The electron polarization was transferred to the 1 H nuclear spins by microwave irradiation (µw) with a frequency centered at 192.65 GHz and modulated at a rate of 500 Hz over a bandwidth of 160 MHz. The polarization was transferred from 1 H to 13 C spins every 4 min using microwave-gated CP as described in references . The polarization of the 13 C spins during this process was monitored using 5 • rf pulses applied every 30 s. As shown in Appendix A , the signal at zero field is proportional to the difference in polarization between the 1 H and 13 C spins P ( 1 H) -P ( 13 C) and so the signal is maximal if the respective polarizations have opposite signs. Therefore, once the polarization plateaued in the solid state, the microwave source was turned off and the 13 C polarization was inverted using an adiabatic frequency swept inversion pulse (chirp pulse) of 1 ms duration and 80 kHz bandwidth. At this point, the 1 H and 13 C spins had positive and negative polarization, respectively, which maximizes the signal at zero field.