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, omega(ex), larger than the N-15 hyperfine separation, A(0), predicted 50 years ago but previously not observed. For spectra with omega(ex) /gamma A(0) < 1, where gamma 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 omega(ex). In stark contrast, when omega(ex)/gamma A(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 = omega(ex)/C, where C is the molar concentration, is continuous through omega(ex)/gamma A(0) = 1.