Rotation of a Charged Spin Probe in Room-Temperature Ionic Liquids

X-band electron paramagnetic resonance spectroscopy has been used to investigate the rotational diffusion of a stable, positively charged nitroxide 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (Cat-1) in a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons. The rotation of Cat-1 is anisotropic with the preferential axis of rotation along the NO^• moiety. The Stokes-Einstein-Debye law describes the mean rotational correlation time of Cat-1, assuming that the hydrodynamic radius is smaller than the van der Waals radius of the probe. This implies that the probe rotates freely, experiencing slip boundary condition, which is solvent-dependent. The rotational correlation time of Cat-1 in RTILs can very well be fitted to a power-law functionality with a singular temperature, which suggests that the apparent activation energy of rotation exhibits non-Arrhenius behavior. Compared to the rotation of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDTO), which is neutral, the rotation of Cat-1 is several times slower. The rotational anisotropy, the ratio of the rotational times of pDTO and Cat-1, and the apparent activation energy indicate the transition from a homogeneously globular structure to a spongelike structure when the alkyl chain has four carbons, which is also observed in molecular dynamics computational studies. For the first time, we have been able to show that the rotational correlation time of a solute molecule can be analyzed in terms of the Cohen-Turnbull free volume theory. The Cohen-Turnbull theory fully describes the rotation of Cat-1 in all ionic liquids in the measured temperature range.