Denysenkov, Vasyl P, and Thomas F Prisner. “Liquid-State Overhauser DNP at High Magnetic Fields,” 8:14, 2019.
Over the past few years, dynamic nuclear polarization at high magnetic fields has become one of the most popular technical and methodical research areas in the field of magnetic resonance spectroscopy and imaging. Much work has been devoted to experiments with the polarization transfer from the electron spin to the nuclear spin performed in the solid state, whereas only few examples exist for polarization transfer in the liquid‐state, which is driven by the Overhauser effect. The technical requirements to perform such experiments at high magnetic fields will be discussed in this review article, with special emphasis on double‐resonance structures, which allow performing such experiments in microwave‐absorbing solutions. Experimental procedures to individually evaluate the factors determining the proton Overhauser dynamic nuclear polarization (DNP) enhancements of several organic solvents using nitroxide radicals are described and compared to theoretical predictions. We show that at high magnetic fields the models based on Stokes–Einstein motion underestimate the coupling factor and therefore the DNP efficiency. This failure of the classical model is most obvious for DNP results obtained with lipid bilayer samples.