## 1 Introduction  
Ionic liquids (ILs) are typically composed of bulky organic cations and inorganic or organic anions. They represent fascinating materials in the liquid as well as the solid state, with numerous potential applications in electrochemistry and chemical engineering [1; 2]. Interionic interactions between cations and anions are known to govern the molecular and macroscopic properties of 1,3-dialkylimidazolium-based ionic liquids. In particular, the C(2) position plays a crucial role in this respect, because the charge distribution in the aromatic ring leads to enhanced polarization and hence a slightly acidic nature of the C(2)-H bond. Consequently, the C(2) proton is the favorable hydrogen-bond donor. A comprehensive overview has recently been given by [PERSON] et al. [3].  
Most studies to date have focused on the interionic interactions of ionic liquids, in which the 1,3-dialkylimidazolium cation is combined with nonsymmetrical anions, exhibiting rather well-defined moieties for establishing hydrogen bonds. The list of investigated anions includes bis(trifluoromethylsulfonyl)imide [4; 5; 6; 7], nitrate [8], alkyl sulfates [5; 9], dicyanamide [5; 10], thiocyanate [5], acetate [11; 12], hydrogen sulfate [13; 14], and trifluoromenthanesulfate [15; 16]. On the other hand, interactions with highly symmetrical anions have been studied less. Examples include the halogenides [17; 18; 19], tetrafluoroborate [20; 21], and hexafluorophosphate [15; 22]. However, systematic studies with varied anions are rare [23] or have had a different focus, e.g., on the rotational isomerism of the cation [24].  
To study the effects of the C(2) position, it has commonly been analyzed in its protonated and methylated forms [4; 8; 19; 25; 26; 27]. The methylation at C(2) basically removes the possibility to form hydrogen bonds with the anion at this site. The impact of C(2) methylation on the viscosity of imidazolium ionic liquids has been studied extensively, leading to several theories. [PERSON]'s defect theory [7] suggests that the absence of a strong and directional hydrogen bond at C(2) produces a reduced disruption of order. On the other hand, [PERSON]'s reduced entropy theory [28] argues that the methyl group at the C(2) position blocks low energy configurations. [PERSON] et al. [4] showed that electron density changes can alter the position and strength of interionic interactions and lead to reduced configurational variations. [PERSON]'s dynamic theory [29] suggests that the free-energy barrier for the anion to move relative to the imidazolium cation increases the viscosity.  
The present study aims to shed further light on the role of the C(2) position in imidazolium ionic liquids regarding the interionic interactions with highly symmetrical anions. For this purpose, FTIR, Raman, and H-NMR spectroscopies are applied to the [I\({}^{-}\)], [BF\({}_{4}\)\({}^{-}\)], and [PF\({}_{6}\)\({}^{-}\)] compounds of 1-methyl-3-propylimidazolium [MPrIm\({}^{+}\)]. In addition, the thermal behavior is investigated. To study the influence of the hydrogen bonding effects at C(2), the three anions have also been combined with the 1,2-dimethyl-3-propylimidazolium [MMPrIm\({}^{+}\)] cation. In this cation, the C(2) position is methylated to remove the possibility of acting as a hydrogen bond donor. The chemical structures and the numbering scheme are illustrated in Figure 1. To the best of the authors' knowledge, this work represents the first combined NMR and vibrational spectroscopic study of protonated and methylated imidazolium ILs with systematically varied spherical anions.