Enhancing NMR Signals in Liquids by Fluorine-19 Overhauser Dynamic Nuclear Polarization (DNP) and Hyperpolarization...

Nuclear magnetic resonance (NMR) plays an essential role in life and material science but suffers from low sensitivity due to the small energy differences it aims to detect. Therefore, techniques that can increase spin polarization, and thus sensitivity, in NMR are gaining tremendous attention. Overhauser effect dynamic nuclear polarization (OE-DNP) can increase multidimensional NMR signals of small molecules in liquid solutions; however, at high magnetic fields, the direct mechanism to ¹³C is site selective, while it is poorly efficient for ¹H. Here, we report OE-DNP hyperpolarization of fluorine-19 (¹⁹F) nuclei at 9.4 T, thus expanding the methodological toolbox of ¹⁹F NMR. Signal enhancements up to ∼20 under steady-state conditions can be used to either rapidly detect ¹⁹F resonances or to enhance through-bond J-coupled ¹³C nuclei by two orders of magnitude. The latter corresponds to a 10⁴ gain in signal acquisition. We demonstrate this with ¹⁹F→¹³C INEPT (insensitive nuclei enhanced by polarization transfer) spectra of the drug flutrimazole at ¹³C natural abundance, where quaternary carbons become readily detectable, opening perspectives for accelerated NMR on fluorinated drugs or small molecules. The results are corroborated by a theoretical analysis of ¹⁹F OE-DNP coupling factors.