Molecular Adducts of Isoniazid: Crystal Structure,Electronic Properties,and Hirshfeld Surface Analysis

Three molecular adducts of the antituberculosis drug isoniazid (INH) are synthesized with γ-resorcylic acid (γRA), phloroglucinol (PG), and gallic acid (GA). The new solid phases are preliminarily characterized by the thermal analysis (DSC/TGA) and powder X-ray diffraction. The formation of new solid phases is confirmed by single crystal X-ray diffraction, infrared (FT-IR) and Raman spectroscopy. All three new solid crystalline forms are stabilized by various hydrogen bonding interactions such as N⁺···H–O^–, N···H–O, O···H–O, and π–π stacking. The FT-IR analysis puts forward that the solid form of INH1 is a salt whereas the INH2 and INH3 molecular complexes are cocrystals. We have also investigated the density of states (DOS), band structure, and atomic orbit projected density of state (PDOS) of title compounds by adopting the density functional theory (DFT) technique in the local density approximation (LDA). The electronic structure calculations show that energy states are delocalized in the k-space due the hydrogen and covalent bonds in the crystals. The frontier molecular orbital (FMO) analysis reveals that charge transfer takes place within the compounds. The Hirshfeld analysis shows that H–H and N?H–O hydrogen bonding interactions are dominant in all three molecular adducts of INH.