## 3 Results
The OH bond, therefore, appears less polarized in the mixture than in the neat alcohol. On the other hand, the excess signatures in the fingerprint region are predominantly positive. The majority of the corresponding modes originate from the triflate anion, indicating that the re-arrangement of the molecular network mainly affects the anion. This is reasonable, as the sulfonate oxygen atoms can easily establish hydrogen bonds with the hydroxyl groups of the alcohol molecules. For a clearer picture of how the signatures vary with the alcohol content, selected representative signatures in the excess spectrum are integrated and plotted vs. the propanol mole fraction, \(x\)(PrOH). Figure 4 illustrates three example profiles: the OH stretching band from the excess IR and Raman spectra, and the SO stretching band from the excess IR spectrum. They all reveal rather similar profiles exhibiting extreme values around \(x\)(PrOH) = 0.66. The experimental data were fitted to a third-order polynomial (solid line) in which the values for \(x\)(PrOH) = 0 and \(x\)(PrOH) = 1 were set to zero. The resulting maxima and minima of these polynomials are all located around \(x\)(PrOH) = 0.66, hence pointing to the 2:1 stoichiometry again. Interestingly, this molar ratio also stood out in a previous study, in which the vapor-liquid equilibrium (VLE) of the binary mixtures was studied [11]. In that work, the VLE data were fitted using a modified Raoult's law, and the activity coefficients were derived. The profile (see Figure 4d) also peaks around \(x\)(PrOH) = 0.66. This shows a clear relationship between the molecular and macroscopic behavior of the solution. The above analyses of the data sets strongly indicate that the mixture stoichiometry of 2:1 PrOH:C4 mimTfo plays a distinct role and is worth further investigation. In order to obtain a clearer picture of the molecular interactions in the solutions, we analyze the OH stretching band. This is due to two main reasons: first, only the alcohol exhibits an OH group and therefore the signals in the range 3200-3700 cm\({}^{-1}\) will exclusively originate from PrOH; second, the hydroxyl group is the predominant site at which hydrogen bonding will take place. Figure 3: Excess ATR-FTIR spectra (**a**) and excess Raman spectra (**b**) of the neat substances and the binary mixtures. The OH stretching band in liquid alcohols is rather broad [18]. However, the overall band can be decomposed into narrower sub-peaks that are characteristic of distinct hydrogen bonding states. The results of the spectral decomposition of five selected example mixtures are displayed in Figure 5. The red solid line represents the experimental data, the thin solid black lines are the derived sub-peaks, and the dashed line shows the sum of the sub-peaks. The two small sub-peaks at the high- and low-wavenumber ends of the OH band are of limited interest in the discussion of the molecular interactions. The high-wavenumber peak can be attributed to those OH groups that are not involved in hydrogen bonding interactions. They are commonly referred to as \"free\" OH groups, but it should be noted that their frequency is still affected by the dielectric surrounding medium [32]. The sub-peak at the low-wavenumber end of the OH band basically serves as a compensation for the overlap from the neighboring CH stretching band. The two main contributions to the overall band are located at 3315 and 3431 cm\({}^{-1}\) in the neat alcohol. The positions change only marginally in the mixtures. However, the peak intensities and the ratio between them varies substantially. The sub-peak at 3431 cm\({}^{-1}\) can be assigned to OH groups that participate in a single hydrogen bonding interaction, while the 3315 cm\({}^{-1}\) peak originates from OH groups involved in two hydrogen bonds. In the latter, the hydrogen acts as a HB donor while the oxygen atom acts a HB acceptor. This is very common in neat alcohols where chain- and ring-like structures can be found, forming large HB networks [18].