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

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 sequence NMR data where it was found that all H₂O molecules have an interproton distance of 167 pm and the hydrogen atoms have a chemical shift asymmetry parameters of 0.5–1.0. DFT energy calculations consistent with the ¹H­{³¹P} REDOR NMR results were performed to determine the most stable H⁺-proton and H₂O-proton structures in Keggin anion dimers (KA₂^–6). For the MgHPW, SrHPW, and BaHPW salts, the H⁺-protons are found to be in static, surface sites with multiple hydrogen-bonding interactions with oxygen atoms of the 1165 pm KA₂^–6 dimer in the monoclinic unit cell. Each hydrogen atom of the rotating H₂O molecules of MgHPW and BaHPW has multiple hydrogen-bonding interactions with one KA^–3 of the 1400 pm KA₂^–6 dimer, whereas each hydrogen atom of the static H₂O molecules of SrHPW has multiple hydrogen-bonding interactions with one KA^–3 of the 1165 pm KA₂^–6 dimer. Similar analysis for CaHPW could not be performed probably because the H⁺-protons and H₂O-protons are very mobile in this salt and no ¹H­{³¹P} REDOR dephasing was observed. For the BeHPW salt, the H⁺-proton and H₂O-proton resonance lines are not resolved but analysis of the ¹H NS two-pulse sequence NMR data shows that there is one H⁺-proton for each H₂O molecule in this salt