Hydrogen-bonding interactions between [BMIM][BF4] and dimethyl sulfoxide

• It is first used excess spectra to investigate the interactions between [BMIM][BF4] and DMSO.
• C2–H is the main interaction site in investigated complexes.
• The two turning points of the wavenumber shift changes of C2–H may indicate the dilution process.
• Hydrogen-bond interactions are the main cause influencing the wavenumber shift changes.

Mixtures of Ionic liquids and small polar organic solvent are potential green solvents for cellulose dissolution under mild conditions. In this work, the interactions between a representative imidazolium-based ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and dimethyl sulfoxide (DMSO) were investigated in detail by attenuated total reflection infrared spectroscopy (ATR-IR) and density functional theory calculations (DFT). The main conclusions are: (1) C2–H is the main interaction site in forming cation–anion, cation–DMSO, and [BMIM][BF4]–DMSO complexes. (2) The two turning points of the wavenumber shift changes of C2–H may indicate that the dilution process can be divided into several stages: from larger ion clusters to smaller ion clusters, then to ion pairs, and finally to individual ions. The solvent molecules cannot break apart the strong Coulombic interaction between [BMIM]+ and [BF4]− but can break apart the ion clusters into ion pairs when the mole fraction of DMSO is less than 0.9. When the mole fraction of DMSO is greater than 0.9, ion pairs can be broke into ions. (3) The hydrogen-bonds of the aromatic C–Hs in [BMIM]+ are strengthened in the dilution process while those of the alkyl C–Hs of [BMIM]+ are weakened. (4) The aromatic C–Hs of the [BMIM]+ cation strength before the weakening of the alkyl C–Hs. These in-depth studies on the properties of the ionic liquid-DMSO mixed solvents may shed light on exploring their applications as mixed solvents in cellulose dissolution and other practices.