## 2 Materials and Methods  
1-Ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-TfO) with a purity of 99% was purchased from Iolitec. A scheme of the ions composing the ionic liquid is reported in Figure 1. To avoid contaminations of undesired water from the atmosphere, the sample was stored in a glove box filled with pure Ar. Before the infrared measurements, the sample was additionally heated in a vacuum of the order of 10\({}^{-5}\) mbar for 48 h at about 80 \({}^{\circ}\)C to remove eventual water contaminations. Infrared absorbance measurements were conducted as a function of temperature in the range of 120-330 K at the AILES beamline of Soleil Synchrotron, by means of a Bruker IFS125 HR spectrometer with a resolution of 1 cm\({}^{-1}\). A few droplets of the neat sample were placed between the two diamond optical windows of a vacuum-sealed cell for liquids. The temperature was varied by the Cryomech cryostat available at AILES with a temperature rate of 5 K/min. The spectra were recorded in the mid-infrared range using a KBr beam splitter and a wide range low noise MCT [45]; in the far-infrared range, spectra were acquired by means of a Si-coated mylar beamsplitter and a bolometer. Absorbance measurements as a function of pressure at room temperature were performed at the SMIS beamline of Soleil Synchrotron, by means of a Nicolet iS50 spectrometer (Thermo-Fisher, Waltham, MA, USA) coupled with a home-made horizontal microscope dedicated to high-pressure experiments. The microscope has custom long working distance Schwarzschild objectives which allowed performing transmittance measurements in the diamond anvil cell (DAC). Spectra were recorded in the mid-infrared range with a KBr beamsplitter and a resolution of 1 cm\({}^{-1}\). DAC was equipped with diamond anvils of type IIas with a culet diameter of 400 \(\upmu\)m and a stainless steel gasket pre-indented to a thickness of 50 \(\upmu\)m, where a hole of 150 \(\upmu\)m was drilled to be used as a sample chamber. NaCl was used as a pressure-transmitting medium and to reduce the sample thickness and avoid signal saturation [46]. Pressure was measured in-situ through the standard ruby luminescence technique [47]. The luminescence was excited and collected using a home-built optical setup aligned with the [PERSON] objectives and made of dichroic mirrors, a 532 nm green laser, a camera, a lamp and a fiber collimator connected to an Ocean Optics HR4000 spectrometer. Before acquiring each spectrum, we waited for about five minutes to achieve pressure stabilization. All calculations were performed by means of the Spartan software [48]. Possible geometries were generated at the molecular mechanics level. All structures were afterwards optimized by means of DFT calculations. For the single ions EMI and TfO, the B3 LYP functional was employed considering the ions as isolated in vacuum; this particular functional is considered as a benchmark for this type of calculation [12]. For the ionic couple EMI-TfO, the \(\omega\)B97X-D functional, considering both the empirical dispersion corrections and the presence of a polar solvent (dimethylformamide, \(\varepsilon_{\mathrm{r}}\) = 37.22), was used. Indeed, this last combination of functional, dispersion forces and polar solvent has recently provided a good description of the absorbance spectra of diethyl methyl ammonium methanesulfonate (DEMA-MS) and diethyl methyl ammonium trifluoromentanesulfonate (DEMA-TfO) [49]. The introduction of the polar solvents represents a further improvement compared to previous similar DFT calculations [35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50]. For all calculations, the 6-31G** basis set was used. After optimization of the geometry of single ions or of the ionic couple, the infrared vibrational frequencies and intensities were obtained with the same level of theory and basis set adopted for the optimization.