A primer to polarizing agent design: Quantum mechanical understanding of cross effect magic-angle spinning Dynamic Nuclear Polarization

Dynamic Nuclear Polarization (DNP) is transforming the landscape of solid-state characterization for both biological solids and functional materials. By transferring electron spin polarization to coupled nuclear spins under microwave irradiation, DNP increases NMR sensitivity by several orders of magnitude. However, the mechanism of DNP transfer and its efficiency under magic-angle spinning (MAS) significantly differs from that under static conditions. This primer article provides a comprehensive and pedagogical explanation of the theoretical aspects of MAS-DNP, with a specific focus on the cross-effect mechanism. A clear understanding of the nuances of MAS-DNP is crucial for improving its efficiency and extending its application to high magnetic fields and fast MAS conditions. To this end, the article proposes a guideline for synthetic chemists to develop DNP polarizing agents under these experimental conditions.