Application of amplitude-modulated radiofrequency fields to the magic-angle spinning NMR of spin-7/2 nuclei

摘要

We report pulse sequences for the sensitivity enhancement of magic-angle spinning and multiple-quantum magic-angle spinning spectra of spin-7/2 systems. Sensitivity enhancement is obtained with the use of fast amplitude-modulated (FAM) radiofrequency pulses. In one-dimensional magic-angle spinning experiments, signal enhancement of 3 is obtained by a FAM pulse followed by a soft 90°pulse. In two-dimensional multiple-quantum magic-angle spinning experiments, FAM pulses are used for both the excitation of multiple-quantum coherences and for their conversion into observable single-quantum coherences. The observed signal enhancements are 2.2 in 3Q experiments, 3.1 in 5Q experiments, and 4.1 in 7Q experiments, compared to the conventional two-pulse scheme. The pulse schemes are demonstrated on the ~(45)Sc NMR of Sc_2(SO_4)_3·5H_2O and the ~(139)La NMR of LaAlO_3. We also demonstrate the generation of FAM pulses by double-frequency irradiation.