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DNP in liquids is driven by electron-nuclear cross-relaxation, known as Overhauser effect (O-DNP). When relaxation is dominated by scalar hyperfine interaction, the enhancements can reach three orders of magnitudes, as recently reported for $^{13}$C-DNP at 3.4 T [1].
Hereby we present a systematic study performed at different magnetic fields on model systems doped with nitroxide radical (TEMPONE) as polarizing agent [2]. $^{13}$C signal enhancements on organic small molecules in liquids at room temperature were observed as high as 800 at 1.2 Tesla and 600 at 9.4 Tesla. An accurate determination of Overhauser parameters allowed us to disclose the primary role of the scalar hyperfine interaction to the $^{13}$C nuclei as mediated by molecular collisions in the sub-picoseconds timescale.
Experimental measurements performed at 1.2 T, 9.4 T, and 14 Tesla allowed us to complete the characterization of the polarization transfer efficiency over a broad frequency range and described it by the subtle combination of dipolar and scalar relaxation.
Furthermore, we recognized that a proper choice of polarizing agent/target system is essential to optimize the efficiency of scalar O-DNP. Indeed, fullerene-nitroxide derivatives are superior to TEMPONE radical as polarizing agent at low fields, while halogens atoms (Cl, Br) bound to the target C nucleus seems to favor the scalar interaction.
The observation of sizable DNP of $^{13}$CH$_2$ and $^{13}$CH$_3$ groups in organic molecules at 9.4 T preserving NMR resolution opens perspectives for a broader application of this method as a tool to address $^{13}$C-NMR sensitivity issues at high fields.
[1] Liu G., Levien M., Karschin N., Parigi G., Luchinat C., and Bennati M. Nat. Chem. 9, 676-680 (2017)
[2] Orlando T., Dervisoglu R., Levien M., Tkach I., Prisner T.F., Andreas L.B., Denysenkov V., Bennati M. Angew. Chem. Int. Ed. 58, 1402-1406 (2019)