Optimization of two-dimensional multiple-quantum MAS NMR experiments for I = 3/2 nuclei on a moderate-field spectrometer

To test the efficiency of the two-dimensional multiple-quantum magic-angle spinning (2D MQMAS) method for half-integer quadrupolar nuclei on a moderate-field spectrometer, the experiments were performed for RbNO3, Na5P3O10, and Na2C2O4 at a static magnetic-field strength of 4.7 T by using a two-pulse sequence and a three-pulse z-filter sequence. The procedure for data-processing has been carefully considered. In both types of experiment? high resolution was obtained in the isotropic (nu'(1)) dimension of the 2D spectra, allowing a clear distinction for the crystallographically different sites of Rb-87 and Na-23 in the samples even with the moderate-field spectrometer. A large improvement was observed in the lineshapes of triple-quantum-filtered single-quantum MAS cross sections by use of the three-pulse z-filter sequence, and a further improvement was achieved by applying rotor-synchronized acquisition in the tl domain. A new approach to lineshape fitting for the cross sections, which uses the information in the isotropic dimension as a constraint for the NMR parameters, was introduced, and the validity of this method was demonstrated. The MQMAS method was also applied to NaCl crystallites, and the resulting 2D spectrum indicated the possible use of such crystallites as a chemical shift reference sample in practice, not only for the MAS (nu(2)) dimension but also for the isotropic (nu'(1)) dimension. The effects on resolution in the isotropic dimension of changes in the applied magnetic field are discussed.