A Characterization of the Linear P−O−P Bonds in M4+(P2O7) Compounds: Bond-Angle Determination by Solid-State NMR

Abstract

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 × 3 × 3 cubic superstructure (Pa3 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 31P 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 (∼100 μs). A static disorder of “normal” bent units, postulated previously, is clearly excluded.