Substituent Effect on the Aromaticity of 1,3-Azole Systems

Abstract

The effects of substituent type and position on the aromaticity of certain derivatives of oxazole, imidazole and thiazole have been theoretically investigated by using density functional theory at the levels of B3LYP/6-31G(d,p) and B3LYP/6-31++G(d,p) methods. The second heteroatom substitution decreases aromaticity of furan, pyrrole and thiophene. The decreased aromaticity is gained back to some extent by the substitution of strong electron withdrawing groups or atoms (NO2 and F). Nucleus-independent chemical shift (NICS) data have been considered to determine the aromaticity of the systems. The most effective substitution to enhance the aromaticity has been calculated to be at position 4. The variation of the bond lengths of the main skeleton supports the findings through NICS calculations. The frontier molecular orbital energies have also been reported to draw a general correlation between these energies and the aromaticity of the system.