Comparison of Polarities of Room-Temperature Ionic Liquids Using FT-IR Spectroscopic Probes

The solvent properties, especially the polarity, of room-temperature ionic liquids (RTILs) of the general type [Cnmim]X, where [Cnmim] is the 1-CnH2n+1-3-methylimidazolium cation and X is [BF4]−, [PF6]−, [NO3]−, [SCN]−, or [NTf2]− (Tf = CF3SO2), have been investigated using Fourier-transform infrared (FTIR) spectroscopy. Comparing the experimental shifts in the νC=O stretching frequencies of acetone and N,N-dimethylformamide (DMF) as probe molecules suggests that the polarities of most common RTILs are between that of DMF and methanol. A more sensitive probe, Fe(CO)5, can distinguish small polarity differences between RTILs with similar structures. The effects of varying the structure of RTILs by introducing functional groups and different anions on their polarity are also compared.