QTAIM study of substituent effects on the intramolecular hydrogen bond in 3,3′-dihydroxy-4,4′-[5-methyl-1,3-phenylenebis(nitrilomethylidyne)]-bis-phenol

The effects of various substituent groups on the hydrogen bond energy in the 3,3′-dihydroxy-4,4′-[5-methyl-1,3-phenylenebis(nitrilomethylidyne)]-bis-phenol (L(CH3)pnp) molecules is one of the factors controlling intramolecular proton transfer process in the molecule’s functional group. In this work, we focused on the influence of different groups on this phenomenon, into the framework of the atom in molecules (AIM) theory. In addition, the calculations of transition state were performed to evaluate the proton transfer’s energy barrier of the proton transfer in the Schiff base molecule Lpnp. Investigation the effect of the electron withdrawing groups including, CHO, CN, CF3, NO2, COOH, COCH3, F, Cl and that of electron donating species consisting of NH2, OH, C2H5, CH3 on the covalent nature of the intramolecular hydrogen bond shows different results: decrease in the electron withdrawing ability reduces the covalent nature whereas, electron donating substituents increase it. The topological properties such as the bond critical point (BCP), ring critical point (RCP), the delocalization index DI, and the integrated properties of the interatomic surface δ(O,H), δ(N,H) and H(G + V) can all be considered as indicators to determine the strength of the intramolecular hydrogen bond.