Title: Interaction character and proton transfer mechanism of [HPro(C<sub>1</sub>)]<sup>+</sup> and [Sac]<sup>-</sup> in the gas phase: A theoretical study
Abstract: While protic ionic liquids (PILs) are today emerging as important materials with an amazing diversity in properties and applications, but not much is known about the detailed interaction mode and the proton transfer mechanism. The amino acid ionic liquids [HPro]<sup>+</sup>[Sac]<sup>-</sup> and [HProC<sub>1</sub>]<sup>+</sup>[Sac]<sup>-</sup> were systematically studied by DFT/B3LYP and MP2 methods with 6-311+G(d,p) basis set. The stable geometries and interaction modes of the cation, anion, ionic pair, neutral pair, as well as the dimmer [HPro]<sub>2</sub><sup>+</sup>[Sac]<sub>2</sub><sup>-</sup> and [HProC<sub>1</sub>]<sub>2</sub><sup>+</sup>[Sac]<sub>2</sub><sup>-</sup> were obtained and characterized. In the single [HPro]<sup>+</sup>[Sac]<sup>-</sup> and [HProC<sub>1</sub>]<sup>+</sup>[Sac]<sup>-</sup> units, proton transfer from cation to anion can be observed and testified by the TS and IRC calculations. The lower energy barriers indicated that there exists the dynamic balance between the ionic pairs and neutral pairs. The interaction intensity between the fragments will significantly be weakened after H-transfer reaction. In the dimmer, proton transfer does not occur. The components are stabilized by completely ionic-ionic interaction in [HProC<sub>1</sub>]<sub>2</sub><sup>+</sup>[Sac]<sub>2</sub><sup>-</sup>, and by jointly ionic-ionic and neutral-zwitterionic interactions in [HPro]<sub>2</sub><sup>+</sup>[Sac]<sub>2</sub><sup>-</sup>. The proton transfer, the existence of zwitterion, and the esterification may play important roles in the physicochemical property change of ILs, which have been discussed. The H-bond chemical nature in the single ion-pair and dimmer were analyzed by the theory of atoms in molecules (AIM).