SURVIVAL OF DIPOLAR SPLITTINGS BETWEEN EQUIVALENT NUCLEI IN HIGH-SPEED MAS-NMR-INTERPRETATION OF THE P-31 COUPLING PATTERNS FOR TETRAPHOSPHORUS DECAOXIDE

It has been demonstrated experimentally that homonuclear dipolar coupl ings may influence the line shapes in solid-state NMR spectra even at magic-angle sample spinning (MAS) speeds that are much larger than the dipolar couplings themselves. The effect occurs for chemically equiva lent or nearly equivalent nuclei if the MAS speeds are smaller than, o r of the same order of magnitude as, the chemical shift anisotropy, an d is sensitive to even very small chemical shift differences. For tetr aphosphorus decaoxide with dipolar couplings of about 900 Hz between p hosphorus nuclei and a chemical shift anisotropy of about 60 kHz at 11 .74 T, calculations show that even a hypothetical spinning speed of 10 0 kHz is not sufficient to remove dipolar line broadening completely. At technically feasible spinning speeds of 8-14 kHz, line splittings a re observed that can be traced back to a spin system of three crystall ographically equivalent nuclei and one nonequivalent nucleus. These co upling patterns are analysed quantitatively with the help of spectrum simulations. Information is obtained on the symmetry of the molecule a nd of its environment in the crystalline state. The findings imply tha t multipulse decoupling techniques may be needed in addition to high-s peed MAS to obtain utmost resolution in P-31 solid-state NMR spectrosc opy for a sizeable number of phosphorus compounds.