The aromaticity and electronic properties of monosubstituted benzene, borazine and diazadiborine rings: an ab initio MP2 study

A comparative Moller-Plesset perturbation theory-based study to analyze the substituent's effect on the aromaticity and electronic properties of benzene, borazine and diazadiborine has been performed considering electron-donating groups (EDGs) such as -NH2 and -CH3 as well as electron-withdrawing groups (EWGs) such as -NO2 and -CF3 at various positions. The lowest energy structures of all monosubstituted rings follow the same trend of NICS values such that there is slight change in the aromaticity due to EDGs and EWGs. However, N-substituted borazine rings are more aromatic but less stable than B-substitution. More interestingly, the aromaticity of substituted diazadiborine rings increases with the increase in the energy of the isomers due to change in the position of substituent. Therefore, the aromaticity does not account for the relative stability of these species. The electronic properties of lowest energy isomers of benzene, borazine and diazadiborine are affected by the substituent in accordance with the aromaticity. For instance, the substitution of EDGs decreases and that of EWGs increases the ionization potential for all ring systems. The electronic properties of -NO2 (strong EDG)substituted rings are particularly noticeable due to their large electrophilic index values and larger ionization potentials. This comprehensive report is intended to provide new insights into the energetics, aromaticity and related properties of substituted six-membered homocyclic as well as heterocyclic rings.