Relation between the substituent effect and aromaticity in imidazole derivatives: A comparative study

The energies and aromaticity of a series of R substituted imidazoles [R = NH2, OH, H, CH3, F, Cl, CN, NO, NO2], their anions and protonated forms in the gas phase have been calculated with the DFT/B3LYP and MP2 methods at the 6-311++G(d,p) level. We have analyzed the change of local aromaticity using several aromaticity indicators (Pozharsky Index, HOMA, NICS, ASE and pEDA) and found a considerable ring aromaticity for imidazoles, imidazolate anions and their protonated forms. In each class anion forms have the most aromaticity and in neutral forms, 1-H imidazoles have less aromaticity character. The HOMA and structural AI show good correlation with each other but NICS (0) values showed a poor correlation with other scales. The geometrical indices seem to be better correlate with substituent effect.