Structural Influence of Primary Amine Donors on the Physicochemical Properties of Hydrogen-Bonded Complexes with Anisole by Experimental and Theoretical Approaches

Abstract The structural influence of primary amine donors and thermodynamics on the hydrogen-bonded complexes formed in the ternary liquid mixtures of an aromatic ether, anisole (ANIS), as a common acceptor and one of the four structurally different primary amines [viz., n-hexylamine (NHA), cyclohexylamine (CHA), aniline (ANI), and o-toluidine (OT)] in n-hexane have been explored through ultrasonic, viscometry, and volumetric studies. The prominent variations in acoustical parameters and excess acoustical parameters have been observed in n-hexane solutions containing very low equimolar concentrations of ANIS and amines at 297.15–306.15 K temperatures and at 101 kPa. These results unravel strong solute–solute association between unlike molecules and the dominance of aromatic amine association with ANIS compared to aliphatic/alicyclic amine systems. The thermodynamic stability of these complexes was assessed by data obtained from Van’t Hoff plots. UV–visible spectra recorded for the four systems confirm the complex formation, and the stability constants (K) deduced from Benesi–Hildebrand plots support the acoustic observations. The possible non-covalent interactions apart from classical N–H---O bonding and the structural fluctuations due to these interactions have been predicted through a quantum computational method. The trend in interaction energy computed by Density functional theory (DFT) supports the trend in experimental K and the hierarchy of strength of complexes. The ANIS–ANI complex is stronger, while the ANIS–OT complex is weaker among the four systems analyzed. The results of the present work will be helpful in the design of multicomponent solvent systems required for pharmaceutical and chemical industries and deep eutectic solvents.