Comparisons of NH…O and OH…O hydrogen bonds in various ethanolammonium–based protic ionic liquids

Abstract

The effect of a number of terminal OH groups in tertiary ammonium cations (triethanolammonium, ethyldiethanolammonium, diethylethanolammonium, and triethylammonium) on structural and energetic characteristics of the ion pairs resulted from the acid–amine interaction with six different sulfonic acids was investigated. The ion pairs have been studied in detail from energetic and structural point of view by means of the B3LYP-GD3 dispersion-corrected density functional theory with further analysis of electron density function based on the Bader method. The topological properties of the electron density distributions for NH…O and ОH…O bonding interactions have been considered as well as their energies estimated by Espinosa approach. It was shown that the presence of OH groups in the cation leads to increasing structural stability of the ion pairs due to additional hydrogen bonding with acid anion. While the number of hydrogen bonds between cation and anion in the ion pair raises, the strength of each NH…O and ОH…O interaction is becoming weaker. The stronger interactions of the NH proton of the cation are evident in all the ion pairs studied. A thorough analysis of the interaction data on the ion pairs allows us to make the conclusion that the interaction energy of the ion pairs increases in the cation order TEA < DEEOA < EDEOA < TEOA, with the sequence of anions being TfO < NBsu < SA < Bsu < MTN < MsO.