High-Pressure ESR Spectroscopy: On the Rotational Motion of Spin Probes in Pressurized Ionic Liquids

[Image: see text] We report high-pressure (up to 50 MPa) ESR-spectroscopic investigations on the rotational correlation times of the nitroxide radicals 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPOL), and 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl (ATEMPO) in the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF(4)), 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF(6)), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)), 1-methyl-3-octylimidazolium tetrafluoroborate (omimBF(4)), and 1-methyl-3-octylimidazolium hexafluorophosphate (omimPF(6)). The activation volumes (38.5–56.6 Å(3)) determined from pressure dependent rotational diffusion coefficients agree well with the pressure dependent viscosities of the ionic liquids. Experimentally, the fractional exponent of the generalized Stokes–Einstein–Debye relation is found to be close to one.