Pulse Radiolysis Studies of Reaction Kinetics in Ionic Liquids

摘要

Reaction rate constants were determined by pulse radiolysis in the ionic liquids 1-butyl-3-methylimidazoliumhexafluorophos-phate and tetrafluoroborate, N-butylpyridinium tetrafluoro-borate, and methyltributylammonium bis(trifluoromethyl-sulfonyl)imide (R_4NNTf_2). Oxidation of chlorpromazine and Trolox by CCl_3O_2~·radicals was studied in all the ionic liquids and oxidation of chlorpromazine by Br_2~·~- radicals was studied in R_4NNTf_2. Reduction of quinones and other compounds was studied both in R_4NNTf_2 and in the pyridinium ionic liquids. The ionic liquids behave as solvent with much lower polarity than water and also inhibit reactions due to their high viscosity. However, electron transfer from the N-butylpyridi-nyl radical to various acceptors is more rapid than the diffusion-controlled limit, suggesting an electron hopping mechanism. Electron transfer between methyl viologen and quinones takes place several orders of magnitude more slowly in this ionic liquid than in water or 2-PrOH and the direction of the electron transfer is solvent dependent. In contrast, addition and abstraction reactions of ~·*CF_3 radicals in R_4NNTf_2 are only slightly slower than those in water and acetonitrile.