Figure 8.

(a, b) Simulated REDOR curves for a $\mathrm{CN}$ spin pair, with a value of 2  $\mathrm{kHz}$ for the anisotropy of the dipolar coupling tensor $\frac{{\mathit{\delta }}_{\mathrm{IS}}}{\mathrm{2}\mathit{\pi }}$ at a carbon resonance frequency of 150  $\mathrm{MHz}$ in a 1.3  $\mathrm{mm}$ MAS probe (a) and a 3.2  $\mathrm{mm}$ MAS probe (b), assuming a spinning frequency of 20  $\mathrm{kHz}$ . The nominal rf field strengths were set to 100  $\mathrm{kHz}$ (62.5  $\mathrm{kHz}$ ) for $\mathrm{C}$ and 62.5  $\mathrm{kHz}$ (50  $\mathrm{kHz}$ ) for $\mathrm{N}$ in the 1.3  $\mathrm{mm}$ (3.2  $\mathrm{mm}$ ) probe. For all four cases (C1–C4), the resulting recoupling efficiencies are significantly lower than the theoretical REDOR curve (dashed line). Amplitude modulations (C2 and C4) lead to a further marginal deterioration of the recoupling efficiency. The remaining two cases (C1 and C3) are indistinguishable. (c, d) Simulated REDOR curves for a $\mathrm{HN}$ spin pair with a $\frac{{\mathit{\delta }}_{\mathrm{IS}}}{\mathrm{2}\mathit{\pi }}$ of 24  $\mathrm{kHz}$ at a proton resonance frequency of 600  $\mathrm{MHz}$ in a 1.9  $\mathrm{mm}$ MAS probe, assuming a spinning frequency of 40  $\mathrm{kHz}$ . The nominal rf field strengths were set to 125  $\mathrm{kHz}$ for $\mathrm{H}$ and 50  $\mathrm{kHz}$ for $\mathrm{N}$ . A scaling of the effective dipolar coupling is achieved by shifting one pulse (c; delay until first pulse $t_{\mathrm{1}}=\mathrm{2.5}$   $\mathrm{\mu }\mathrm{s}$ ) or both pulses (d; delay until first pulse $t_{\mathrm{1}}=\mathrm{16}$   $\mathrm{\mu }\mathrm{s}$ ) per rotor period. No significant deviation in any of the four cases (C1–C4) from the theoretical REDOR curves (dashed lines) is observed, indicating the robustness of these REDOR implementations towards rf inhomogeneity.