# Intermolecular Interactions in Mixed Choline Acetate and Maleic Acid Systems
## 3 Results
### Infrared Spectroscopy Results  
Figure 4 shows the experimental FIR absorbance spectra (panel a) measured at 300K for the three considered concentrations, and the theoretically computed spectra (panel b, c and d for ChAc-MA 2-1,ChAc-MA 1-1 and ChAc-MA 1-2, respectively). The experimental spectra of the three samples are qualitatively very similar, as they all show an absorption band around 100 cm\({}^{-1}\), which is particularly pronounced in the 1-1 and 2-1 compositions. The high frequency side of this band presents a shoulder around 180 cm\({}^{-1}\) which is detectable as a well distinguishable band in the ChAc-MA 1-2 mixture. At higher frequencies, all the three spectra show a wide band between 300 and 400 cm\({}^{-1}\) whose maximum shifts to lower frequencies with increasing acid content, and a couple of minor bands between 400 and 500 cm\({}^{-1}\), which become closer at higher acid contents and overlap in the ChAc-MA 1-2 mixture. Figure 3: NCI surface computed for 2-1 (**a**), 1-1 (**b**) and 1-2 (**c**) compositions. All calculations are referred to the most stable configuration of each composition. (Isovalue = 0.75). The band around 100 cm\({}^{-1}\) appeared challenging to be assigned: the computational models of the three mixtures suggest that it is generated by the superposition of several contributions related to skeletal movements of the components and intermolecular interactions. In particular, numerous signals are noticeable in the computational spectra of the 2-1 composition around this frequency. This arises from the great number of degrees of freedom within the molecular cluster, resulting in a higher number of possible vibrational modes. As described by [PERSON] and coworkers for protic ionic liquids [24], the band around 100 cm\({}^{-1}\) can be interpreted as originating from dispersion forces between the components of mixtures. This interpretation is supported by the calculation of Non-Covalent Interactions reported in Figure 3: the extended green region observable in NCI surfaces corresponds to van der Waals interactions, or more broadly, to regions characterized by dispersion force interactions. Although a strictly quantitative interpretation of intermolecular interactions by means of the NCI method is not feasible, we noted that in the 2-1 composition, the green-colored zones are broader and their extension tends to decrease as the amount of choline acetate in the mixtures decreases. Indeed, in the experimental spectra, we observed that the intensity of the dispersion forces band seems to follow the same described trend, since it is the most intense band of the measured frequency range in the 2-1 composition, while its intensity is comparable to that of the other bands in the 1-1 composition and becomes broader and less intense than the other absorptions in the 1-2 mixture. The computational investigation predicted the presence of a robust hydrogen bonding network between the components comprising the mixtures. Since infrared spectroscopy is a proven method to detect hydrogen bonding in liquid systems [24; 25; 52] from an experimental perspective, we expected to observe well-defined signals associated with hydrogen bond stretching in the region below 200 cm\({}^{-1}\). Instead, the experimental FIR spectra exhibited only broad bands between 185 and 175 cm\({}^{-1}\). The broad shape of the 2-1 composition is consistent with the experimental results. Figure 4: Room temperature experimental (**a**) and computational (**b**–**d**) FIR absorbance of ChAc-MA mixtures. these bands, notably pronounced in the 1-2 composition, presented a notable challenge in precisely determining the peak maximum. The computational spectra revealed a prominent signal slightly below 200 cm\({}^{-1}\): our DFT calculations suggested they can be attributed to the stretching of hydrogen bonds between maleic acid and acetate. These frequencies are compatible with the occurrence of strong hydrogen bonding as confirmed by the comparison with similar results in profile and aprotic ionic liquids [24, 25, 52].