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Influence of alkyl chain length on the diffusion of a nitroxide radical in imidazolium-based ionic liquids

Solutions of a nitroxide radical in imidazolium-based ionic liquids with different alkyl chain length were studied by electron paramagnetic resonance (EPR) spectroscopy. The radical was perdeuterated 2, 2, 6, 6-tetramethyl-4-oxopiperidine-1-oxyl (pDTEMPONE) and the ionic liquids were four members of the 1-alkyl-3-methylimidazolium tetrafluoroborate family ([Cnmim][BF4]) with ethyl (n=2), butyl (n=4), hexyl (n=6), and octyl (n=8) groups. The EPR spectra of solutions with various radical concentrations were recorded at temperatures between 0°C and 120°C. The EPR spectra were fitted to the spectral function obtained from the modified Bloch equations for the magnetization of radicals. The fitted values of EPR parameters that quantify spin dephasing, coherence transfer and hyperfine splitting were analyzed as functions of radical concentration at each measured temperature. Using linear fitting, we determined linear concentration coefficients of these EPR parameters. The concentration coefficients are affected by the relative motion of radicals, which modulates the Heisenberg spin exchange and dipole-dipole interactions between radicals. Assuming continuous diffusion of radicals, the diffusion coefficients of pDTEMPONE were calculated from the concentration coefficients by numerical solving of the kinetic equations for the spin evolution of the interacting radical pair [1]. Upon lowering the temperature, the calculated diffusion coefficients of pDTEMPONE in all ionic liquids decrease slower than is predicted by the Stokes-Einstein relation and slower than its rotational diffusion coefficient [2]. Such behavior of tracer diffusion was detected in supercooled liquids and ascribed to the presence of dynamical heterogeneities. Radical diffusion was compared to self-diffusion of ions in these ionic liquids measured by NMR [3]. We found that the diffusion coefficient of the radical and ions are similar for short alkyl length (n=2) and that the diffusion coefficient of the radical decreases much slower than that of ions with increasing alkyl chain length. This relative enhancement of radical diffusion with the lengthening of alkyl chain might be related to the aggregation of the alkyl chains with n≥4 in non-polar domains [4]. The fast diffusion of a neutral tracer through these soft non-polar domains [5] can increase its effective diffusion coefficient relative to that of original ions. [1] K.M. Salikhov, et al., Appl. Magn. Reson. 45, 911 (2014). [2] D. Merunka, M. Peric, and M. Peric, J. Phys. Chem. B 119, 3185 (2015). [3] K. R. Harris, M. Kanakubo, N. Tsuchihashi, K. Ibuki, and M. Ueno, J. Phys. Chem. B 112, 9830 (2008) ; H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan, and M. Watanabe, J. Phys. Chem. B 108, 16593 (2004) ; A. Noda, K. Hayamizu, and M. Watanabe, J. Phys. Chem. B 105, 4603 (2001). [4] A. Triolo, O. Russina, H. J. Bleif, and E. Di Cola, J. Phys. Chem. B 111, 4641 (2007). [5] J. C. Araque, S. K. Yadav, M. Shadeck, M. Maroncelli, and C. J. Margulis, J. Phys. Chem. B 119, 7015 (2015).

electron paramagnetic resonance ; nitroxide radical ; ionic liquids ; diffusion

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