  In this paper we investigate a novel set of polarizing agents --
mixed-valence compounds -- by theoretical and experimental methods and
demonstrate their performance in high-field Dynamic Nuclear Polarization (DNP)
experiments in the solid state. Mixed-valence compounds constitute a group of
molecules, in which molecular mobility persists even in solids. Consequently,
such polarizing agents can be used to perform Overhauser-DNP experiments in
solid-state, with favorable conditions for dynamic nuclear polarization
formation at ultra-high magnetic fields.
