Study of heteropolyanions by solid-state nuclear magnetic resonance spectroscopy

Abstract

Solid-state ³¹P and ²⁹Si n.m.r. spectroscopy has been used to characterise a number of heteropolyanion salts. A significantly larger chemical shift anisotropy was seen for the phosphorus in the dimeric P : Mo = 2 : 18 Dawson structure than for the P : Mo = 1 : 12 Keggin unit. Two inequivalent phosphorus sites are seen for the 2 : 18 anion. Solution and solid-state chemical shifts agree very well allowing the identification of various degrees of tungsten substitution in the compound of nominal composition Cs₃[PMo₁₁WO₄₀]. Structures having the composition 1 : 11 ([SiW₁₁O₃₉]^8–) show an observable difference in the solid-state isotropic chemical shift from 1 : 12 species thus allowing the detection of the first stages of decomposition of the Keggin unit.