# Intermolecular Interactions in Mixed Choline Acetate and Maleic Acid Systems
## 3 Results  
### Computational Results  
A careful DFT computational analysis was conducted by varying the number of composing molecules according to the experimental concentration. In agreement with the labeling used for the mixed samples, the corresponding simulated clusters were labeled as reported in Table 1.  
Previous work [22] showed that in the present systems, the coordination of cations and anions in the liquid phase is influenced by strong H-bond interactions between the hydroxyl groups of choline (OH) and the carboxylate oxygens of acetate (O=C-O-); in addition, in all the models of liquid bulks, maleic acids act as hydrogen bond donors (HBDs) [22], being able to donate strong H-bonds to acetate and other acid molecules. For these reasons, we decided to set the input for MM/DFT calculations, paying particular attention to the orientation of the H-bond donors and acceptors groups with the aim of focusing on these interactions. The optimized lowest energy configurations obtained for each composition are reported in Figure 2. Within a range of 6 kJ/mol, with respect to the most stable one, we obtained three possible configurations for the 2-1 mixture (see Figure 2a-c), two configurations for the 1-1 composition (Figure 2d,e) and only one (Figure 2f) for the 1-2 structure. The energy separations between the configurations are reported in Table 2, while the main distances and angles obtained for these configurations are reported in the supporting info; see Table S1. The calculated absolute energies of the six configurations are also reported in Table S2. The 2-1 composition is formulated by an excess of choline acetate. Being a dicarboxylic organic acid, MA serves as a bidentate hydrogen bond donor, allowing for coordination with equal probability at either of its carboxylic groups. In fact, in the most stable configuration (Figure 2a) MA is engaged in a double H-bond, simultaneously coordinating one of the two choline cations (H18--O8, see Figure 2a) and the acetate of the other ion pair (H16--O2). Indeed, the calculated H18--O8 and H16--O2 bond lengths are 1.57 and 1.43 A, respectively, with the corresponding angles close to 180\({}^{\circ}\), and are compatible with the characteristic distances of a strong hydrogen bond [48]. Configuration b (Figure 2b), although separated from the previous one by a small barrier (0.8 kJ/mol), shows an important difference in the coordination: MA is arranged with an intramolecular H-bond between H18 and O4, which induces a closed configuration of the maleic acid due to a rotation of one of the carboxylic acid groups. The other carboxylic group is still engaged in the hydrogen bond donation to one of the two acetates. The increased stability observed in the first configuration may arise from the open structure of the acid, facilitating the more efficient cross-linking and structural stabilization of the whole system. Configuration c (Figure 2c) is the less stable with a considerable energy difference of more than 5 kJ/mol, with respect to the lowest energy value. In this case, the maleic acids again arranged in a closed configuration with an intramolecular H-bond and an intermolecular H-bond with one of the acetates. However, the comparison of the bond distances (Table S1 of the Supporting Information) suggests that the intramolecular H-bond becomes slightly stronger in configuration b than in the c one (since the H18-O4 distances is larger in configuration c), while concomitantly the choline cation is closer to the acetate anion not involved in the intermolecular H-bond. The intermolecular H-bond, instead, becomes stronger in configuration b (since the H16-O5 distances is larger in b). Indeed, the formation of strong intermolecular bonds contributes to the stability of the whole system, as observed for the most stable configuration a, where the main occurring interactions are of this type. Moreover, in all configurations (a, b and c), all the acetates are engaged in at least one H-bond with either choline or maleic acid.