CHEMICAL-SHIFT-SELECTIVE MULTIPLE-QUANTUM NMR AS A PROBE OF THE CORRELATION LENGTH OF CHEMICAL SHIELDING TENSORS

A method for obtaining multiple-quantum NMR spectra of a selected homo geneously broadened region of an inhomogeneously broadened powder patt ern (CHESHIRE) is demonstrated with F-19 NMR results for polycrystalli ne calcium fluorapatite, CaF(PO4)3, and a 9/1 calcium/strontium fluora patite solid solution. The slower growth rate of F-19 multiple-quantum coherences at the same selected chemical shift in the latter compound is interpreted in terms of a decrease in the average correlation leng th of the shielding tensor in the one-dimensional chain of fluoride io ns in the solid-solution, due to the F-19 chemical-shift perturbation caused by the strontium ion substitutions.