Turke, M.-T., et al., Overhauser DNP with 15N labelled Fremy\’s salt at 0.35 Tesla. Phys. Chem. Chem. Phys., 2012. 14(2): p. 502-510.
The effectiveness of dynamic nuclear polarization (DNP) as a tool to enhance the sensitivity of liquid state NMR critically depends on the choice of the optimal polarizer molecule. In this study the performance of 15N labelled Fremy\’s salt as a polarizing agent in Overhauser DNP is investigated in detail at X-band (0.35 T, 9.7 GHz EPR, 15 MHz 1H NMR) and compared to that of TEMPONE-D,15N employed in previous studies. Both radicals provide similar maximum enhancements of the solvent water protons under similar conditions but a different saturation behaviour. The factors determining the enhancement and effective saturation were measured independently by EPR, ELDOR and NMRD and are shown to fulfil the Overhauser equation. In particular, following the theory of EPR saturation we provide analytical solutions for the dependence of the enhancement on the microwave field strength in terms of saturation transfer between two coupled hyperfine lines undergoing spin exchange. The negative charge of the radical in Fremy\’s salt solutions can explain the peculiar properties of this polarizing agent and indicates different suitable application areas for the two types of nitroxide radicals.