A characterization of the linear p-o-p bonds in M4+(P2O7) compounds:: Bond-angle determination by solid-state NMR

Many compounds of the type M4+(P2O7) contain, in their room-temperature structure, both linear and bent pyrophosphate groups The commonly found packing motif is usually described as a 3 x 3 x 3 cubic superstructure (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 configuration. Using a novel variant of two-dimensional (2D) exchange NMR spectroscopy, we have measured, in a the cubic room-temperature structure of SiP2O7, the angle between the P-31 chemical-shift-anisotropy (CSA) tensors in covalently bonded P-O-P unit. The selectivity to covalently bonded pairs is achieved by using the scalar coupling interaction as the driving force for polarization exchange during the mixing time of a two-dimensional exchange experiment. The NMR data show that the "linear" pyrophosphate groups in SiP2O7 are in fact linear on the NMR time scale (similar to 100 mu s). A static disorder of "normal" bent units, postulated previously; is clearly excluded.