High-field31P N.M.R. investigations of the chemical shielding and indirect dipolar coupling of polycrystalline fluorophosphates

Abstract

³¹P chemical shielding and indirect dipolar coupling data of the polycrystalline fluorophosphates K₂PO₃F, Na₂PO₃F, BaPO₃F and K₂P₂O₅F₂ were obtained from broad-line and magic angle sample spinning ³¹P N.M.R. measurements at 109·3 MHz. The N.M.R. spectra are interpreted in terms of the model established by Vander Hart and Gutowsky. For interpreting the spectrum of K₂P₂O₅F₂ the theory was extended to the case of nonaxial ³¹P chemical shielding tensors. The values of the ³¹P shielding anisotropy Δσ and the asymmetry parameter η of the fluorophosphates are explained on the basis of the assumption that in phosphates the character of the chemical bond between phosphorus and bridging oxygen atoms (P-O_B bond) is very similar to that between phosphorus and fluorine atoms (P-F bond). In addition, the directions of the principal axes of the shielding tensors with respect to the coordinate system of the fluorophosphate ions are discussed. As a result, further support was obtained for the assumption of the similarity of P-O_B and P-F bonds in phosphates.