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

Abstract

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.