Fig. 6.

Field dependence curve on 2 M ¹³C urea in glycerol-d₈–D₂O–H₂O (60 : 30 : 10 volume ratio) with 15 mM TOTAPOL biradical. The NMR field was swept while the gyrotron remained at fixed frequency, 263.343 GHz. Each field position was measured with ¹³C CPMAS experiment with microwaves on and off for calculating the DNP signal enhancement. 4 scans, 10 s recycle delay, and 2 dummy scans per experiment, ω_r/2π = 5.1 kHz, 105 K sample temperature. The transmitter frequency of the ¹H and ¹³C channel was adjusted for each field point. Error bars on the DNP signal enhancement measurements are ±2%.