Aromaticity in heterocycles: new HOMA index parametrization

Abstract

A new parametrization for the Harmonic Oscillator Model of Aromaticity (HOMA) index to deter- mine aromaticity of heterocycles is introduced. The new HOMA for Heterocycle Electron Delocalization (HOM- HED) is based on the experimental data from electron diffraction X-ray for the reference molecules used to esti- mate the simple, double, and optimal bond lengths. Bond length of ''pure'' single and double bonds of non-conju- gated systems or systems without p-electrons and/or n-electron delocalization were considered. The HOMHED index was determined for a series of five and six hetero- cycles with C-C, C-N, C-O, C-S, N-N, N-O, and N-S bonds. The p-electron delocalization of these heterocycles was determined by Krygowski-reformulated HOMA and HOMHED and it was proved that HOMHED worked in line with HOMA for all heterocycles, except those con- taining oxygen, which were found to be weak aromatic from Krygowski rHOMA calculations.