DFT study on the structure and cation–anion interaction of amino acid ionic liquid of [C 3mim] +[Glu] −

Abstract

A novel amino acid ionic liquid (AAIL), 1-propyl-3-methylimidazolium glutamic acid ([C 3mim] +[Glu] −), was systematically studied by the density functional theory (DFT) at the B3LYP/6-311++G∗∗ level. The stable geometries of the cation, anion, and [C 3mim] +[Glu] − ion-pair with no imaginary frequencies were obtained and characterized. The nature of the cation–anion interaction was analyzed by the atoms in molecules theory (AIM), harmonic vibrational frequency and the natural bond orbital analysis (NBO). The interaction energies between the cation and the anion were calculated. The results show that the multiple intermolecular hydrogen bonds together with the electrostatic interaction play an important role in stabilizing the cation–anion system. The favorable sites for the cation–anion interaction have been determined. The micro-structure changes and the charge redistribution of the ions during the formation of the ion-pair have been investigated. The impact of the intramolecular hydrogen bonds on the intermolecular interaction energy has been discussed.