Heteronuclear dipolar decoupling effects on multiple‐quantum and satellite‐transition magic‐angle spinning NMR spectra

We here report on the influence of heteronuclear dipolar decoupling on the 27 Al 3QMAS, 5QMAS, and the double‐quantum filter–satellite‐transition magic‐angle spinning (DQF‐STMAS) spectra of a strongly dipolar‐coupled system, gibbsite. The requirements for heteronuclear dipolar decoupling increase with the order of coherence evolving in the indirect dimension of a two‐dimensional (2D) experiment. The isotropic line width of the high‐resolution 2D spectra, in samples like gibbsite, is composed of four parts: the distribution of isotropic shifts (δ ISO , δ QIS ), the homogeneous broadening related to the proton–proton flip‐flop terms, the 27 Al 27 Al homonulcear dipolar couplings, and the 1 H 27 Al heteronuclear dipolar couplings. It is shown that, even in the case of gibbsite, where a strong proton–proton bath exists, the main resolution limiting factor in these experiments resides in the 1 H 27 Al dipolar interaction. Copyright © 2008 John Wiley & Sons, Ltd.