Experimental Observation of a Peculiar Effect in Electron Paramagnetic Resonance Spectra for Nitroxides Undergoing...

We report the experimental observation of an EPR spectral manifestation of spin exchange frequencies, ω, larger than the ¹⁵N hyperfine separation, A₀, predicted 50 years ago but previously not observed. For spectra with ω/γA₀ < 1, where γ is the gyromagnetic ratio of the electron, the spectrum consists of two "normal" spin modes each with one absorption and one dispersion component separated by A < A₀. A decreases with ω. In stark contrast, when ω/γA₀ > 1, the spectrum consists of two absorption spin modes, one of which is negative (emissive). We show that the experimental behavior of the spin modes agrees with theory: (a) the doubly integrated intensity of the first-derivative spectra remains constant because the increased intensity of the positive spin mode minus the negative emissive mode remains constant; (b) the value of the spin exchange rate constant K = ω/C, where C is the molar concentration, is continuous through ω/γA₀ = 1.