Similarities and Differences in the Effect of Dipole–Dipole and Exchange Interactions on the Shape of the EPR Spectrum of Dilute Solutions of Paramagnetic Particles

Spin exchange caused by the exchange interaction during bimolecular collisions of paramagnetic particles in dilute solutions causes several effects: broadens the resonance lines of the EPR spectrum, changes the resonance frequencies, changes the shape of the resonance lines of the spectrum, and causes the effect of the exchange narrowing of the spectrum. The well-established belief is that the dipole–dipole interaction between paramagnetic particles only broadens the resonance lines. According to the new paradigm of spin exchange, the dipole–dipole interaction causes effects similar to the effects of spin exchange. In this article, a detailed quantitative analysis of the effect of the dipole–dipole interaction on the shape of the EPR spectra of dilute solutions of paramagnetic particles is carried out for the model system. It is shown that the contribution of the dipole–dipole interaction to the spin coherence transfer between particles makes it possible to more accurately determine the rate of spin exchange and, as a result, the rate of bimolecular collisions of molecules from the analysis of the shape of the EPR spectra. An experimental protocol is proposed that definitely highlights the contribution of the dipole–dipole interaction to the transfer of spin coherence.