EPR Line Shifts and Line Shape Changes Due to Spin Exchange Between Nitroxide Free Radicals in Liquids 10. Spin-Exchange Frequencies of the Order of the Nitrogen Hyperfine Interaction: A Hypothesis

The behavior of Electron paramagnetic resonance spectra due to 15N and 14N nitroxide free radicals undergoing spin exchange in liquids at high frequencies ωex , of the same order of magnitude as the nitrogen hyperfine coupling constant A0 is investigated. The well known features are reconfirmed: (1) at low values of ωex where the lines broaden, shift toward the center of the spectrum, and change shape due to the introduction of a resonance of the form of a dispersion component; (2) at values of ωex comparable to A0, the line merge into one; and (3) at values much larger than A0, the merged line narrows. It is found that each line of a spectrum may be decomposed into an admixture of a single absorption and a single dispersion component of Lorentzian shape. These two- or three-line absorption-dispersion admixtures, for 15N and 14N, respectively, retain their individual identities even after the spectrum has merged and has begun to narrow. For both isotopes, the average broadening and integrated intensities are equal to the predictions of perturbation theory although, in the case of 14N the outer lines broaden faster than the central line and intensity moves from the outer lines to the center line. In fact, the outer line intensity becomes zero and then negative at higher values of ωex which is compensated by the center line becoming more intense than the overall integrated intensity. For both isotopes, the dispersion components and the line shift depart from the perturbation prediction. The results are presented in terms of measurable quantities normalized to A0 so that they may be applied to any two- or three-line spectrum.