Quantum crystallography and topological analysis of imine compounds via HAR, QTAIM, and 3D IRI mapping

Abstract A series of three ligands L 1 -L 3 {where L 1 = N, N’-bis(3-methoxysalicylidene (propylen-2-ol)-1,3-diamine; L 2 = N, N’-bis(3-ethoxy-salicylidene) (propylen-2-ol)-1,3-diamine); L 3 = N, N’-bis(5-methylsalicylidene)(propylen-2-ol)-1,3-diamine are reported. Ligands were structurally characterized by NMR spectroscopy. The crystal structures of L 2 ·2EtOH and L 3 were determined by X-ray diffraction using aspheric Hirshfeld Atom Refinement model. Within the framework of quantum crystallography, HAR-derived geometries provide highly accurate hydrogen atom positions that serve as a reliable experimental basis for electron-density analysis. Both Schiff bases possess tautomeric character in their molecular structure, displaying ketoenamine/zwitterionic or enolimine forms stabilized by intra- or intermolecular hydrogen bonding. The nature and strength of these interactions were analyzed using electron-density based topological methods (QTAIM, Laplacian analysis) supported by complementary visualization tools (electron localization function ELF, reduced density gradient, RDG, deformation density maps and interaction region indicator, IRI), allowing detailed insight into proton localization and hydrogen-bond stabilization in the crystal state.