High-frequency dynamic nuclear polarization in rotating solids.

Nuclear magnetic resonance (NMR) spectroscopy is among the most powerful tools for determining molecular structure, yet its intrinsic sensitivity has long constrained what can be studied. Dynamic nuclear polarization (DNP) addresses this fundamental limitation by transferring the much larger polarization of electron spins to nearby nuclei, amplifying NMR signals by several orders of magnitude. When combined with magic angle spinning (MAS), a technique that averages anisotropic interactions in solid samples to yield high-resolution spectra, DNP transforms solid-state NMR into a practical tool for investigating complex biological assemblies, functional materials, and surfaces that are inaccessible to solution methods. Here, we review the current state and near-term future of high-field MAS DNP. We describe the principal continuous-wave polarization transfer mechanisms and their distinct dependencies on magnetic field strength, spinning frequency, and microwave power. We survey the development of polarizing agents, from early nitroxide biradicals to asymmetric and hetero-biradical designs that maintain efficiency at high magnetic fields above 18 T. We discuss the instrumentation that makes high-field DNP possible, particularly the gyrotron oscillator and emerging solid-state microwave sources. We discuss time-domain pulsed DNP as the solution to circumvent the field-scaling limitations of continuous-wave methods. Last, we highlight applications in structural biology, materials science, and surface chemistry where DNP-enhanced sensitivity has enabled measurements not otherwise feasible.