Measuring radical diffusion in viscous liquids by electron paramagnetic resonance

Abstract Relative diffusion of free radicals in solution modulates the Heisenberg spin exchange and dipole-dipole interactions among them, which affects their electron paramagnetic resonance (EPR) spectra. The radical concentration dependence of EPR parameters can, in turn, give information about radical diffusivity in a liquid. We studied the diffusivities of the 14 N- and 15 N-labeled perdeuterated TEMPONE radicals at various temperatures in three viscous liquids: 1‑ethyl‑3‑methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid, propylene carbonate, and ethylene glycol. By fitting EPR spectra at various radical concentrations, we obtained the concentration coefficients of EPR parameters. The concentration coefficients were related to the radical diffusivity by solving the kinetic equations for the spin evolution of a radical pair, considering the radicals as continuously diffusing spherical objects in the hard-core pair potential. We tested the method by comparing the calculated radical diffusivities of isotopically substituted TEMPONE radicals. Temperature dependences of radical diffusivities were discussed in terms of the Stokes-Einstein relation. Additionally, the radical diffusivities were compared to the self-diffusivities of the studied liquids. At lower temperatures, the radical diffusivities follow the self-diffusivities, while at the higher temperatures, the radical diffusivities start deviating from the self-diffusivities.