Measuring radical diffusion in viscous liquids by electron paramagnetic resonance

Abstract

Relative diffusion of free radicals in solution modulates the Heisenberg spin exchange and dipoledipole 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.

Molecular Dynamics in Ionic Liquid/Radical Systems

“…These values of C slip are in good agreement with literature data for nitroxides ( C slip = 0.1–0.2) and trityl radicals ( C slip = 0.65) . The rotation correlation times for nitroxides (70 and 230 ps for TEMPO and 4-benzoyloxy-TEMPO, respectively) are in the range of values reported in the literature in similar systems. ,,− Thus, parameters obtained from EPR measurements, such as rotational correlation times, constants of anisotropy hyperfine interaction, and g -factors (see SI, Table S1) are used for further fitting of NMRD data. Moreover, EPR line-shape parameters are necessary to fit the DNP spectra in order to quantify the DNP spectra and distinguish between OE and SE contributions, which further allows the calculation of the coupling factor.…”

“…In order to observe CE, the presence of two coupled electron spins is required. The possibility that this coupling may occur is supported by evidence of mesostructure formation in ILs, 5,59,71 as well as radical pairing and nonaveraged dipolar interactions. 73 The residual (incompletely averaged) dipolar coupling between electron spins, as well as between electron and nuclear spins, provides the conditions for hyperpolarization via CE, which is possibly reflected in a small contribution in the observed DNP enhancement at the studied conditions.…”

An analysis of radical diffusion in ionic liquids in terms of free volume theory

“…The EPR spectrum of 14 N-pDTEMPONE has three EPR lines because the nitrogen nucleus 14 N has the spin I = 1. The original spectral function for the absorption EPR spectra of 14 N-labeled radicals interacting by HSE and DD interactions can be calculated from the modified Bloch equations, 28,29,31 and it is given by…”

“…Finally, we have recently been able to successfully apply the HSE-DD separation EPR method to measure the diffusion coefficients of the 14 N-and 15 Nlabeled perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-Oxyl ( 14 N-pDTEMPONE and 15 N-pDTEMPONE) radicals in 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (Emim TFSI) ionic liquid, glass-forming liquid propylene carbonate, and hydrogen-bonding liquid ethylene glycol. 31 The translational diffusion of 14 N-pDTEMPONE in those liquids was satisfactorily explained by the fractional Stokes-Einstein relation. The diffusion coefficient values of the radicals were also found to approach the values of the self-diffusion coefficients of the liquids at lower temperatures, while at the higher temperatures, the values of the diffusion coefficient of the radicals were smaller than the values of the self-diffusion coefficients in all three liquids.…”

Rotation of a Charged Spin Probe in Room-Temperature Ionic Liquids

“…31 We also studied the translational diffusion of the 14 N-and 15 N-labeled pDTO at various temperatures in 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide (EmimTFSI) ionic liquid. 29 The temperature dependence of the translation diffusion coefficient of pDTO in EmimTFSI was well explained by the fractional power-law modification of Stokes−Einstein law. In another study, 30 the Heisenberg spin exchange dipole−dipole separation method 36 was used to measure the translational diffusion coefficients of pDTO as a function of temperature in two series of imidazolium RTILs, one based on the BF 4 anion and the other based on the bis(trifluoromethylsulfonyl)imide (TFSI) anion.…”