Quantum mechanical investigation of the choline chloride/carboxylic acid deep eutectic solvents

Deep eutectic solvents (DES) have received substantial applications and development in the field of green chemistry during the last decade. Using computational density functional theory (DFT), DESs composed of Choline Chloride (ChCl), as the hydrogen bond acceptor (HBA), and six carboxylic acids (CA) including Glycolic acid (Gly), Glutaric acid (Glu), Oxalic acid (Oxa), Lactic acid (La), Itaconic acid (Ita), and Levulinic acid (Lev), as the hydrogen bond donner (HBD), were investigated. Geometry optimization and vibrational frequency assignment were performed by applying three DFT functionals, i.e., B3LYP-D3(BJ), M06-2X, and omega B97XD in combination with Pople's 6-311+g(d,p) basis set. Then, using of the QTAIM, NBO, and the NCI analyses, different sorts of interactions have been evaluated to indicate the most stable structure between ChCl and various CAs, and the hydrogen bonding in the studied DESs. The results confirm that the Cl ion in choline chloride created a strong hydrogen-bond with HBDs, and the mixed ionic-covalent interaction is the main interaction in construction of the DESs of ChCl with the studied carboxylic acids.