Toward Generalized Solution-State ¹H DNP NMR via Particle-Mediated Cross-Relaxation

This study discusses a potential route for enhancing ¹H NMR signals in the liquid phase at high magnetic fields for samples on the 100 μL volume scale using dynamic nuclear polarization (DNP). The approach involves dispersing an inert powder that is both rich in protons and capable of undergoing DNP with good efficiency at noncryogenic temperatures, and letting the solid ¹H polarization thus enhanced pass from the dispersed particles onto the surrounding liquid via spontaneous cross relaxation effects. To this end, BDPA-doped polystyrene (PS) particles in the μm range were suspended in 30 μL of heptane, loaded into 3.2 mm sapphire rotors, and spun at ≈500 Hz for homogeneity purposes in a 14.1 T magnet. Irradiation with ≈13 W at 395 GHz while maintaining temperature in the 185-220 K range thus enhanced the PS proton polarization ≈12-fold within ≈2 s; after ca. 6 s of irradiation, this resulted in ca. 3-fold enhancements of the heptane proton resonances, while preserving their ≤2 Hz line widths. The conditions over which such particle-mediated transfer occurs were explored over a range of sample composition, deuteration and molecular weight; best results were obtained when polarizing a ball-milled powder made of deuterated-PS/PS/BDPA = 86.4/9.6/4.0 suspended on perdeuterated heptane-d₁₆. While the solution-state enhancements provided by this approach are still relatively modest, its generality could open new avenues in DNP-enhanced ¹H NMR that do not sacrifice on the volumes, on the high-resolution conditions, or on the multiscan averaging that is customary in contemporary applications.