The structure and interaction properties of two task-specific ionic liquids and acetonitrile mixtures: A combined FTIR and DFT study.

The mixtures of ionic liquid (IL) and acetonitrile (CHCN) can be used as reaction media, supercapacitors and thermally stable electrolytes. The macroscopic properties of ILs-CHCN mixtures have been extensively studied. However, some fundamental questions regarding the microscopic properties of ILs-CHCN mixtures still remain to be answered. In this work, the structure properties and hydrogen-bond interactions of two task-specific ILs, i.e., 1-propylnitrile-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([PCNMIM][TfN]) and 1-(2'-hydroxylethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2OHMIM][TfN]), and CHCN were studied using the combination of Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations. The aromatic C‒H stretching vibration region of the cation was an area of special focus. Excess infrared spectroscopy with enhanced resolution was applied to analyse the original infrared spectra. It is found that: (1) The two ILs form stable hydrogen-bonds with CHCN. (2) Ion cluster, ion cluster-acetonitrile, and ion pair-acetonitrile are identified in the mixture. Acetonitrile cannot break apart the strong electronic interaction between the cation and anion in the examined concentration range. (3) The hydrogen-bonds are weak strength, closed shell and electrostatic dominant interactions. (4) The preferred interaction site of [PCNMIM] cation is the hydrogen atom at the C2 site, while that of [C2OHMIM] cation is the hydrogen atom in the hydroxyl group.