Detailed Analysis of Manifestations of the Spin Coherence Transfer in EPR Spectra of 14N Nitroxide Free Radicals in Non-Viscous Liquids

Abstract

Manifestations of the spin coherence transfer induced by the exchange and dipole–dipole interactions between spin probes in dilute solutions in the electron paramagnetic resonance (EPR) spectra have been studied. The perturbation theory of manifestations of the spin coherence transfer in the EPR spectra of nitroxide free radicals elaborated by one of the authors Salikhov (Applied Magnetic Resonance 38:237–256, 2010) has been generalized with allowance for the super hyperfine structure of the EPR spectra. For ¹⁴N nitroxide radicals, the total EPR spectrum was presented as a sum of three independent components in the case of slow and intermediate spin coherence transfer rates. The shapes of these components were found. The side components of the EPR spectrum contain the absorption and dispersion contributions and, as a result, have the asymmetric (mixed) shapes. These asymmetric components can be presented as J = J_absorption ± pJ_dispersion. The p value is found for the arbitrary super hyperfine structure of the spectrum. In the slow and intermediate spin coherence transfer rate regime, the parameter p is independent of the super hyperfine interactions in the nitroxide radicals, but the shapes of J_absorption and J_dispersion terms depend on the super hyperfine structure of the nitrogen components of the nitroxide EPR spectrum and on the spin coherence transfer rate. It is confirmed theoretically that a good strategy to evaluate the spin coherence transfer rate from the EPR spectra is using the dispersion contribution to the shape of the EPR spectra of nitroxide free radicals. An algorithm is suggested and tested for determining the spin coherence transfer and spin decoherence rates.