Quantifying Site-Specific Proton Dynamics in Phosphate Solid Acids by H-1 Double Quantum NMR Spectroscopy

Solid-state magic angle spinning (MAS) NMR was used to investigate changes in proton dynamics in phosphate solid acids that exhibited increased proton conductivity between room temperature and 110 degrees C. Double quantum dipolar recoupling methods were used to quantify site-specific changes in proton-proton dipolar coupling as a function of temperature. The static dipolar coupling and moqonally induced changes to it were compared. This was accomplished by calculating (from crystal structures) and measuring (from the initial parts of the DQrecoupling curves) the root-sum-square of the dipolar coupling, a geometry-independent measure of dipolar coupling strength referred to as the ``apparent dipolar coupling'', D-app. The analysis of KH2PO4 and RbH2PO4 showed that the experimentally determined apparent dipolar couplings were reduced from the calculated values at increased temperatures in dynamic systems. Higher proton conductivity was associated with greater reduction of the apparent dipolar coupling as measured by dipolar recoupling NMR methods. Most interestingly, in its monoclinic phase, RbH2PO4 has two chemically distinct proton environments, one disordered and one ordered, which are resolved by H-1 MAS NMR. These sites exhibit different dipolar coupling responses as a function of temperature, revealing that proton conduction in this temperature range arises from motions involving only one of the sites. This site-specific dynamics is measured directly for the first time, using a combination of MAS to resolve the H-1 sites and dipolar recoupling experiments to probe the temperature dependence of the H-1-H-1 dipolar interactions.