Rotational and Translational Mobility of Nitroxide Spin Probes in Ionic Liquids and Molecular Solvents

Abstract

Rotational and translational movements of 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine (TEMPOL) spin probe in the room temperature ionic liquid (RTIL) 1-octyl-3-methylimidazolium tetrafluoroborate (omimBF4) and in two molecular solvents, 1-propanol and isopropyl benzene (cumene), have been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Rotational correlation times τ c of spin probes and the intermolecular spin exchange rate constants k e were measured from EPR spectra at different temperatures and TEMPOL concentrations, and compared with the published data. The τ c values were calculated both by known equations and from the EPR spectra simulation. Rotation movements of TEMPOL in omimBF4 cannot be described by the model of the isotropic Brownian diffusion, which is valid for conventional solvents. The correct modeling of EPR spectra in RTIL can be achieved with the assumption of different rotational mobility of the spin probe around different molecular axes. The rotational, D rot, and translational, D tr, diffusion coefficients were calculated from τ c and k e values. The Debye–Stokes–Einstein law is valid in all three solvents while the dependence of D tr on T/η is not linear in Stokes–Einstein coordinates. The effective activation energy E rot a of the rotational movements in omimBF4 is noticeably higher than the corresponding values for conventional solvents, while the effective activation energies E tr a of the translational movements are comparable in all solvents studied.