Radical Diffusion Crossover Phenomenon in Glass-Forming Liquids.

We studied the diffusivities of a nitroxide radical at various temperatures in six glass-forming molecular liquids by electron spin resonance. By comparing the radical diffusivities and solvent self-diffusivities, we found that the radical diffusivities are lower than the self-diffusivities at high temperatures and approach them at low temperatures in all liquids. This crossover behavior was considered as evidence that a single-molecule diffusion process transforms into a collective process with temperature lowering.

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.