized electron density over the central ring. To account for this reduction in the delocalized electron density, a correction is introduced based on Pauling's electronegativity equation. When the corrected interaction coordinates are used in the computation of AIBIC, we get a new index-aromaticity index based on interaction coordinates corrected. This new index, when computed for a variety of heterocyclic systems, yields results in line with the expectations, and its usefulness in quantifying aromaticity appears to be very promising.