π-electron content of rings in polycyclic conjugated compounds – A valence bond based measure of local aromaticity

Abstract

© 2017 Elsevier B.V. In 2004 Randić and Balaban employed Kekulé structures to assess the π-electron content (EC) of rings in benzenoid hydrocarbons. In this paper we showed how ab initio Valence Bond (VB) calculations based on the set of Kekulé structures of a given polycyclic conjugated molecule can be used to obtain more accurate π-electron contents of rings (VB-EC) as the true computed weights of the Kekulé structures are used, instead of postulating that all Kekulé structures have equal weights as in the EC formula. In the case of benzenoid hydrocarbons, the values of VB-EC and EC are very close. The main difference is found for linear polyacenes, for which, contrary to the EC, the VB-EC values predict that the inner rings are more aromatic than the terminal rings. The original EC method has also the disadvantage that it cannot be applied to heterocyclic conjugated molecules. It was shown that the VB-EC method can be used to assess the local aromaticity in aza derivatives of naphthalene. The calculated VB-EC values were compared with several other aromaticity indices, namely: energy effect (ef), harmonic oscillator model of aromaticity (HOMA) index, six center delocalization index (SCI) and nucleus independent chemical shifts (NICS). The best correlation was found between the VB-EC and SCI, implying that these two indices carry similar information on the local aromaticity.