Enhancement of the central-transition signal in static and magic-angle-spinning NMR of quadrupolar nuclei by frequency-swept fast amplitude-modulated pulses

We here report on using fast amplitude-modulated (FAM) pulse trains with constantly incremented pulse durations (SW-FAM) for signal enhancement in one-dimensional nuclear magnetic resonance spectra of quadrupolar nuclei with half-integer spin. In such systems, a FAM pulse train leads to a redistribution of populations across the spin levels, which results in a substantial gain for the central-transition signal. Compared to fixed-duration FAM pulse trains, SW-FAM delivers about the same signal enhancement for spinning samples, but gives much better performance in the static case. This is demonstrated for several compounds, observing the nuclei 23Na (I=3/2), 27Al (I=5/2), and 45Sc (I=7/2).