The importance of precise timing in pulsed, rotor-synchronous MAS NMR

Magic-angle spinning NMR experiments with synchronously applied pulse trains are used to measure a variety of different anisotropic interactions, such as heteronuclear dipolar couplings and chemical-shift anisotropies. When the sample rotation speed and the pulse-sequence timings are not properly synchronized, the resulting spectra may contain artifacts and anisotropic interactions may be measured incorrectly. Sample-rotation/pulse-sequence asynchrony can result from short-term fluctuations in spinning speed, even when the average spinning speed is carefully regulated. Here we examine three simple, spin-echo pulse sequences under conditions where the pulse trains are incommensurate with the sample rotation.