The rotational and translational diffusion of negatively charged and uncharged spin probes in five imidazolium-based room-temperature ionic liquids (RTILs), 1-ethyl-3methylimidazolium tetrafluoroborate, emimBF 4 , 1-butyl-3-methylimidazolium tetrafluoroborate, bmimBF 4 , 1-octyl-3-methylimidazolium tetrafluoroborate, omimBF 4 , 1-octyl-3-methylimidazolium hexafluorophosphate, omimPF 6 , and 1-octyl-3-methylimidazolium chloride, omimCl, has been studied by means of electron paramagnetic resonance spectroscopy. Detailed analyses of the spin-Hamiltonian parameters and spin exchange interactions have been carried out. The temperature dependences of the line broadening induced by the electronic dipole−dipole interaction and the electron spin exchange coupling are determined. The translational mobility of spin probes is semiquantitatively characterized and successfully explained in the framework of a hypothesis based on the assumption of polar and unpolar domains within the RTILs.

Temperature dependences of electron paramagnetic resonance (EPR) spectra of an imidazoline nitroxide biradical spin probe in a series of room-temperature ionic liquids in the temperature range 124–390 K have been quantitatively simulated. The unusual asymmetric EPR spectrum shape previously observed in these systems [Kokorin et al., Appl. Magn. Res. 48 (2016) 287] is shown to originate from anisotropic rotational diffusion of the probe molecule. All experimental spectra were quantitatively reproduced in simulation using a unified set of geometrical and magnetic parameters of the spin probe, which were found to be fully consistent with the biradical geometry obtained from density functional theory calculations. Temperature dependences of rotation diffusion coefficient of the probe characterize the molecular mobility of the ionic liquid, whereas the temperature dependences of the spin-exchange integral J and of the isotropic hyperfine interaction constant, a N, are shown to reflect the intramolecular conformation motions of the biradical probe.

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.