Computational insight into the contribution of para-substituents on the reduction potential, proton affinity, and electronic properties of nitrobenzene compounds

New insight is provided into the chemistry of 12 para-substituted nitrobenzene compounds, using the high-level computational method G3(MP2) and DFT methods. The results show that the chemical properties of the nitrobenzene molecules, such as reduction potential, ionization energy, proton affinity, pK(a), interaction energy of the fragments, hyperpolarizability, exaltation index, band gap, UV electron excitation, and QTAIM properties, are controlled by the strong coupling between the nitro group (NO2) and the nature of the various para-substituents via the benzene ring as their conducting link. As the electron donating tendency of the para-substituent increases in the molecules, the electron cloud around the nitro group also increases, resulting in contraction of the N-C bonds and elongation of the N=O bonds, consequently leading to gradually increasing electron conductivity, polarizability, and ionization energy but lower proton affinity, thereby progressively impeding the reduction potential of the molecules. The experimental reduction potential was reproduced to a high degree of accuracy, with a mean absolute deviation (MAD) of 0.048V, depending on the computational method used and the choice of the free energy circle. Additionally, the experimental electron affinity and proton affinity of the 12 molecules were reproduced to a high degree of accuracy.