Paramagnetic Metal Ions for Dynamic Nuclear Polarization

Paramagnetic metal ions have been utilized as polarizing agents already in the early days of dynamic nuclear polarization (DNP) and have been more recently introduced for magic-angle spinning (MAS) DNP at high magnetic field. In this article, a comprehensive overview is given about the concepts relevant to DNP with high-spin metal ions. The theoretical basis covering the peculiar electron spin dynamics including spin-orbit coupling and zero-field splitting is reviewed, and prerequisites for efficient DNP are introduced. Subsequently, special considerations about the relevant DNP mechanisms (i.e., solid effect and cross effect) are derived. The practical aspects particular to high-spin metal ion DNP are discussed, focusing on differences with respect to conventional (i.e., radical) polarizing agents. In the final section, several demonstrations of MAS DNP on model systems as well as samples relevant to structural biology and materials research are presented. At last, an outlook is given about the prospects of metal ion DNP in light of recent and future advances in modern DNP.