Reaction kinetics in ionic liquids: Pulse radiolysis studies of 1-butyl-3-methylimidazolium salts

Radiolysis of liquid 1-butyl-3-methylimidazolium (BMI⁺) salts leads to formation of several species with similar absorption spectra. To identify and characterize these species, experiments were carried out with BMI⁺ in solution under various conditions. BMI⁺ is reduced by e_aq^- very rapidly (k = 1.9 × 10¹⁰ L mol^-1 s^-1) but not by (CH₃)₂CO^- radicals (k ≤ 1 × 10⁵ L mol^-1 s^-1). The reduced species exhibits an absorption peak at 323 nm (ε = 5.9 × 10³ L mol^-1 cm^-1). It decays via radical-radical reactions but does not react with O₂ or with methyl viologen. BMI⁺ reacts rapidly also with OH radicals (k = 3.7 × 10⁹ L mol^-1 s^-1) to produce a mixture of isomeric OH-adducts. The spectrum of these adducts is similar to that of the reduced species, it exhibits a peak at 322 nm (ε = 2.5 × 10³ L mol^-1 cm^-1) but with a pronounced shoulder at 380 to 420 nm. The adducts react with O₂ and with OH^-. In the latter reaction they lose H⁺ to form a hydroxylated analogue of the electron adduct. Irradiation of liquid BMI⁺ salts produces BMI^., BMI^.2+, and various radical adducts. In the presence of CCl₄ and O₂, CCl₃O₂^. radicals are formed and these radicals oxidize Trolox and chlorpromazine with rate constants of (1 to 3) × 10⁷ L mol^-1 s^-1. Even after correcting for the effect of viscosity, the rate constants are much lower than those determined in aqueous solutions and close to those measured in low-polarity organic solvents. The low rate constants in BMIPF₆ and BMIBF₄ probably indicate a high degree of ion-association in these ionic liquids. On the other hand, the activation energy of the reaction is close to that measured in aqueous solutions but higher than that in alcohols. This is discussed in terms of highly structured solvent/solute domains.