Fig. 3.

Schematic representation of the main aspects of Overhauser DNP. A) Single-B_o and shuttle Overhauser DNP methods. B) Energy level diagram illustrating the Overhauser DNP mechanism. EPR transitions are saturated (red-double-headed arrows), causing an increase in double (W₂) and zero (W₀) quantum relaxation rates and an enhanced nuclear polarization. W_1S and W_1I denote single quantum transitions for electrons and nuclei, respectively. C) Experimental and predicted magnetic field dependence of Overhauser DNP enhancement of water relative to an unpolarized sample [66, 67]. Experimental DNP data are denoted as black dots. The theoretical field dependence of the coupling factor (assuming leakage and saturation factors to be 1) based on a force-free model is shown in yellow. MD simulations are shown in red. (Reprinted from Ref [67].) D) ¹H Overhauser DNP spectrum of water following high-power 260 GHz microwave irradiation at 9.2 T, leading to a >80-fold polarization enhancement relative to a nonpolarized control, corresponding to an Overhauser enhancement of −83 [71]. The change in chemical shift is due to sample heating. (Reprinted from Ref [71].)