Solid-State 1H NMR Study of Structures and Dynamics of Proton Sites in Group II Salts of 12-Tungstophosphoric Acid and Related Compounds

Abstract

The group II salts of 12-tungstophophoric acid were studied in the limiting state of hydration using a suite of 1H solid-state NMR experiments, theoretical analysis, and density functional theory (DFT) energy calculations. Various nonspinning (NS) 1H NS NMR, magic-angle spinning (MAS) 1H MAS NMR, and rotational-echo double-resonance (REDOR) 1H{31P} REDOR NMR experiments were performed. Chemical shift and spinning sideband patterns and comparison of 1H MAS two-pulse sequence NMR results with predictions of average Hamiltonian theoretical calculations definitively show that the limiting hydrated form of the group II salts of 12-tungstophosphoric acid contain only H+-protons and H2O-protons. Density matrix methods were used to derive expressions for the 1H NMR signal for a two-pulse NMR experiment as a function of the second pulse length under both NS and MAS conditions for H+-protons and H2O-protons. NMR structural parameters were obtained from the fit of theoretical expressions to the 1H NS two-pulse seque...