A comparison of ether- and alkyl-imidazolium-based ionic liquids diluted with CH3CN: A combined FTIR and DFT study

Abstract

Ether-based functionalized ionic liquids (ILs) show great potential in electrochemical devices and energy applications, but their microscopic behaviour is seldom investigated. In this work, the microscopic structures of two imidazolium-based ILs (1-methoxyethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EOMIMFSI) and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIMFSI)) and their hydrogen bonding interactions with acetonitrile (CH3CN) at various concentrations are investigated by using attenuated total reflection infrared spectroscopy (ATR-FTIR), excess infrared spectroscopy and quantum chemical calculations. Additionally, the effects of introducing a functional group, i.e., an ether group, on the properties of the IL are explored. The main conclusions are as follows: (1) [EOMIM]+ can form an intra-molecular hydrogen bond with C2H or C5H, and the former is more stable. (2) The introduction of an ether group to the cation weakens the hydrogen bonding interactions between the cation and anion/CH3CN. (3) The introduction of an ether group to the cation changes the interaction sites between the ILs and CH3CN. In the EMIMFSI–CH3CN system, C2H is the main interaction site. In the EOMIMFSI–CH3CN system, the interaction site with CH3CN changes from C2H in the [EOMIM]+–CH3CN complex to C4H in the ion pair–CH3CN/ion cluster–CH3CN complex. (4) Some complexes such as ion clusters, small ion clusters and ion pairs in the two systems are identified by excess infrared spectroscopy and quantum chemical calculations.