Signal Enhancement of the Central-Transition Signal in Solid-State NMR of Quadrupolar Nuclei with Spin I =5/2 Using Fast Amplitude-Modulated (FAM) Pulse Trains: A Computational Study

ABSTRACT

In solid-state NMR of quadrupolar nuclei with half-integer spin I, a variety of fast amplitude-modulated (FAM) pulse trains have been used to enhance the intensity of the central-transition signal, by transferring spin population from the satellite transitions. Here, we describe the results of extensive numerical calculations using the Simpson program package to evaluate and compare the signal enhancing performance of FAM-I, SW(τ)-FAM, and SW(1/τ)-FAM sequences under both static and MAS conditions for nuclei with I =5/2. The comparative calculations were performed for a two-dimensional grid of the quadrupolar interaction parameters, namely λ =0.5 ... 4 MHz, η =0 ... 1, and a range of MAS spinning speeds, i.e. 0 kHz (static), 2.5 kHz, 5 kHz and 10 kHz. In agreement with previously published experimental data, the best signal enhancement of static spectra is delivered by the SW(1/τ)-FAM sequence, whereas under MAS conditions, SW(τ)-FAM and FAM-I give better results.

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