Multiple-quantum MAS NMR of quadrupolar nuclei. Do five-, seven- and nine-quantum experiments yield higher resolution than the three-quantum experiment?

The question of whether or not higher-order (five-, seven- and nine-quantum ) multiple-quantum magic angle spinning (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 dispersi on is shown theoretically to be greatly increased in higher-order MQMAS spe ctra, but it is argued that whether or not this translates into an increase in resolution depends upon the ratio of the homogeneous to inhomogeneous c ontributions to the isotropic linewidth. Experimentally, it is demonstrated using three-, five- and seven-quantum Sc-45 MAS NMR and three- and five-qu antum Al-27 MAS NMR of crystalline samples that higher-order MQMAS experime nts can yield a real and useful increase in resolution but that, owing to t he presence of inhomogeneous broadening in the isotropic spectra, this incr ease is less than the theoretically predicted value. A number of practical issues relating to resolution in MQMAS NMR are also pointed out. (C) 2000 E lsevier Science B.V. All rights reserved.