The structural and hydrogen bonding properties of ionic liquid-co-solvent binary mixtures: the distinct behaviors of...

In practical applications, ionic liquids are often mixed with co-solvents. Understanding their structures and the interactions between them is a prerequisite for expanding their range of applications. In this work, spectroscopic and theoretical methods were employed to explore the structure and hydrogen bonding behaviors of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI)/1-ethyl-3-methylimidazolium thiocyanate (EMIMSCN) and co-solvents. It can be concluded that the hydrogen bonds associated with C2-H and C4,5-H are enhanced with the addition of co-solvents in the EMIMTFSI-DMSO system, while those associated with C4,5-H are weakened in the EMIMSCN-DMSO system. Infrared and excess spectra in the v(imidazolium C-H) range of EMIMSCN-CD₃CN/CD₃COCD₃ systems further indicate that the abnormal change of hydrogen bonds associated with C4,5-H can be attributed to [SCN]^-. These differences can be explained by the change of the primary interaction site. For EMIMTFSI, the primary interaction site in ion pairs and ion clusters is always C2-H, while for EMIMSCN, the primary interaction site in ion pairs is C2-H, and in ion clusters, it becomes C4,5-H. In the EMIMTFSI-DMSO system, the co-solvent primarily interacts with C4,5-H, while in the EMIMSCN-DMSO/CH₃CN/CH₃COCH₃ systems, it primarily interacts with C2-H. In addition, several complexes are identified through excess infrared spectra and DFT calculations.