Reorientation dynamics and ion diffusivity of neat dimethylimidazolium dimethylphosphate probed by NMR spectroscopy

NMR spectroscopy at two magnetic field strengths was employed to investigate the dynamics of dimethylimidazolium dimethylphosphate ([C₁C₁IM][(CH₃)₂PO₄]). [C₁C₁IM][(CH₃)₂PO₄] is a low-melting, halogen-free ionic liquid comprising of only methyl groups. ¹³C spin-lattice relaxation rates as well as self-diffusion coefficients were measured for [C₁C₁IM][(CH₃)₂PO₄] as a function of temperature. The rotational correlation times, τ _c, for the cation and the anion were obtained from the ¹³C spin-lattice relaxation rates. Although from a theoretical point of view cations and anions are similar in size, they show different reorientation mobilities and diffusivities. The self-diffusion coefficients and the rotational correlation times were related to the radii of the diffusing spheres. The analysis reveals that the radii of the cation and the anion, respectively, are different from each other but constant at temperatures ranging from 293 to 353 K. The experimental results are rationalised by a discrete and individual cation and anion diffusion. The [(CH₃)₂PO₄]^- anion reorients faster compared to the cation but diffuses significantly slower indicating the formation of anionic aggregates. Relaxation data were acquired with standard liquid and magic-angle-spinning NMR probes to estimate residual dipolar interactions, chemical shift anisotropy or differences in magnetic susceptibility within the sample.