Figure 2.

As a function of the MAS rotor position angle, for one orientation of a nitroxide biradical: (a) ESR frequencies (blue and red lines) with microwave frequency indicated by dashed line; (b) central four energy levels of the three-spin system, colored to match Fig. 1; (c) electron and nuclear polarization from density matrix calculations during the first rotor period with hyperfine coupling, after equilibration of microwaves and electrons for 5T_1e. Parameters used for the calculations are listed in Table 1. Three types of energy level avoided crossings are circled in parts (a) and (b): (1) ESR frequency crosses microwave frequency; (2) ESR frequency difference crosses NMR frequency; (3) ESR frequencies cross. The inset of part (b) shows an expanded view of the last avoided crossing of the rotor period. For part (c), 100 sets of random T_2e fields were averaged.