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Abstract:
Proton wide-line, multiple-pulse, T1 and T1ϱ measurements on single crystals of PMADH, and deuteron EFG measurements and bandshape analyses of spectra recorded from deuterated crystals of PMADH are reported. The wide-line and multiple-pulse proton results indicate that the water molecules in PMADH are flipping about their twofold symmetry axes. Both T1 and T1ϱ were measured as a function of crystal orientation and temperature. Comparison of the experimental data with model calculations for T1ϱ based on the established flipping motions of the water molecules shows that is dominated by this process whereas is not. The T1ϱ data thus enable determination of the rate of the H2O flips as a function of temperature. EFGs of the water deuterons in deuterated PMADH, measured at low and high temperatures, confirm the occurrence of the flips for D2O in PMADH. The flips constitute an exchange process for the water deuterons. Bandshape analyses of single-crystal deuteron spectra recorded at temperatures covering the full range of exchange rates allowed determination of the flip rates of the D2O molecules. The activation energies for the H2O and D2O flips are the same, Ea = 10 kcal/mol, within the limits of accuracy of the experiments. The frequency factors in the Arrhenius relation are 8.3 X 1013 sec−1 (H2O) and 2.6 X 1013 sec−1 (D2O).
