Synergistic Zundel–Eigen mechanism for proton transport in protic ionic liquids unraveled by machine learning force fields and NMR spectra

Abstract Elucidating proton transfer in nonaqueous media is essential for the rational design of advanced chemical processes and energy materials, yet its molecular mechanism remains poorly understood. Here, we combine machine-learning potentials (MLPs) molecular dynamics, path-integral simulations, metadynamics, and ¹H NMR spectroscopy to investigate proton-transfer dynamics in choline-amino acid protic ionic liquids. We show that proton migration occurs across a continuum of transient solvation motifs, including Eigen-like, Zundel-like, and intermediate configurations, with intermediate states being predominant. In contrast to aqueous systems, where Eigen and Zundel motifs are often comparably populated, these ionic liquids exhibit a markedly higher population of Zundel-like proton-sharing dimers than Eigen-like monomers, revealing a unique excess-proton solvation landscape in nonaqueous ionic environments. Free-energy analyses establish that molecular flexibility and steric hindrance cooperatively govern proton-transfer barriers, thereby explaining the pronounced structure-property relationships across different amino acid anions. Elevated temperature promotes proton exchange by accelerating hydrogen-bond-network reorganization, whereas nuclear quantum effects reduce the effective free-energy barrier through proton delocalization and zero-point motion, facilitating room-temperature proton transfer. 1H NMR measurements provide qualitative experimental evidence for rapid exchangeable-proton dynamics, consistent with the simulated local proton-exchange trends. Together, these results establish a molecular framework linking local proton solvation, ionic-liquid architecture, and proton-exchange dynamics, extending the concept of Grotthuss-like proton relay beyond water and offering design principles for high-performance proton-conducting ionic liquids.