Procedures for labeling the high-resolution axis of two-dimensional MQ-MAS NMR spectra of half-integer quadrupole spins

The increasing development and application of the multiple-quantum MAS NMR for half-integer quadrupole spins has led to various RF pulse sequences for improving the excitation of multiple-quantum coherences and their conversion to single-quantum coherences. As a result, several conventions for labeling the F1 dimension of a 2D MQ-MAS spectrum appear in the literature. The corresponding relations for extracting the isotropic chemical shift, the quadrupole coupling constant, and the asymmetry parameter from experimental data are not always provided. We analyze these various conventions systematically and propose a new one, similar to that introduced by J.-P. Amoureux and C. Fernandez (2000, Solid State NMR 10, 339-343). These various conventions are illustrated with Al-27 (I = 5/2) nuclei in aluminum acetylacetonate Al(CH3COCHCOCH3)(3). Another experimental problem often met, the aliasing of peaks in the 2D spectrum, is analyzed and illustrated with Al-27 (I = 5/2) in NH4Y zeolite and Na-23 (I = 3/2) in sodium pyrophosphate Na4P2O7. (C) 2002 Elsevier Science (USA).