Reorientational motions of hydrogenic species in 12-tungstophosphoric acid 14-hydrate: a neutron scattering study

Abstract

Variable-temperature incoherent quasielectric neutron scattering (QENS) measurements have been used to investigate motions of hydrogenic species in 12-tungstophosphoric acid 14-hydrate (H₃PW₁₂O₄₀·14H₂O). Reorientational motions are classifiable as those of H₂O (free and coordinated to hydrated protons) and H₃O⁺[free and in non-centrosymmetric H⁺(H₂O)_n units]. Scattering spectra are satisfactorily modelled assuming quasielastic broadening to arise from a three-fold reorientation of H₃O⁺ about an ionic C₃ axis with a (model independent)E_a= 19 ± 1 kJ mol^–1. Slower reorientation of H₂O and H self-diffusion would not give discernible broadenings on the instrument used.