A CHARACTERIZATION OF THE LINEAR P-O-P BONDS IN M4- BOND-ANGLE DETERMINATION BY SOLID-STATE NMR((P2O7) COMPOUNDS )

Many compounds of the type M4+(P2O7) contain, in their room-temperatur e structure, both linear and bent pyrophosphate groups The commonly fo und packing motif is usually described as a 3 x 3 x 3 cubic superstruc ture (Pa (3) over bar with Z = 108) where 11% of the P-O-P angles are predicted by diffraction methods to be in the unusual linear configura tion. Using a novel variant of two-dimensional (2D) exchange NMR spect roscopy, we have measured, in a the cubic room-temperature structure o f SiP2O7, the angle between the P-31 chemical-shift-anisotropy (CSA) t ensors in covalently bonded P-O-P unit. The selectivity to covalently bonded pairs is achieved by using the scalar coupling interaction as t he driving force for polarization exchange during the mixing time of a two-dimensional exchange experiment. The NMR data show that the ''lin ear'' pyrophosphate groups in SiP2O7 are in fact linear on the NMR tim e scale (similar to 100 mu s). A static disorder of ''normal'' bent un its, postulated previously; is clearly excluded.