Multiple-quantum cross-polarization in MAS NMR of quadrupolar nuclei

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.