A theoretical study was employed to investigate the interaction energy between the five amino acids based anions ([AA] -) (AA=Gly, Ala, Val, Leu, ILe) and N7,N9-dimethyladeninium cation ([dMA]+). The stable geometries of the cation, anions, and [dMA][AA] ion pairs were obtained and characterized. The structural characteristics of the ionic liquids (ILs) were examined by analyzing the thermodynamic functions using five different positions for interaction on the [dMA]+ cation. The obtained results showed that under the investigated conditions, three positions on the cation can produce more stable structures of the [dMA][AA] ion pairs. Moreover, the nature of the cation–anion interaction and underlying the formation mechanism of this type of the ionic liquids were investigated, using the quantum theory of atoms in molecules and the natural bond orbital analysis. The obtained results showed that increase in the side chain length of the amino acid anion decreases the calculated interaction energy of the ionic liquid systems. Finally, for the most stable configurations of ion pairs, there is a good correlation between the electron densities at the bond critical points of the hydrogen bonds and the corresponding vibrational frequencies.

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