Double Quantum ¹H MAS NMR Studies of Hydrogen-Bonded Protons and Water Dynamics in Materials

Two-dimensional double quantum (DQ) ¹H MAS NMR was used to investigate different proton environments in a series of alkali (Na, K, Rb, Cs) [Nb₆O₁₉]^8- Lindqvist salts, with the water and hydrogen-bound intercluster protons being clearly resolved. Through the analysis of the DQ ¹H NMR spinning sideband pattern, it is possible to extract both the mean and distribution of the motionally averaged intramolecular homonuclear ¹H−¹H dipolar coupling for the different water environments and the intercluster protons. Motional order parameters for the water environments were then calculated from the averaged dipolar couplings. The influence of additional intermolecular dipolar couplings due to multispin interactions were simulated and discussed.