Mechanism of dynamic nuclear polarization in high magnetic fields

Solid-state NMR signal enhancements of about two orders of magnitude (100-4 00) have been observed in dynamic nuclear polarization (DNP) experiments pe rformed at high magnetic field (5 T) and low temperature (10 K) using the n itroxide radical 4-amino TEMPO as the source of electron polarization. Sinc e the breadth of the 4-amino TEMPO EPR spectrum is large compared to the nu clear Larmor frequency, it has been assumed that thermal mixing (TM) is the dominate mechanism by which polarization is transferred from electron to n uclear spins. However, theoretical explanations of TM generally assume a ho mogeneously broadened EPR line and, since the 4-amino TEMPO line at 5 T is inhomogeneously broadened, they do not explain the observed DNP enhancement s. Accordingly, we have developed a treatment of DNP that explicitly uses e lectron-electron cross-relaxation to mediate electron-nuclear polarization transfer. The process proceeds via spin flip-flops between pairs of electro nic spin packets whose Zeeman temperatures differ from one another. To conf irm the essential features of the model we have studied the field dependenc e of electron-electron double resonance (ELDOR) data and DNP enhancement da ta. Both are well simulated using a simple model of electron cross-relaxati on in the inhomogeneously broadened 4-amino TEMPO EPR line. (C) 2001 Americ an Institute of Physics.