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

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.