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γ‑Radiolysis of Room-Temperature Ionic Liquids: An EPR Spin-Trapping Study

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journal contribution
posted on 10.12.2019, 19:49 by Peter Tarábek, Alexandra Lisovskaya, David M. Bartels
The radiolytic stability of a series of room-temperature ionic liquids (ILs) composed of bis­(trifluoromethylsulfonyl)­imide anion (Tf2N) and triethylammonium, 1-butyl-1-methylpyrrolidinium, trihexyl­(tetradecyl)­phosphonium, 1-hexyl-3-methylpyridinium, and 1-hexyl-3-methylimidazolium (hmim) cations was studied using spin-trap electron paramagnetic resonance (EPR) spectroscopy with a spin-trap α-(4-pyridyl N-oxide)-N-tert-butylnitrone (POBN). The trapped radical yields were measured as a function of POBN concentration and as a function of radiation dose by double integration of the broad unresolved lines. Well-resolved motionally narrowed EPR spectra for the trapped radicals were obtained by dilution of the ILs with CH2Cl2 after irradiation. The trapped radicals were identified as mainly carbon-centered alkyl and CF3, and their ratio varies greatly across the series of ILs. Expected nitrogen-centered radicals derived from Tf2N were not observed. The hmim liquid proved most interesting because a large part of the trapped radical yield (entirely carbon-centered) grew in over several hours after irradiation. We also discovered a complicated narrow-line stable radical signal in this neat IL with no spin trap added, which grows in over several hours after irradiation and decays over several weeks.