Infrared spectra of bis(trifluoromethanesulfonyl)imide based ionic liquids: Experiments and DFT simulations

We measured the far- and mid-infrared spectra of three ionic liquids having bis(trifluoromethanesulfonyl)imide anions and three different cations of the families of pyrrolidinium and ammmonium ions. The molecular vibrations of the individual ions were calculated by means of DFT theory at the B3LYP/6-31G** level: we found good agreement between the experimental and the computed frequencies. The infrared lines are ascribable to molecular vibrations of the single ions, suggesting an extremely weak interaction between anions and cations. The spectral lines found experimentally between 760 and 1050 cm⁻¹ are fingerprints for different cations. The comparison with the calculated frequencies allows the assignment of the experimental spectral lines to specific molecular vibrations of anions and for the first time of the specific cations of the measured ionic liquids.