Proton Exchange in Ionic Liquids

The protic ionic liquid 1-methylimidazolium acetate [Im₁H][OAc] is in equilibrium with its neutral species 1-methylimidazole Im₁ and acetic acid HOAc as shown in Figure 1.
The program protex uses a single topology approach with two discrete λ-states to allow for the proton exchange. For imidazoles and acetate, we model the neutral species Im₁ and HOAc and the cation Im₁H⁺ and anion OAc⁻, respectively. In principle, it is also possible to model the protonated acetic acid (Ingenmey et al., 2018 (link); Jacobi et al., 2022 (link)), which might be necessary for the Grotthus conductivity mechanism, but we restrict ourselves to the simple protonation scheme by Jacobi et al. (2022) (link) for the sake of simplicity. The deprotonation of the Im₁H⁺ or HOAc is modeled by turning the hydrogen (HP) into a dummy atom (DM) which is part of the acetate OAc⁻ and imidazole Im₁ molecule.
In contrast to common alchemical mutations for proton transfer, the presented approach is not limited to partial charge mutations (Mey et al., 2020 (link)). As the atom types are changed to fit the DGenFF force field nomenclature (Chatterjee et al., 2019 (link); Lin and MacKerell, 2019 ; Kumar et al., 2020 (link)) of the charged/neutral species, all bonded and non-bonded parameters are modified. Tables 1, 2 outline these changes in the atom types, Lennard-Jones parameters, partial charges q_iβ, and polarizabilities α_iβ. In imidazolium, both ring nitrogens share the same atom type NC. The neutral imidazole Im₁ has lone pairs at the unsubstituted ring nitrogen (NB). As a consequence of protonation, the charge of these lone pairs is set to 0 e, turning off all their interactions.
All these changes ensure that the molecules behave according to their charge state. This is particularly crucial for ionic liquids as the Coulombic interactions are neither short-ranged nor restricted to ion pairs and lead to cage-like structures (Schröder, 2011 (link); Szabadi et al., 2022 (link)).