Multiple-Quantum MAS NMR of Quadrupolar Nuclei. Do Five-, Seven- and Nine-Quantum Experiments Yield Higher Resolution than the Three-Quantum Experiment?

Abstract The question of whether or not higher-order (five-, seven- and nine-quantum) MQMAS experiments yield isotropic NMR spectra of half-integer quadrupolar nuclei with higher resolution than the basic three-quantum MAS experiment is examined. The frequency dispersion is shown theoretically to be greatly increased in higher-order MQMAS spectra, but it is argued that whether or not this translates into an increase in resolution depends upon the ratio of the homogeneous to inhomogeneous contributions to the isotropic linewidth. Experimentally, it is demonstrated using three-, five- and seven-quantum 45Sc MAS NMR and three- and five-quantum 27Al MAS NMR of crystalline samples that higher-order MQMAS experiments can yield a real and useful increase in resolution but that, owing to the presence of inhomogeneous broadening in the isotropic spectra, this increase is less than the theoretically predicted value. A number of practical issues relating to resolution in MQMAS NMR are also pointed out.

(a) Three-quantum, (b) five-quantum and (c) seven-quantum 45Sc MAS NMR spectra of Sc2(SO4)3.5H2O recorded using the phase-modulated split-t1 experiment. The isotropic projections are shown on the right. The asterisks in (c) indicate spinning sidebands.