Proton Dynamics and Transport in a Protic Ionic Liquid Revealed by Multinuclear Solid-State NMR

Variable-temperature solid-state NMR (1H, 13C, 19F, and 15N), thermogravimetric, and electrical conductivity studies were employed to investigate molecular dynamics and proton transport in protonated 1-methylimidazolium triflate ([HMIM][Tf]). Two distinct dynamic processes were identified: the reorientation of the imidazolium ring (∼103 s-1) and a much faster proton exchange within the hydrogen-bond network. Temperature-dependent spectral changes indicate dynamic averaging of cationic and anionic environments in the liquid phase. Conductivity measurements revealed two distinct transport regimes, separated by a transition at approximately 88 °C. In the liquid phase, lower activation energy (24.5 kJ mol-1) indicates efficient proton transport, while conductivity decreases in the plastic crystalline phase due to reduced mobility. The NMR results indicate that proton exchange occurs on a substantially faster time scale than the reorientational ion dynamics of the imidazolium cation, providing evidence for a Grotthuss-type proton transport mechanism operating beside the vehicular transport mechanism characteristic of ionic liquids.