Stability of Superoxide Ion in Imidazolium Cation-Based Room-Temperature Ionic Liquids

The stability of superoxide ion (O-2(center dot-)) generated chemically by dissolving KO2 in dried dimethyl sulfoxide solutions containing imidazolium cation [e.g., 1-ethyl-3-methylimidazolium (EMI+) and 1-n-butyl-2,3-dimethylimidazolium (BMMI+)] based ionic liquids (ILs) was investigated with UV-visible spectroscopic. NMR, and voltammetric techniques and an ab initio molecular orbital calculation. UV-visible spectroscopic and cyclic voltammetric measurements reveal that the O-2(center dot-) species reacts with BMMI+ and EMI+ cations of ILs to form hydrogen peroxide. The pseudo first order rate constant for the reaction of BMMI+ and O-2(center dot-) species was found to be about 2.5 x 10(-3) s(-1). With a molecular orbital calculation, the O-2(center dot-) species is understood to attack the 2-position (C-2) of the imidazolium ring (i.e., BMMI+) to form an ion pair complex in which one oxygen atom is bounded to C-2 and the other to the hydrogen atom of -CH3 group attached to C-2. Eventually, the ion pair complex of BMMI+ cation and O-2(center dot-) species undergoes a ring opening reaction as evidenced with H-1 NMR measurement.