Cross-Polarization from Quadrupolar Nuclei to Silicon Using Low-Radio-Frequency Amplitudes during Magic-Angle Spinning

Abstract

The dynamics of cross-polarization from the central transition of a quadrupolar nucleus (27Al or 23Na) to a spin-1/2 nucleus (29Si) 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 29Si NMR sensitivity and several applications are demonstrated.