Pulse radiolysis studies of functionally substituted imidazolium-based ionic liquids

Abstract

The reactions of imidazolium-based ionic liquids having different substituent groups on the ring with hydrated electrons (e aq-), hydroxyl radicals ( •OH), and sulfate anion radicals (SO 4•-) were investigated using nanosecond pulse radiolysis techniques. The spectra of these ionic liquids on reaction with e aq- all exhibited a similar peak at about 320 nm, and a typical peak for e aq- in aqueous ionic liquid solutions. The reaction rate constants for 1,3-disubstituted imidazolium-based ionic liquid hexafluorophosphates (BMIPF 6) with e aq- were deduced to be 10 10 L mol -1 s -1, however the values were lower for trisubstituted ionic liquids. For example, the rate constant for 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BMMIBF 4) was 5.5×10 9 L mol -1 s -1. Imidazolium-based ionic liquids reacted with hydroxyl radicals via adducts to produce a mixture of isomeric OH adducts, and the pk a value of the OH adducts was deduced to be 8.4±0.4 for 1-butyl-3-methylimdazolium tetrafluoroborate (BMIBF 4). Moreover, imidazolium cations were also oxidized by SO 4•- to produce bivalent cation radicals, which exhibit a peak at 320 nm, and these rate constants are of the same order of magnitude, i. e., 10 9 L mol -1 s -1, except that for 1-(2-hydroxyethyl)-3-methylimdazolium tetrafluoroborate with SO 4•- (k=2.8×10 8 L mol -1 s -1). Theoretical calculations were carried out to estimate the structures of the products of reduction by e aq-1 and the results were related to the experimental data. © 2012 The Author(s).