Pi-electron delocalization in aza derivatives of naphthalene and indole

The influence of increasing number of nitrogens and their sequences on π-electron delocalization of aza derivatives of naphthalene and indole has been investigated by use of different quantitative descriptors including the geometry-based harmonic oscillator model of aromaticity (HOMA), magnetic index nucleus independent chemical shift (NICS) and electronic indices. Moreover we used HOMO–LUMO gap and anisotropy of magnetic susceptibility as global descriptors. It is observed that N–N bond destabilizes both endosubstituted naphthalenes and indoles and the most stable isomers are those having smallest number of such units. All studied indices except HOMA indicate aromaticity lowering with an increase of the number of nitrogen atoms in a ring, however, the anomaly high aromatic characters are observed for the cases when N–N bond is formed. In addition, analysis of atomic charge density shows that the maximum local aromaticity belongs to position isomer in which the average over atomic charges on heteroatoms is less in the individual rings.