Parametrizing reaction probabilities for proton transfers in protic ionic liquids

Protic ionic liquids are promising electrolytes for electrochemical applications owing to their intrinsic proton conductivity, but quantitative understanding of the underlying proton transfer processes remains limited. Here, we present a systematic quantum-mechanical investigation of proton transfer in a series of 1-methylimidazolium carboxylates, with the specific goal of parametrizing reaction probabilities for use in reactive molecular dynamics simulations. Density functional theory scans were performed to map relaxed potential energy surfaces along two collective variables, the donor-acceptor distance and the proton transfer coordinate. The resulting energy profiles were accurately represented by Morse potentials. From the donor-acceptor distance scans, the distance-dependent reaction probabilities were fitted using a hyperbolic tangent function. Analysis of the proton transfer coordinate revealed small or even negligible energy barriers for the proton transfer reactions, which in turn resulted in low empirical valence bond coupling energies between the reactant and product states. Quantum tunneling effects appear to play only a minor role in these processes. Consequently, the proton transfer reaction probabilities are predominantly governed by thermally activated hopping events, which are captured within a classical kinetic model framework.