The mechanism of electron-nuclear polarization transfer from the TEMPO free radical to proton spins relies on a thermal mixing mechanism. In DNP experiments at lower fields, the solid effect (direct irradiation of simultaneous electron-nuclear transitions) is typically exploited. However, at higher fields, the solid effect efficiency drastically decreases because it relies on a ncmina]ly forbidden, second-order transition. Thermal mixing is driven by direct irradiation of EPR transitions, and thus does not have the stringent requirements on microwave power which the solid effect possesses. However, while thermal mixing at low fields typically involves a homogeneous EPR line, the TEMPO EPR line at 5 T is inhomogenously broadened, primarily by g-anisotropy. We have performed a number of experiments which elucidate the mechanism at high field, suggesting that electron-electron cross relaxation allows spin diffusion across the EPR line, rendering it effectively homogeneous at high radical concentrations. Electron-electron double resonance (ELDOR) experiments and EPR saturation recovery experiments both provide clear evidence of cross-relaxation across the TEMPO line.
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