Little-known Aspects of Overhauser DNP at Zero and Low Magnetic Fields Stimulated by Parallel Electron Pumping of Nitroxide Radicals Solutions

摘要

We present comprehensive investigation of Overhauser dynamic nuclear polarization in solutions of the nitroxide radical TANO at high-frequency saturation field of electron paramagnetic resonance transitions with its parallel orientation, related to the static magnetic field up to 0.1 mT. Experimental observation requires specific technique of cycle-polarized application, which is described in detail. It was shown, that these results cannot be interpreted in terms of standard theoretical methods, taking into account the electron magnetic relaxation as conditioned by hyperfine isotropic interaction of NO fragment. We suggest an original description of the effect, based on consideration of the anisotropic hyperfine interaction, modulated by rotational diffusion of the NO fragment. Thus developed theoretical approach enables to determine the combined contribution of isotropic and anisotropic interactions and investigate the rotational diffusion process, which can serve as fundamental basis for a new branch of studying regarding molecular diffusion of the spin labels.