Heisenberg Spin Exchange Between Nitroxide Probes Diffusing in a Percolation Network

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Abstract
Heisenberg spin exchange (HSE) between nitroxide (Tempone) spin probes has been measured as a function of concentration in the aqueous phase of the hydrated ion exchange membrane Nafion 117. The observed fast-motional electron paramagnetic resonance spectra were analyzed in terms of the perturbation expressions given by Bales and Peric based on the earlier monograph of Molin et al., as well as by the full stochastic Liouville equation lineshape calculation of Freed and coworkers. Differences between the methods for determining HSE from the spectrum are presented and discussed. In acidic Nafion membranes, the spin probes are deactivated over time, allowing simultaneous measurement of the decay kinetics and HSE as a function of paramagnetic probe concentration. Both processes deviate from the behavior that would be expected based on classical diffusion and chemical kinetics in isotropic media. The results are discussed in terms of currently available models for diffusion and reaction in a percolation network.

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