Interaction between phosphomolybdic anion and imidazolium cation in polyoxometalates-based ionic liquids: a quantum mechanics study.

In this study, interaction between the phosphomolybdic anions ([PMoO](3-)) and 1-butyl-3-methyl imidazolium cations ([Bmim](+)) has been systematically studied by the density functional theory at the PBE-D3/TZP level. The stable geometries of the ion pairs with no imaginary frequencies were obtained and characterized. Multiple H-bonds formed between the cation and anion were revealed with the type of C-H · · · O. The interaction energy between the constituent [PMoO](3-) anion and [Bmim](+) cation is obviously larger than those in common ionic liquid. This is the possible reason for the relatively higher melting point of polyoxometalates (POMs)-based ionic liquids. It was observed that the interaction between the ion pairs was mainly contributed from the electrostatic interaction between [PMoO](3-) and [Bmim](+). The nature of the H-bonds was analyzed by the atoms in molecules (AIM) theory, harmonic vibrational frequency, the natural bond orbital (NBO), and the non-covalent interaction (NCI) approaches. The charge transfer and the orbital interaction between the ion pairs have also been identified, which may have an important influence on the electronic property of the ion pairs.