Multiple-Quantum Cross Polarization in MAS NMR of Quadrupolar Nuclei

Abstract Using 27Al (I = 5/2) NMR of aluminium acetylacetonate, we show that it is possible to cross polarize from a spin I = 1/2 nucleus (1H) directly to the central triple-quantum transition of a half-integer quadrupolar nucleus (27Al) in a powdered sample under MAS conditions. The optimum conditions for this multiple-quantum cross polarization (MQCP) are investigated experimentally and compared with existing theoretical results. The new technique is applied to the recently introduced two-dimensional MQMAS experiment for recording high-resolution NMR spectra of half-integer quadrupolar nuclei.

Triple-quantum cross-polarized 27Al NMR spectra of aluminium acetylacetonate as a function of (a) the 27Al radiofrequency field strength, (b) the cross-polarization contact time and (c) the MAS rate.