## 3 Results
### NMR Spectra  
The H-NMR data can shed further light on the effects of C(2) methylation. Table 3 summarizes the measured peaks and their assignments based on [4; 40; 41]. Signatures of the C(2) and C(10) protons can only be found in the nonmethylated and methylated ILs, respectively. In all three IL pairs, the C(4) and C(5) protons experience an upfield shift upon methylation, which indicates increased electron density. This increase can be explained by the inductive effect of the added methyl group and by charge transfer via the new hydrogen bond. This charge transfer seems to be rather small for iodide, as the upfield shifts are small. Interestingly, the shifts in the protons at the aromatic ring and those in the alkyl side chains are very similar. This indicates that the overall charge distribution at the protons is not significantly altered and almost independent of the position at which the iodide ion interacts with the cation. This is different for the two molecular anions, [BF\({}_{4}\)\({}^{-}\)] and [PF\({}_{6}\)\({}^{-}\)]. The NMR spectra show more significant upfield shifts. Moreover, these shifts are stronger for the aromatic protons than for the aliphatic ones. The NMR data suggest that [BF\({}_{4}\)\({}^{-}\)] and [PF\({}_{6}\)\({}^{-}\)] transfer more charge to the cation than iodide. This is counterintuitive, as the charge transfer usually decreases with increasing anion size [28; 42]: iodide has an ionic radius of 220 pm, while [BF\({}_{4}\)\({}^{-}\)] and [PF\({}_{6}\)\({}^{-}\)] exhibit radii of 230 and 245 pm, respectively. Hence, the observed behavior must have another reason. Furthermore, it must be kept in mind that H-NMR provides information about the electron density around the hydrogen atoms. Consequently, the observed shifts do not necessarily allow conclusions to be drawn on the overall interionic charge transfer. [I\({}^{-}\)] can be considered as a sphere, which is geometrically difficult to interact with the cation at multiple sites simultaneously. On the other hand, [BF\({}_{4}\)\({}^{-}\)] and [PF\({}_{6}\)\({}^{-}\)] possess several electronegative atoms, which may establish additional Coulomb or dispersion forces with other moieties of the cation, e.g., the alkyl side chains. This situation is shown schematically in Figure 5, taking [I\({}^{-}\)] and [BF\({}_{4}\)\({}^{-}\)] as examples. The [BF\({}_{4}\)\({}^{-}\)] and [PF\({}_{6}\)\({}^{-}\)] anions seem large enough to reach the terminal methyl group of the propyl side chain. This would explain why the C(9) protons in the iodide IL show virtually no chemical shift, while significant upfield shifts are observed for [BF\({}_{4}\)\({}^{-}\)] and [PF\({}_{6}\)\({}^{-}\)]. However, it must be noted that the molecular arrangement displayed in Figure 5 is not exclusive. In the real liquid, a distribution of several cation/anion configurations will be present simultaneously [43], and the sketch must be seen as one example of what the data suggest exists in larger amounts. In all three cases, methylation of the C(2) positions results in a replacement of the highly directional interaction between the cation and the anion by a less exclusive one that furthermore allows additional interactions with the ring and the side chains. This is particularly true for the molecular anions, \(\left[\text{BF}_{4}{}^{-}\right]\) and \(\left[\text{PF}_{6}{}^{-}\right]\). Consequently, the overall molecular system gains symmetry when the C(2) position is methylated. Such an increase in symmetry at the molecular level manifests as altered macroscopic properties. For example, viscosity increases significantly from 35 to 195 cP in the \(\left[\text{I}{}^{-}\right]\) ILs and from 103 to 330 cP in the \(\left[\text{BF}_{4}{}^{-}\right]\) ILs [29]. The increased symmetry also allows easier formation of a solid crystalline phase, hence an increase in the melting point temperature. For example, in the \(\left[\text{PF}_{6}{}^{-}\right]\) ILs it rises from 40 to 78 \({}^{\circ}\)C upon methylation [44].