Rotation-Synchronized Homonuclear Dipolar Decoupling

Abstract

Rotor-synchronized pulse sequences for homonuclear dipolar decoupling are analyzed for the particular case of the magic-sandwich pulse sequence. The decoupling efficiency of this sequence in the presence of sample spinning is considered for the slow- and the fast-spinning regime and compared with the results of the experiments. For arbitrary ratios of the cycle time of the pulse sequence tC and the rotor period τR, the decoupling properties of the pulse sequence are largely reduced by the influence of the sample spinning. However, the decoupling efficiency is recovered, if special synchronization conditions tC/τR are met and a rotational dipolar magic echo is formed. These synchronization conditions are derived by secular averaging theory for weak and strong dipolar solids and confirmed experimentally for pulse sequences consisting of one or four magic-sandwich pulse trains. For the pulse sequence with one magic sandwich, the first synchronization condition is met for tC/τR = 3, whereas for the sequence consisting of four magic sandwiches, the synchronization condition is t(c)/τR = 14. These results form the basis for the application of combined rotation and multiple-pulse sequences at high spinning speeds. Similar synchronization conditions are expected to apply to other sequences for decoupling, recoupling, multiple-quantum excitation, and dipolar-filter experiments, when performed under fast MAS.