Intra- and inter-molecular interactions in choline-based ionic liquids studied by 1D and 2D NMR

Abstract

Choline acetate [Ch]+[OAc]− and choline trifluoroacetate [Ch]+[TFA]− mixed with 30 wt% H2O have been characterized by variable temperature 1D and 2D 1H/19F NMR. It was found that the cation, anion and water molecules participate in a strong local H-bond network, but there was no evidence for the formation of large clusters of ions. Weaker association effects were observed for the trifluoroacetate than for the acetate, originating from the higher anion acidity of TFA. Rotational correlation times, which were extracted by temperature dependent spin-lattice relaxation measurements, indicated that cations, anions and H2O exhibit collective reorientation even at room temperature. At the same time, no significant long-range correlations in the translational motion of the ions could be observed from PFG diffusion measurements. Moreover, 1D and 2D NOE experiments showed that both ions share close proximity to water molecules for a period on the order of 0.2 s at room temperature. It is deduced that the ions exist in a solvent-shared ion pair (SIP) configuration, with strongly correlated short-range dynamics between the choline cation and the H2O molecules, indicating a similar behavior of these ILs to a deep eutectic solvent with water as Lewis acid. A reason for this observation, which is supported by a high entropy term in the activation process particularly at low temperatures, could be a fairly static configuration with water near the –N(CH3)3 center of the choline, and relaxation caused by pseudorotation due to reordering of the H-bond network.