Bennati, Marina, and Tomas Orlando. “Overhauser DNP in Liquids on 13C Nuclei” 8 (2019): 8.
https://doi.org/10.1002/9780470034590.emrstm1581.
Dynamic nuclear polarization (DNP) in solution is known since the early days of magnetic resonance to deliver information about molecular motion and electron–nuclear spin relaxation. The method has emerged as a potentially powerful tool to increase sensitivity of high-field solution NMR. In this article, we summarize our recent insights into the mechanism of Overhauser DNP for 13C nuclei, which led to the observation of NMR signal enhancements in liquids up to three orders of magnitude and at magnetic fields of 3.4 T. In contrast to the well-studied case of Overhauser DNP with 1H nuclei, the 13C DNP mechanism can lead to signal enhancements increasing toward high magnetic fields. A key feature of these large high-field enhancements is the underlying scalar relaxation mechanism, which is responsive to correlation times on the subpicosecond time scale. Experiments using microwave (mw) cavities with exquisite control of mw irradiation fields have allowed for disentangling the counteracting effect of dipolar and scalar mechanisms as well as the impact of the 13C chemical environment. These mechanistic insights open up new perspectives for Overhauser liquid DNP as a general tool to increase sensitivity in high-field liquid NMR.