15N to 13Cα coherence transfer efficiencies calculated for EXPORT using the parameters in Fig. 1b. (a–c) 2D 15N vs 13C offset plots for EXPORT (c), an OCNCA optimal control sequence28 (b), and DCP (a)29. (d–f) 2D 15N vs 13C rf field strength plots (scaling factors relative to the nominal values) for DCP (d), EXPORT with high digitization of the rf field (100 points over 1 rotor period) (e), and EXPORT with lower digitization (20 points over 1 rotor period) (f). Simulations were made using SIMPSON25 with parameters30 for a directly bonded 15N-13Cα spin system, powder averaging with 5 γCR and 144 REPULSION angles,31 and a spinning frequency of 12 kHz at 16.4 T. Broken lines mark carrier frequencies.