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An NMR study of 1H, 31P, and 23Na relaxation and molecular dynamics in the polycrystalline disodium salt of N-phosphocreatine hydrate

1988; Elsevier BV; Volume: 78; Issue: 1 Linguagem: Inglês

10.1016/0022-2364(88)90159-x

1557-8968

E. C. Reynhardt, K. Jurga, E.R. Andrew,

Advanced Battery Materials and Technologies

Two samples of the disodium salt of phosphocreatine containing 4.5 and 1.5 water molecules for each phosphocreatine molecule, respectively, have been studied by employing techniques in solid-state NMR. Proton spin-lattice relaxation times in the laboratory frame, T1(H), have been measured as a function of frequency and temperature (333 K > T > 80 K). The spin-lattice relaxation times in the rotating frame, T1ϱ(H), have been measured at H1 = 23 G while the proton second moments, M2(H), have been extracted from the shape of the free induction decays. In addition T1(31P) and T1(25Na) have been measured as functions of temperature at 81 and 53 MHz, respectively. In both samples twofold reorientations of 1.5 H2O molecules dominate the relaxation results to a large extent over the temperature region covered. In the sample containing 4.5 H2O molecules the second moment results are in agreement with a model in which two water molecules rotate freely about their symmetry axes at lower temperatures. Atom-atom potential energy calculations support the conclusions drawn from the NMR results. The threefold reorientation of the methyl group has been identified in the sample containing 4.5 H2O molecules. The depth of the T1(H) minimum is however more shallow than expected, most probably due to a distribution of correlation times. At higher temperatures the sample with 1.5 H2O molecules exhibits relaxation minima which have been assigned to a large-amplitude vibration or wobbling motion of the PO3 group.