Diffusion-Controlled Electron-Transfer Reactions in Ionic Liquids

Abstract

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 (BuPyBF 4 ) and the following tetraalkylammonium (R 4 N + ) salts of bis-(trifluoromethylsulfonyl)imide ( -NTf 2 ): methyltributylammonium (MeBu 3 NNTf 2 ), 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.

Influence of Chiral Ionic Liquids on the Excited-State Properties of Naproxen Analogs

“…This was attributed to the difference between microviscosity and bulk viscosity in ILs, which was originally suggested by Skrzypczak and Neta. 28 Third, no exciplex emission was observed in any of the ILs studied, which is striking given that the DMA-pyrene system is well-known for exciplex formation in conventional bulk solvents. The lack of exciplex formation led the authors to conclude that it is the microscopic and not the bulk polarity which could be related to the E T (30) scale that determines the formation of exciplex in ILs.…”

Bimolecular Photoinduced Electron Transfer in Imidazolium-Based Room-Temperature Ionic Liquids Is Not Faster than in Conventional Solvents

“…13−18 Indeed, the ET reactions in conventional solvents were observed to be diffusion controlled, while quenching rate constants up to 2 orders of magnitude larger than diffusion were found in RTILs. These extraordinary findings have been given different explanations, ranging from accelerated diffusion 13,14 and ET occurring in the alkyl chain regions of the RTIL, 15 to a tentative statement on too simplified a data analysis. 9,12 The interpretations of the results obtained in RTILs are very often based on the assumption that the measured rate constant is simply equal to the diffusion rate constant and should therefore be inversely proportional to the solvent viscosity, which is indeed very high for RTILs.…”

A Kinetic Study of the Reaction between N,N‐Dimethyl‐p‐toluidine and its Electrogenerated Radical Cation in a Room Temperature Ionic Liquid

“…For the fastest reactions, one might predict with reasonable confidence a diffusion-controlled rate curtailed in proportion to the greater viscosity ionic liquids are known to possess, although work by Neta et al cautions against the over-interpretation of classical formulae commonly applied to molecular solvents for estimating such rates. [10] Activated processes, however, could be either driven or impeded depending on specific solvation effects, and while the dimerisation of benzaldehyde radical anions and the reaction of 1,2-dimethylimidazole with benzyl bromide in RTILs display rate constants comparable to those in polar, aprotic solvents, [9,11] Hapiot has reported both a significant decrease and increase in the rates of various electrodimerisation reactions, rationalised in each case using ion-association arguments. [5,7] In all these examples, however, the rate constants did not differ by more than one order of magnitude from the equivalent value in acetonitrile, where reported.…”