Structural, SC-XRD and spectroscopic investigation of schiff base derivatives: A joint experimental and DFT investigation

Abstract

In this study, three Schiff base derivatives of 1,3-bis[(E)-(4-dimethylaminobenzylidene)amino] propan-2-ol (BDP), 1,3-bis[(E)-(4-methoxybenzylidene)amino] propan-2-ol (BMP) and 1,3-Bis[(E)-(4-chlorobenzylidene)amino] propan-2-ol (BCP) were synthesized and resolved into their single crystal structures. Interestingly, the crystal structures of compound BDP and BMP preserved the asymmetric unit whereas compound BCP showed two symmetry independent units. Moreover, three derivatives retained the polymeric framework through hydrogen bonds(OH⋯N) between adjacent molecules in the solid state. Equilibrium structure and electronic properties including natural bond orbital (NBO), frontier molecular orbitals (FMO) and molecular electrostatic potential (MEP) were calculated using M06-2X/6-31G (d,p) level of theory. A good agreement was observed between density functional theory (DFT) optimized geometry and experimental SC-XRD structure.NBO calculation indicated that hyper conjugative interaction promotes the hydrogen bonds in all derivatives. The FT-IR, FT-Raman and UV-VIS spectroscopic analysis were carried out at B3LYP/6-31G (d,p) level of theory. The FT-IR, MPA and MEP also affirmed hydrogen bonds in the dimers, which supports the SC-XRD results. The stabilization energy for hydrogen bonds and delocalization of lone pair electron from N, O and Cl to π* orbital of benzene ring was enhanced in the following order BDP < BMP < BCP. The maximum absorptions for BDP and BMP containing influence of electron-donating groups (EDG) were red shifted in comparison to BCP with withdrawing group (EWG). Furthermore, frontier molecular orbital (FMO) analysis was performed to obtain insights about influence of the EDG and EWG. The FMO calculations showed that the EHOMO levels of compound with EWG was found to be lower than those of compounds with EDG, while, the energy gap between HOMO and LUMO for compound with EWG was higher as compared to the compounds with EDG.