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

The effects of substituent type and position on the aromaticity of certain derivatives of three 1,3- azoles (oxazole, imidazole and thiazole) have been investigated theoretically by using density functional theory at the level of B3LYP/6-31G(d,p) method. The second heteroatom substitution decreased the aromaticities of furan, pyrrole and thiophene. The decreased aromaticity has been found to be gained back to some extent by the substitution of strong electron withdrawing groups or atoms (NO₂ and F). NICS data have been considered in order to judge the aromaticities of the systems. The most effective substitution to enhance the aromaticity has beencalculated to be at position- 4. The variation of the bond lengths of the main skeleton supported 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.