Abstract
Cross-polarization from a spin I = 1/2 nucleus (e.g., ¹H) to a spin S = 3/2 nucleus (e.g., ²³Na) or a spin S = 5/2 nucleus (e.g., ²⁷Al or ¹⁷O) in static powder samples is investigated. The results of conventional (single-quantum), three-quantum, and five-quantum cross-polarization experiments are presented and discussed. Based on a generalization of an existing theory of cross-polarization to quadrupolar nuclei, computer simulations are used to model the intensity and lineshape variations observed in cross-polarized NMR spectra as a function of the radiofrequency field strengths of the two simultaneous spin-locking pulses. These intensity and lineshape variations can also be understood in terms of the spin S = 3/2 or 5/2 nutation rates determined from experimental quadrupolar nutation spectra. The results of this study are intended as a preliminary step towards understanding single- and multiple-quantum cross-polarization to quadrupolar nuclei under MAS conditions and the application of these techniques to the MQMAS NMR experiment.

(a) Cross-sections parallel to the F2 axis, for the F1 nutation frequencies shown, of the two-dimensional 27Al NMR three-quantum nutation spectrum of aluminium acetylacetonate. (b) Three-quantum cross-polarized 27Al NMR spectra of a static sample of aluminium acetylacetonate as a function of the I-spin r.f. field strength, ω1I, with ω1S/2π ~ 70 kHz.