Improved heteronuclear decoupling performance under fast MAS by Slightly Desynchronized Phase Alternated Cycles (SDPACs)

A new version of the recently introduced Rotor-Synchronized Phase-Alternated Cycle (ROSPAC) heteronuclear decoupling pulse sequence [Simion et al., J. Chem. Phys. 157, 014202 (2022)] is proposed, where the delay between the pulses is not perfectly rotor synchronized, dubbed slightly desynchronized phase alternated cycles. Its efficiency in terms of the spectral line's intensity and robustness toward 1H offset and flip angle is analyzed by experimental measurements at 100 kHz magic-angle spinning and theoretically explored by using a generalized theoretical framework based on Floquet theory. The best decoupling was found for a delay between the pulses that has a deviation of about 10% from the perfect synchronization condition, with an enhancement of the signal's intensity of up to 20% compared to the original ROSPAC sequence.