Separate-local-field NMR spectroscopy on half-integer quadrupolar nuclei

New approaches to the characterization of resonances in the solid-state NMR spectroscopy of half-integer quadrupolar nuclei are explored, on the basis of the acquisition of heteronuclear separate-local-field spectra on rotating solids. In their two-dimensional version, these experiments correlate for each chemical site a second-order quadrupolar MAS powder pattern with the dipolar MAS sideband pattern to nearby heteronuclei. As 3D NMR sequences, such 2D anisotropic correlation spectra become separated for inequivalent chemical sites along a third, isotropic dimension. Extending in such manner separate-local-field NMR approaches to quadrupoles facilitates the assignment of inequivalent resonances to specific structural environments, and provides new tools for the investigation of dynamics in solids. Details about these 2D and 3D NMR experiments are given, and their application is illustrated with 1H-23Na recoupling experiments on mononucleotides possessing multiple bound cations.