Experimental Observation of a Peculiar Effect in Electron Paramagnetic Resonance Spectra for Nitroxides Undergoing Fast Spin Exchange: Emission.

We report the experimental observation of an EPR spectral manifestation of spin exchange frequencies, ω ex , larger than the 15 N hyperfine separation, A 0 , predicted 50 years ago but previously not observed. For spectra with ω ex / γA 0 < 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 abs < A 0 . A abs decreases with ω ex . In stark contrast, when ω ex / γA 0 > 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 ex = ω ex / C , where C is the molar concentration, is continuous through ω ex / γA 0 = 1.