23Na Magic-angle spinning nuclear magnetic resonance of central and satellite transitions in the characterization of the anhydrous, dihydrate, and mixed phases of sodium molybdate and tungstate

Abstract

23Na Magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of pure phases for Na 2MoO 4, Na 2MoO 4·2H 2O, Na 2WO 4, and Na 2WO 4·2H 2O have led to the determination of accurate values for the quadrupole coupling parameters and isotropic chemical shifts for all Na sites. The analysis of the spectra involves a combination of simulations of the line shapes for the central transitions and the manifold of spinning sidebands for the satellite transitions. The spectral parameters for the pure phases represent a prerequisite for a correct assignment and quantitative evaluation of 23Na MAS spectra at different magnetic field strengths observed for mixtures of the anhydrous and dihydrate phases. Such phase mixtures are observed, for example, for some commercial samples of Na 2MoO 4 or may be generated by (i) exposure of the anhydrous phases to a humid atmosphere or (ii) gently heating the dihydrates. The quadrupole coupling parameters for the two Na sites in the dihydrates are tentatively assigned to the two crystallographically distinct Na atoms in the asymmetric unit by calculations of an approximate dependency of the electric field gradient tensor on the local geometry for the Na sites.