title: Application of amplitude-modulated radiofrequency fields to the magic-angle spinning NMR of spin-7/2 nuclei.
abstract: We report pulse sequences for the sensitivity enhancement of magic-angle spinning and multiple-quantum magic-angle spinning spectra of spin-72 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 degrees 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 45Sc NMR of Sc2(SO4)3 x 5H2O and the 139La NMR of LaAlO3. We also demonstrate the generation of FAM pulses by double-frequency irradiation.
authors.affiliation: Department of Chemistry, University of Southampton, Southampton SO17 1BJ, UK.
authors.forename: P K
authors.initials: PK
authors.lastname: Madhu
authors.forename: Ole G
authors.initials: OG
authors.lastname: Johannessen
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Pike
authors.forename: Ray
authors.initials: R
authors.lastname: Dupree
authors.forename: Mark E
authors.initials: ME
authors.lastname: Smith
authors.forename: Malcolm H
authors.initials: MH
authors.lastname: Levitt