Measuring radical diffusion in viscous liquids by electron paramagnetic resonance
Measuring radical diffusion in viscous liquids by electron paramagnetic resonance
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DOI：
10.1016/j.molliq.2019.01.006
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发表时间：
2019-03
影响因子：
6
通讯作者：
D. Merunka;M. Peric
中科院分区：
文献类型：
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作者：
D. Merunka;M. Peric
自由基在溶液中的相对扩散调制了自由基之间的海森堡自旋交换和偶极-偶极相互作用，从而影响它们的电子顺磁共振（EPR）谱。EPR参数的自由基浓度依赖性，反过来，给出了自由基在液体中的扩散系数的信息。我们研究了14 N-和15 N-标记的全氘代TEMPONE自由基在三种粘性液体中的扩散系数：1-乙基-3-甲基咪唑双（三氟甲基磺酰基）酰亚胺离子液体、碳酸丙烯酯和乙二醇。通过拟合不同自由基浓度下的EPR谱，得到了EPR参数的浓度系数。通过求解自由基对自旋演化的动力学方程，将自由基视为在硬核对势中连续扩散的球形物体，得到了浓度系数与自由基扩散系数之间的关系.我们通过比较同位素取代TEMPONE自由基的自由基扩散系数来测试该方法。自由基扩散系数的温度依赖性进行了讨论的斯托克斯-爱因斯坦关系。此外，自由基扩散率进行了比较，所研究的液体的自扩散率。在较低的温度下，自由基扩散率遵循自扩散率，而在较高的温度下，自由基扩散率开始偏离自扩散率。
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 the14N- and15N-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.