Radio-frequency pulse sequences for one-dimensional 13C MAS NMR measurements (a), 2D-PME measurements (b), and 2D-RPME measurements (c). Radio-frequency-driven recoupling periods consist of one 13C π pulse per MAS rotor period. Dephasing delays τ are 1 ms. Phase cycles are: φ1 = x, −x; φ2 = x, x, −x, −x; φ3 = x or y for real or imaginary signals in t1; φ4 = x, x, y, y, −x, −x, −y, −y; φ5 = x, x, x, x, y, y, y, y, −x, −x, −x, −x, −y, −y, −y, −y. Receiver phase cycles are x, −x, y, −y, −x, x, −y, y in panel a and x, −x, −x, x, y, −y, −y, y, −x, x, x, −x, −y, y, y, −y in panels b and c.