1,3,4-oxadiazoles: evaluation of aromaticity and atomic charge distribution

The aromaticity and atomic charge distribution were investigated for the mono- and disubstituted 1,3,4-oxadiazoles. Using an electron density-based approach (B3LYP), the structures of the molecules were estimated, then the changes of π-electron delocalization of the heterocyclic ring were estimated based on geometric (HOMA) as well as magnetic properties (NICS). Aromaticity of the oxadiazole ring varies to some extend depending on the electron character of the substituent, however, the two methods do not correlate well. The HOMA approach suggests a non-aromatic character for the oxadiazole ring, while NICS predicts a relatively high aromaticity. Additionally, two computational methodologies were used for quantitative measures of atomic charges, i.e. AIM and GAPT. Results of these computations reveal accumulation of electron density inside the heterocyclic ring due to substitution by the aryl groups and its significant removal in the presence of the imino group. However, both procedures suggest different explanation of this fact.