Ionic Dynamics in [C4mim]NTf2 in the Glassy and Liquid States: Results from 13C and 1H NMR Spectroscopy

The ionic dynamics of the room temperature ionic liquid 1-butyl-3-methylimdiazolium bis((trifluoromethyl)sulfonyl)amide ([C(4)mim]NTf2) is studied using C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy over a wide temperature range encompassing the glassy and liquid states. The temperature dependence of the C-13 spin-lattice relaxation time is analyzed with four different models to derive the rotational dynamics of the RTIL in the nano to picosecond range. It was found that the extended model-free approach bridges the data obtained from the BPP and DC models, and describes ion dynamics of the RTIL well. Three different motions are observed based on the approach: an overall ion rotation, a slow and a fast local rotational motion. The time scale of the slow local rotational motion, particularly of the imidazolium ring carbons, is strongly coupled to the time scale of the overall ion rotation, above the melting point. Below the melting point these two time scales show strong decoupling and the local rotation displays nanosecond dynamics in the glassy state. The analyses of the second moment (M-2) of the H-1 and C-13 nuclides indicate that, in addition to the axial rotations of the two methyl groups (cation) and the CF3 group (anion), all C-13 sites including the imidazolium ring carbon and CF3 show evidence of mobility, even in the glassy state.