Combination of FTIR and DFT to study the structure and hydrogen-bond properties of alkylammonium-based ILs and DMSO mixtures

•Structure and hydrogen-bond features of ammonium ILs and DMSO mixtures are studied.•Mixing process is non-ideality with hydrogen-bond formation in four IL–DMSO systems.•Four excess peaks are identified by 2D-COS.•They are related with the ion cluster/ion pair–DMSO and DMSO dimer/monomer. Ionic liquids (ILs) are often used in combination with molecular solvents as working fluids in many applications. The addition of molecular solvents disturbs the structure and dynamics of ILs. Thus, understanding the structure and interactions in molecular solvents and IL mixtures is particularly important. In this work, the structure and hydrogen-bond features of N-alkyl-N-trimethylammonium bis(trifluoromethylsulfonyl)imide ([N-111n][Tf2N], n = 3, 4, 6 and 8) and dimethylsulfoxide (DMSO) binary systems were studied via the combination of Fourier transform infrared spectroscopy (FTIR) and quantum chemical calculations. The CD systematic stretching vibrational (vs(CD)) region of the methyl groups in DMSO-d6 was specially focused on using excess infrared and two-dimensional correlation spectroscopy. In the mixing process, the DMSO dimer and monomer were gradually reduced with increasing ILs. [N-111n][Tf2N] interacts with DMSO by ion cluster–DMSO-d6 complexes and ion pair–DMSO-d6 complexes. The ion cluster–DMSO complexes gradually increase while the ion pair–DMSO complexes decrease due to the strong electrostatic interaction between the cation and anion. In the interaction complexes, closed-shell, electrostatically dominant and weak strength hydrogen-bonds were found between the ILs and DMSO-d6. The hydrogen atoms in the methyl groups directly attached to the nitrogen atom of the [N-111n]+ cation are the preferred interaction sites of the cations.