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 (TEM-PO), 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEM-POL), and 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl (ATEM-PO) 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 angstrom(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.