Anne Kaintz - Bimolecular Electron Transfer in Ionic Liquids: Are Reaction Rates Anomalously High?

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      Publication Details (including relevant citation   information):

      Min   Liang, Anne Kaintz, Gary A. Baker, and Mark Maroncelli,  J.   Phys. Chem. B,   2012, 116, 1370-1384.



      Steady-state and picosecond time-resolved emission spectroscopy   are used to monitor the bimolecular electron transfer reaction   between the electron acceptor 9,10-dicyanoanthracene in its   S1 state and the donor   N,N-dimethylaniline in a variety of ionic   liquids and several conventional solvents. Detailed study of this   quenching reaction was undertaken in order to better understand   why rates reported for similar diffusion-limited reactions in   ionic liquids sometimes appear much higher than expected given   the viscous nature of these liquids. Consistent with previous   studies, Stern–Volmer analyses of steady-state and lifetime data   provide effective quenching rate constants   kq, which are often 10–100-fold larger than   simple predictions for diffusion-limited rate constants   kD in ionic liquids. Similar departures from   kD are also observed in conventional organic   solvents having comparably high viscosities, indicating that this   behavior is not unique to ionic liquids. A more complete analysis   of the quenching data using a model combining approximate   solution of the spherically symmetric diffusion equation with a   Marcus-type description of electron transfer reveals the reasons   for frequent observation of kq   kD. The primary cause is that the high   viscosities typical of ionic liquids emphasize the transient   component of diffusion-limited reactions, which renders the   interpretation of rate constants derived from Stern–Volmer   analyses ambiguous. Using a more appropriate description of the   quenching process enables satisfactory fits of data in both ionic   liquid and conventional solvents using a single set of physically   reasonable electron transfer parameters. Doing so requires   diffusion coefficients in ionic liquids to exceed hydrodynamic   predictions by significant factors, typically in the range of   3–10. Direct, NMR measurements of solute diffusion confirm this   enhanced diffusion in ionic liquids.

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