14.1 T Liquid-State ¹⁹F Overhauser Dynamic Nuclear Polarization in an Analytical Organic Setting

Liquid-state ¹⁹F Overhauser dynamic nuclear polarization (DNP) was applied to several organic compounds using 1,3-bis(diphenylene)-2-phenylallyl (BDPA) as the polarizing agent. ¹⁹F DNP enhancement factors ranging between 3- and 37-fold were achieved upon microwave irradiation at 395 GHz/14.1 T. These results were obtained using a conventional liquid-state NMR probehead, modified to incorporate a waveguide for the microwaves capable of delivering ≈13-30 W of power emanating from a gyrotron source, as well as a field-sweeping coil for precise tuning of the Overhauser effect resonance condition. Experiments were conducted on ≈70 μL sample volumes, yielding spectral resolutions that were comparable to those of standard liquid-state ¹⁹F NMR. These studies were complemented with quantitative determinations of BDPA's T_1e and T_2e relaxation times in these liquids, using pulsed-echo and inversion-recovery measurements at 94 GHz. From these data, saturation factors, leakage factors and BDPA coupling factors, could be extracted for each fluorinated system. The resulting coupling factors ranged from -0.0067 to -0.14, depending on the analyte and solvent system used. While heating issues remain to be resolved, these results open new vistas for performing high-field ¹⁹F DNP NMR on organic liquids, on sample volumes of relevance in chemical analysis. Such approaches could significantly broaden the applicability of liquid-state ¹⁹F DNP for small-molecule NMR in analytical and pharmaceutical chemistry, and eventually for the detection and characterization of environmentally persistent PFAS compounds.