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 form ation of several species with similar absorption spectra. To identify and c haracterize these species, experiments were carried out with BMI+ in soluti on under various conditions. BMI+ is reduced by e(aq)(-) very rapidly (k = 1.9 x 10(10) L mol(-1) s(-1)) but not by (CH3)(2)CO- radicals (k less than or equal to 1 x 10(5) L mol(-1) s(-1)). The reduced species exhibits an abs orption peak at 323 nm (is an element of = 5.9 x 10(3) L mol(-1) cm(-1)). I t decays via radical-radical reactions but does not react with O-2 or with methyl viologen. BMI+ reacts rapidly also with OH radicals (k = 3.7 x. 10(9 ) L mol(-1) s(-1)) to produce a mixture of isomeric OH(.)adducts. The spect rum of these adducts is similar to that of the reduced species, it exhibits a peak at 322 nm (is an element of = 2.5 x 10(3) L mol(-1) cm(-1)) but wit h a pronounced shoulder at 380 to 420 nm. The adducts. react with O-2 and w ith OH-. In the latter reaction they lose H+ to form a hydroxylated analogu e of the electron adduct. Irradiation of liquid BMI+ salts produces BMI., B MI.2+, and various radical adducts. In the presence of CCl4 and O-2, CCl3O2 . radicals are formed and these radicals oxidize Trolox and chlorpromazine with rate constants of (1 to 3) x 10(7) L mol(-1) s(-1). Even after correct ing for the effect of viscosity, the rate constants are much lower than tho se determined in aqueous solutions and close to those measured in low-polar ity organic solvents. The low rate constants in BMIPF6 and BMIBF4 probably indicate a high degree of ion-association in these ionic liquids. On the ot her hand, the activation energy of the reaction is close to that measured i n aqueous solutions but higher than that in alcohols. This is discussed. in terms of highly structured solvent/solute domains.