# Intermolecular Interactions in Mixed Choline Acetate and Maleic Acid Systems
## 3 Results
### Computational Results
Indeed, this phenomenon occurs even at other concentrations, and the bonds distances are similar and close to those described for the 2-1 composition (See Table SI). The proton transfer occurs between molecules where the establishment of an acid-base equilibrium is possible. The presence of intramolecular hydrogen bonds in maleic acid, as revealed by almost all computed structures, can promote the proton transfer mechanism. This suggests that the intramolecular H-bond acts as the mechanism that makes proton transfer favorable, providing further confirmation of the significant role of H-bonds in facilitating this process. Furthermore, previous study has experimentally and computationally demonstrated, through vibrational spectroscopy, molecular dynamics simulations and DFT calculations, that in cholinium maleate (ChMal) ionic liquid, the maleate anion tends to arrange itself in a closed structure, establishing a strong intramolecular hydrogen bond [50]. To better describe the mechanism and the thermodynamics related to the PT, we computed the Gibbs energy (\(\Delta G^{0}\)\({}_{\rm r}\)) for two proton transfer reactions. For simplicity, we started by considering the reaction without the presence of the counterion, namely the choline, as indicated in Equation (1). Considering the significant stabilization effect on the ion pair exerted by choline, we extended the calculation introducing the cation, as reported in Equation (2). Both reactions are attempts to describe the proton transfer mechanism. Therefore, the reactants considered are maleic acid and acetate anion (or choline acetate in the case of Equation (2)). When the proton is transferred from maleic acid to acetate, a mechanism is established that leads to the protonation of the acetate, with the formation of acetic acid and maleate. The \(\Delta G^{0}\)\({}_{\rm r}\) was computed at the B3 LYP/6-3 IG** level of theory and was obtained as the difference between the products and reactants. Maleic Acid + [Acetate]\({}^{-}\)\(\rightarrow\)-> Acetic Acid + [Maleate] (1) Maleic Acid + Choline Acetate -The computed Gibbs free energy of both reactions demonstrates that proton transfer is thermodynamically favored, as \(\Delta\)G\({}_{\rm r}^{0}\)(1) is \(-362\) kJ/mol and \(\Delta\)G\({}_{\rm r}^{0}\)(2) is \(-44\) kJ/mol. The notable difference between these two values arises from the strong coordination of acetate to choline in the second reaction, thereby diminishing its reactivity. The proton transfer occurs between molecules where an acid-base equilibrium can be established. The significant difference in the pKa values [51] of maleic acid (pKa1 \(\approx\) 1.9, pKa2 \(\approx\) 6.1) and acetic acid (pKa \(\approx\) 4.76) plays a critical role in this mechanism. Maleic acid, being a much stronger acid than acetic acid, has a higher tendency to donate protons, driving the proton transfer process. The stronger acidity of maleic acid compared to acetic acid facilitates the formation of the maleate anion, making the proton transfer thermodynamically favorable. This contrast in acidity highlights the role of the relative acid strengths in determining the direction of the proton transfer, where the proton moves from the stronger acid (maleic acid) to the weaker acid (acetate). Additionally, the presence of intramolecular hydrogen bonding in maleic acid further promotes this mechanism, as it stabilizes the structure and lowers the energy barrier for proton transfer. An alternative and complementary way to describe and visualize intermolecular interactions can be performed by means of the NCI calculations. The role of NCIs can be investigated by studying the Laplacian of electron density, \(\
abla^{2}\rho\), as the sign of its eigenvalue \(\lambda_{2}\) represents a good way to discriminate van der Waals, hydrogen bonds and repulsive interactions.