| In this work, two kinds of novel amino acid ionic liquids (AAILs) 1-propyl-3-methylimidazolium glutamic acid ([C3mim]+[Glu]-) and proline nitrate ([Pro]+[NO3]-) have been systematically studied by the density functional theory (DFT) at the B3LYP/6-311++G** level. The mechanism of the cation-anion interaction and the related micro-properties of the system have been illuminatedFirst, [C3mim]+[Glu]- was systematically studied. The stable geometries of the cation, anion, and [C3mim]+[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.Then, the proline cation based AAIL [Pro]+[NO3]-was systematically studied at the same theoretical level. The stable geometries of the cation, anion, ion-pair, as well as the ion-pair dimer [Pro]+2 [NO3]2- with no imaginary frequencies were obtained and characterized. In the single [Pro]+[NO3]- unit, proton transfer from Pro]+ to [NO3]- can be observed in some of the configurations and the corresponding proton-transferred products (neutral pairs) are strongly hydrogen-bonded. While in the ion-pair dimer [Pro]2+[NO3]2-, proton transfer does not occur and the components are stabilized by ionic interaction and hydrogen bonding interaction jointly. The proton transfer reaction between the cation and the anion in the single ion-pair unit has been investigated, and the role that the proton transfer reaction may play in the physicochemical property change of the ILs has also been discussed.
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