Different flavors of diffusion in paramagnetic systems: Unexpected NMR signal intensity and relaxation enhancements

The NMR community is well acquainted with different kinds of diffusion but, at the same time, there are several effects that are worth a better understanding for an improved design of molecular imaging and dynamic nuclear polarization experiments. Spin diffusion and chemical diffusion are known to play important roles in determining the NMR signal and relaxation enhancements caused by the presence of paramagnetic molecules in solution. Para- magnetic complexes are used as contrast agents in magnetic resonance imaging, due to their efficacy in selectively increase the relaxation rates of solvent water protons, as well as in dynamic nuclear polarization experiments to increase the NMR signal of desired molecules through polarization transfer from unpaired electrons. In this paper we review some recent, unexpected observations in these two areas, which seem related to spin and/or chemi- cal diffusion, and demonstrate the need for a detailed understanding of the interplay of different phenomena. A deeper understanding of spin and chemical diffusion may thus result very important for an improved design of contrast agents for magnetic resonance imaging and for the optimization of hyperpolarization experiments.