DISTORTED POWDER LINESHAPES IN AL-27 CP MAS NMR-SPECTROSCOPY OF SOLIDS

Al-27 CP/MAS NMR spectra of aluminium acetylacetonate show distinct lineshapes depending on the experimental conditions as the different orientations within the powder cross-polarize with differing efficiencies. Selective polarization transfer to the 1/2<-- --> - 1/2 transition occurs at rf power levels corresponding to 3omega(Al=omega(H)), while non-selective polarization transfer can occur at higher Al-27 rf powers. Two-dimensional quadrupole nutation spectra show that the cross-polarization lineshapes arise principally because the different orientations have different Al-27 nutation frequencies. The results obtained highlight the caution needed in the interpretation of CP/MAS NMR spectra of quadrupolar nuclei.