## 3 Results
### Interaction of Water with the Ionic Liquid  
The interaction of water and the ionic liquid can be investigated by comparing the position of the vibrational bands when different concentrations of solvent are present in the sample. In the previous literature it has been reported that water mainly interacts with the anion of ILs [8; 20; 21; 22; 23; 24; 25; 26] Indeed, Figure 8 displays a comparison of the spectra of PYR\({}_{14}\)-DCA and PYR\({}_{14}\)-DCA + H\({}_{2}\)O at 300 K and 160 K in the region 1250-2300 cm\({}^{-1}\), where one finds the absorption around 1310 cm\({}^{-1}\), that was attributed to the asymmetric stretching mode of the bridging N and C atoms of the anion and the absorptions around 2230 cm\({}^{-1}\), attributed to the triple bond between the C and the terminal N atoms [35; 36; 37; 38]. The three bands due to the anion shift toward higher frequencies in the intentionally hydrated sample by \(\sim\)6 cm\({}^{-1}\). Similar observations were previously reported for other ionic liquids mixed with water and the different behavior of anion and cation vibrations were interpreted as the consequence of the strong interaction of water with the anions [14; 18; 39]. It can be noted in the present study that both the vibration of the central part (C-N-C) and of the extremities (C\(\equiv\)N) of the anion are affected, suggesting that the whole anion interacts with water.  
Figure 7: Temperature dependence of the parameters of the most significant absorption lines of PYR\({}_{14}\)-DCA + H\({}_{2}\)O measured on heating from 160 up to 300 K.  
While it is well known that water mainly interacts with the anion of ionic liquids, in some cases weaker interactions have also been reported between water and the cations of ILs [17; 18; 39]. Figure 9 displays a comparison of the spectra of PYR\({}_{14}\)-DCA and PYR\({}_{14}\)-DCA + H\({}_{2}\)O at 300 K and 160 K in the region 2840-3170 cm\({}^{-1}\), where the absorption bands due to the CH stretching of the cation can be found. Both at room temperature and at 160 K, one can observe that the bands centered around 2876, 2935, 2964 cm\({}^{-1}\)(band A, B and C in the following) maintain their central frequency after the intentional addition of water while the bands around 3015 and 3135 cm\({}^{-1}\) (band D and E in the following) are blue shifted by \(\sim\)6 cm\({}^{-1}\) by increasing the H\({}_{2}\)O content.  
Contrarily to the case of imidazolium-based ionic liquids [40], to the best or our knowledge a detailed attribution of the CH stretching bands of ILs containing the pyrrolidinium cation is  
Figure 8: Comparison of the infrared absorption spectra measured at 160 K and 300 K of PYR\({}_{14}\)-DCA and PYR\({}_{14}\)-DCA + H\({}_{2}\)O in correspondence with three anion vibrational bands. The curves are vertically shifted for clarity.  
Figure 9: Comparison of the infrared absorption spectra measured at 160 K and 300 K of PYR\({}_{14}\)-DCA and PYR\({}_{14}\)-DCA + H\({}_{2}\)O in correspondence with the CH stretching vibrational bands. The curves are vertically shifted for clarity.  
not available. Based on our DFT calculations, we suggest that band A is due to the terminal CH\({}_{3}\) group of the butyl chain, band B and C are ascribable to the whole alkyl chain, band D is attributable to the motions of the pyrrolidinium ring and band E originates from the CH\({}_{3}\) methyl group directly attached to the N atom. Using these attributions, we can suggest that the absorption lines that are more strongly affected by the addition of water are those due to the pyrrolidinium ring and to the methyl group directly attached to the N atom of the cation.