Aromaticity in heterocycles: new HOMA index parametrization

A new parametrization for the Harmonic Oscillator Model of Aromaticity (HOMA) index to determine aromaticity of heterocycles is introduced. The new HOMA for Heterocycle Electron Delocalization (HOMHED) is based on the experimental data from electron diffraction X-ray for the reference molecules used to estimate the simple, double, and optimal bond lengths. Bond length of pure single and double bonds of non-conjugated systems or systems without pi-electrons and/or n-electron delocalization were considered. The HOMHED index was determined for a series of five and six heterocycles with C-C, C-N, C-O, C-S, N-N, N-O, and N-S bonds. The pi-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 containing oxygen, which were found to be weak aromatic from Krygowski rHOMA calculations.