EPR investigation of the intermediate spin exchange regime

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Abstract
The electron paramagnetic resonance spectra of di-tert butylnitroxide (DTBN) are studied as a function of the nitroxide concentration in three hydrocarbon solvents at constant temperature. In the concentration range studied, DTBN undergoes electron spin exchange which leads to broadening, shifting, and eventual collapse of the lines forming the 19-line proton hyperfine pattern. Particular emphasis is given to the spin exchange frequencies ωHE in the region in which they are comparable with the proton hyperfine coupling constants—the intermediate spin exchange regime. Both linewidths and line shapes are found to be in full accord with theory in the intermediate exchange regime which completes the demonstration that theory and experiment are in agreement, the slow and fast exchange regimes having been studied many times in the past. Modeling the spectral lines as Lorentzian–Gaussian convolutions allows a detailed exposition of the collapse of the hyperfine structure as ωHE increases and this collapse, too, is well described by theory. Bimolecular spin exchange rate constants are compared with the prediction of simple hydrodynamic theory.