Triethylamine-Based Salts: Protic Ionic Liquids or Molecular Complexes?

Amines can interact with protic acids with different degrees of proton transfer, which can lead to the formation of both hydrogen-bonded complexes and protic ionic liquids (PILs) in which the hydrogen bond between the cation and anion contributes to the formation of ion pairs. This work is devoted to studying the degree of proton transfer from different acids (hydrochloric, nitric, phosphoric, acetic, propionic, benzoic, and salicylic) to triethylamine (TEA). The results of quantum-chemical calculations based on the density functional theory (DFT) and thermal (phase transition and destruction temperatures) and physicochemical (conductivity, viscosity) characteristics of the compounds show that TEA interaction with acetic and propionic acids leads to the formation of hydrogen-bonded complexes. The B3LYP-GD3 method also shows that the interaction between TEA and benzoic acid is more energetically favorable for the formation of a molecular complex, whereas the obtained experimental data are more characteristic of a protic ionic liquid. For the other acids studied, the calculation and experimental data confirm salt formation. The geometric and energy parameters of the H-bond have been calculated both in the molecular complexes and in the ion pairs. The QTAIM theory was used to localize critical points of the hydrogen bonds and to calculate their properties.