C–H···O interaction between cation and anion in amino acid-based ionic liquids—A DFT study in gas and solvent phase

Abstract

The interaction between anions and cations within amino acid-based ionic liquids (AAILs) are studied in the gas phase and in three different solvents (DMSO, water, and formamide). Structural and topological analyses of ion pairs signify that they interact via C–H···O hydrogen bond. In gas and solvent phase, the aliphatic amino acids (anions) interact strongly with EMIM (1-ethyl-3-methylimidazolium) and BMIM (1-butyl-3-methylimidazolium) cations. Further, the interaction between amino acid and EMIM cation is stronger due to large charge transfer from the electronegative oxygen atoms (carbonyl group) of the amino acids to the C–H bond of the imidazole ring. All the C-H···O bonds observed between the ions are red shifted and strong due to large interaction energy. The major contribution to the interaction energy is from electrostatic and orbital energies. The implicit solvents tend to increase the H···O distance of the AAILs. The increase in the chain length of cations irrespective of phase meagerly decreases the interaction between the ions. From the solvation energy, the reaction between solvents and AAILs are exothermic. AAILs possess higher solvation energy in DMSO. Overall, ionic liquids are highly stable in the gas phase and moderately stable in the solvents due to C-H···O bonds.