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

Abstract

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 EPR spectroscopy with a spin trap α-(4-Pyridyl-1-oxide)-N-tert-butylnitrone (POBN). Total 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 ionic liquid with no spin trap added, which grows in over several hours after irradiation, and decays over several weeks.