Electronic-level insight into the weak interactions of ion pairs in acetate anion-based ionic liquids

Abstract

The weak interactions in ion pairs of acetate anion-based ionic liquids (ILs) were investigated by dispersion-corrected density functional theory (DFT) calculations, where the structural parameters and vibrational frequency changes of ion pairs and isolated ions were systematically analyzed. Four kinds of stable configurations for ILs 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) and 1-butyl-3-methylimidazolium acetate ([Bmim][OAc]) were determined by calculating the interaction energies of different ion pairs. The essence of weak interactions between ion pairs was qualitatively and quantitatively characterized by the electrostatic potential (ESP) of anions and cations, natural bond orbital (NBO), atoms in molecules (AIM), and Generalized Kohn-Sham energy decomposition analysis (GKS-EDA). The results indicated that the red-shifted hydrogen bond (H-bond) of C1-H2⋯O contributed the most to the stability of ion pairs. For the different stable ion pairs, the H-bond interactions formed by acetate anion ([OAc]−) with H protons on the imidazolium ring and side chain were separately divided into medium and weak H-bond, showing partly covalent and noncovalent nature. GKS-EDA results revealed that the interactions of ion pairs were mainly electrostatic dominant, charge transfer and orbital overlap contributions were also significant, which were in accordance with the ESP, NBO, and AIM analyses results. Hopefully, this study could offer an insight into weak interactions principle of ILs and provide a theoretical basis for predicting their physicochemical properties.