Experimental and Theoretical Studies on the Noncovalent Interactions in the Complexes of Diphenyl Ether with Primary Amines in n -Hexane Medium

Acoustic parameters were determined for two ternary liquid systems consisting of equal concentrations of donor–acceptor {[DPE] = [Amine]} from 0.03 mol·kg–1 to 0.31 mol·kg–1 of diphenyl ether (DPE), and one of the two structurally different primary amines [n-hexylamine (NHA), cyclohexylamine (CHA)] in n-hexane at 303.15 K and at atmospheric (0.1 MPa) pressure. Equal concentrations of solutes are a necessary condition for computing the stability constants (K) by ultrasonic method. The influence of various noncovalent interactions, concentration, and the structure of the amines on the physicochemical properties of the two liquid mixtures was analyzed. The acoustic data highlighted the dominance of hydrogen-bonded interactions over the structure-breaking behavior of the components. The optimized geometries of the solutes and complexes in n-hexane, obtained by density functional theory (DFT) analysis, revealed the existence of C–H···O, C–H···N, C–H···π, and N–H···O interactions in the two systems. This was also established by the NCI and RDG plots for the two systems. The experimental K values correlated satisfactorily with the theoretically computed interaction energies. These results provide insights into the influence of the H-donor structure on the formation constants and stability of amine-diphenyl ether (DPE) complexes. The results show that NHA forms stronger complexes than CHA with DPE.