Quantitative Observation of Slow and Fast Exchange in EPR Spectra of Organic Free Radicals

We have measured effects of electron exchange on the EPR spectrum of the organic free radicals DTBN and DNB/DNB·− in both the low-concentration and high-concentration regions. At low concentrations exchange broadens the individual hyperfine lines while at high concentrations exchange narrows the single, merged line. Existing theories are used to calculate the rate constant from both the low-concentration and high-concentration data, and consistent results are obtained. Such quantitative agreement has not been previously reported, to our knowledge. In the region of very high concentration, however, intermolecular dipole—dipole interactions contribute to the line breadth making it more difficult to determine the rate constant.