The dynamics of cross-polarization from the central transition of a quadrupolar nucleus ( 27 Al or 23 Na) to a spin- 1 / 2 nucleus ( 29 Si) during magic-angle spinning and using low-radio-frequency field strengths are analyzed for the mineral low albite. Under these conditions additional complications in the spin-lock behavior of the quadrupolar nucleus and in the cross-polarization process were found experimentally and are examined in detail. A step-by-step procedure for optimizing cross-polarization from the central transition of a quadrupolar nucleus to a spin- 1 / 2 nucleus is described. Significant enhancement of 29 Si NMR sensitivity and several applications are demonstrated.
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Paul et al. (1997) studied this question.
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