On Mechanisms of Dynamic Nuclear Polarization in Solids

Dynamic nuclear polarization (DNP) can dramatically increase the signal in nuclear magnetic resonance (NMR) experiments by transferring spin polarization from electrons to nuclei. We discuss quantum mechanical descriptions of the solid effect and cross effect mechanisms of DNP that typically occur in solid state NMR at high magnetic field, both with and without magic angle spinning (MAS). We present several extensions of recent theoretical descriptions, including (i) treatments of solid effect DNP and cross effect DNP with MAS, in which two nuclei change spin states simultaneously and (ii) a treatment of solid effect DNP in a two-spin system without MAS, in which the ratio of microwave field strength to hyperfine coupling strength can be arbitrary. We also briefly discuss factors that influence DNP efficiency in experiments, beyond the basic principles contained in simple quantum mechanical models.