Knowledge
of the ion motion in room temperature ionic liquids (RTILs)
is critical for their applications in a number of fields, from lithium
batteries to dye-sensitized solar cells. Experiments on a limited
number of RTILs have shown that on macroscopic time scales the ions
typically undergo conventional, Gaussian diffusion. On shorter time
scales, however, non-Gaussian behavior has been observed, similar
to supercooled fluids, concentrated colloidal suspensions, and more
complex systems. Here we characterize the diffusive motion of ionic
liquids based on the <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium
(PYR<sub>14</sub>) cation and bis(trifluoro methanesulfonyl)imide
(TFSI) or bis(fluorosulfonyl)imide (FSI) anions. A combination of
pulsed gradient spin–echo (PGSE) NMR experiments and molecular
dynamics (MD) simulations demonstrates a crossover from subdiffusive
behavior to conventional Gaussian diffusion at ∼10 ns. The
deconvolution of molecular displacements into a continuous spectrum
of diffusivities shows that the short-time behavior is related to
the effects of molecular caging. For PYR<sub>14</sub>FSI, we identify
the change of short-range ion–counterion associations as one
possible mechanism triggering long-range displacements
