Diffusion-controlled electron-transfer reactions in ionic liquids

The reactions of pyridinyl and alkylpyridinyl radicals with duroquinone involve an electron transfer to produce the durosemiquinone radical in a diffusion-controlled process. The rate constants for such reactions in several ionic liquids and molecular organic solvents are determined by pulse radiolysis. The ionic liquids used are N-butylpyridinium tetrafluoroborate (BuPyBF4) and the following tetraalkylammonium (R4N+) salts of his(trifluoromethylsulfonyl)imide (-NTf2): methyltributylammonium (MeBu3NNTf2), hexyltributylammonium (HxBu(3)NNTf(2)), methyltrioctylammonium (MeOc(3)NNTf(2)), and methylbutylpyrrolidinium (MeBuPyrNTf(2)). The molecular solvents used are triethanolamine (TEOA) and cyclohexanol (c-HxOH). The experimental rate constants in the molecular solvents are only slightly higher than the diffusion-controlled rate constants estimated from the viscosity. On the other hand, the experimental values in the ionic liquids are about an order of magnitude higher than the values estimated from the measured viscosity. The reason for this difference is suggested to be due to the voids that exist in ionic liquids and the possibility that diffusion of reacting species takes place through movement of segments of the ions while the viscosity is related to movement of the whole ions.