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

High Resolution Image Download MS PowerPoint Slide Liquid-state 19 F Overhauser dynamic nuclear polarization (DNP) was applied to several organic compounds using 1,3-bis(diphenylene)-2-phenylallyl (BDPA) as the polarizing agent. 19 F 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 19 F 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 19 F DNP NMR on organic liquids, on sample volumes of relevance in chemical analysis. Such approaches could significantly broaden the applicability of liquid-state 19 F DNP for small-molecule NMR in analytical and pharmaceutical chemistry, and eventually for the detection and characterization of environmentally persistent PFAS compounds.