## 3 Results
### Vibrational Spectra  
The full ranges of IR and Raman spectra of all compounds are displayed in Figure 3. The detailed vibrational analysis and tentative assignments are summarized in Tables A1 and A2 in the appendix. The assignments are based on previous work and the references therein [6; 19; 30]. In the following, we focus on a discussion of the interionic interactions. The predominant sites of the protonated cation (R = H) for hydrogen bonding interactions are the aromatic CH groups at C(2), C(4), and C(5). As mentioned above, the C(2) proton of imidazolium compounds is slightly acidic, hence it is expected to be the main donor of hydrogen bonds [31]. The formation of a hydrogen bond at C(2) leads to a change in the strength of the covalent C(2)-H bond, which in turn can be observed as a frequency shift of the corresponding vibrational mode. A red shift (i.e., a shift toward lower frequency or wavenumber) means a weakening of the bond, and a blue shift (i.e., a shift toward higher frequency or wavenumber) means strengthening [32]. Figure 2: TGA (panels (**a**,**c**,**e**)) and differential scanning calorimetry (DSC) (panels (**b**,**d**,**f**)) curves of the C2-protonated versus C2-methylated imidazolium-based ionic liquids. The legends in the diagrams on the right are also valid for the corresponding diagrams on the left; (**a**) and (**b**) show the data of the iodide ILs; (**c**) and (**d**) show the data of the tetrafluoroborate ILs; (**e**) and (**f**) show the data of the hexafluorophosphate ILs. The formation of strong hydrogen bonds is commonly associated with a red shift. Consequently, the C(2)-H stretching vibration should occur at a lower wavenumber than its C(4)/(5)-H counterparts. This has also been suggested by DFT calculations of other imidazolium-based ionic liquids, e.g., in references [6; 16]. The above hypothesis is corroborated by the IR and Raman spectra of the [MPrIm\({}^{+}\)] ILs in Figure 4a-e. The C(4)/(5)-H stretching vibrations are observed as a single degenerate peak at 3163/3143 (IR/Raman), 3163/3154, and 3171/3181 cm\({}^{-1}\) for the [I\({}^{-}\)], [BF\({}_{4}\)\({}^{-}\)], and [PF\({}_{6}\)\({}^{-}\)] compounds, respectively. In contrast, the C(2)-H stretches appear at 2852, 2849, and 3004 cm\({}^{-1}\), indicating an involvement in strong hydrogen bonding interactions. For completeness, we note that assignment of the CH stretching bands of imidazolium-based ILs is a subject of ongoing debate (see, for example, [3; 22; 33; 34; 35]) and a definitive conclusion has not yet been reached. The assignment used in the present study is consistent with our previous work; all IL data we have recorded so far point in this direction. Figure 3: Full range infrared (panels (**a**,**c**,**e**)) and Raman (panels (**b**,**d**,**f**)) spectra; (**a**) and (**b**) show the data of the iodide ILs; (**c**) and (**d**) show the data of the tetrafluoroborate ILs; (**e**) and (**f**) show the data of the hexafluorophosphate ILs. Upon methylation of the C(2) position, the C(2)-H stretching bands disappear and the C(4)/(5)-H are modified in all three types of ionic liquids, suggesting a molecular rearrangement of the anion-cation interactions. Interestingly, the initially degenerate peak of the C(4)/(5)-H stretching modes splits into blue- and red-shifted components. This can be explained by the fact that the anion predominantly interacts with only one of the two protons: a new hydrogen bond is formed at either C(4) or C(5). Consequently, the hydrogen-bonded CH stretch is red-shifted. At the same time, a redistribution of the charge in the aromatic system occurs as a result of the methylation and the hydrogen bond formation. This manifests as a blue shift of the other CH mode. The arrows in Figure 3 mark the initial and resulting bands in the IR and Raman spectra. The strongest effects are found in the spectra of the [I\({}^{-}\)] and [BF\({}_{4}\)\({}^{-}\)] compounds.