Exploring Structural and Spectroscopic Properties of Secondary Amide Derivatives Bearing Bulky and Hydrophilic Substituents

Abstract

The synthesis of four novel amide compounds (R1C(O)NHR2) is reported, which contain both bulky (R1=phenyl and adamantyl) and hydrophilic (R2=hydroxyl, methoxyl) groups. The incorporation of such substituents provides compounds with unique properties that combine steric hindrance and hydrophilicity. The synthesized amides were fully characterized by 1H and 13C nuclear magnetic resonance (NMR) and analysed using infrared (IR) and Raman spectroscopy to investigate their vibrational properties in the solid phase, specifically focusing on the amide-type normal modes of vibrations. Furthermore, the crystal structures of three derivatives were determined by single-crystal X-ray diffraction, which provided insights into their molecular arrangements. The occurrence of N−H⋅⋅⋅O=C hydrogen bonding intermolecular interaction is clearly observed and characterized by Hirschfeld surface analysis. Topology analysis using the Quantum Theory of Atom in Molecules and Crystals (QTAIM−C) were employed to investigate the electronic properties of the isolated amide molecules and to analyse the intermolecular interactions within the crystalline structure. The periodic calculations for the crystal revealed a higher electronic density in the C−N bond, indicating an increased covalent character in this bond compared with the C=O bond. This observation indicates the presence of resonance-assisted hydrogen bonds within the amide core, leading to π electronic delocalization.