Structural and computational studies of new Schiff base ligands derived from 2-(methylamino)- and 2-(ethylamino)benzaldehyde: synthesis, characterization, QTAIM, NCI plot, TD-DFT and energy decomposition analysis

Three novel Schiff base ligands were synthesized through the condensation reaction of 2-(methylamino) or 2-(ethylamino)benzaldehyde with ethylenediamine and 4-nitro-1,2-phenylenediamine, respectively. The resulting Schiff bases were thoroughly characterized using FTIR, ¹H NMR and UV–vis spectroscopies, and their crystal structures were elucidated through single-crystal X-ray diffraction. Additionally, computational methods based on noncovalent interaction index and quantum theory of atoms in molecules were employed to investigate the intra- and intermolecular interactions in the molecular structure and crystal packing. The origin of the electronic transitions in UV–vis spectra of the Schiff base ligands were also investigated using time-dependent density functional theory along with the electron density difference map. Furthermore, energy decomposition analysis combined with natural orbitals for chemical valence theory has been performed to estimate the pairwise interaction energies in the dimer associates aggregated by the intermolecular interactions in the Schiff base ligands.