We measure the far-infrared spectrum of N,N-Dimethyl-N-ethyl-N-benzylammonium (DEBA) bis(trifluoromethanesulfonyl)
imide (TFSI) ionic liquid (IL) in the temperature range between 160 and 307 K. Differential scanning calorimetry
measurements indicate that such IL undergoes a glass transition around 210K. DFT calculations allow us to assign all the
experimental absorptions to specific vibrations of the DEBA cation or of the two conformers of the TFSI anion. We find that the
vibration frequencies calculated by means of the PBE0 functional are in better agreement with the experimental ones than those
calculated at the B3LYP level, largely used for the attribution of vibration lines of ionic liquids. Experimentally we show that, in the
liquid state, the relative concentrations of the two conformers of TFSI depend on temperature through the Boltzmann factor and
the energy separation, ΔH, is found to be ≈2 kJ/mol, in agreement with previous calculations and literature. However, in the glassy
state, the concentrations of the cis-TFSI and trans-TFSI remain fixed, witnessing the frozen state of this phase