Hydrogen dynamics in [Me(H2O)6](ClO4)2 with Me=Mg, Mn, Fe, Ni, and Zn investigated with quasielastic neutron scattering

• Published in: The Journal of chemical physics 2000-08, Vol.113 (8), p.3219-3225
• Main Authors: Nöldeke, C., Asmussen, B., Press, W., Büttner, H., Kearley, G., Lechner, R. E., Rufflé, B.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.1286976

The hydrogen dynamics in the metalhexahydrateperchlorates with Mg, Mn, Fe, Ni, and Zn as metal ions has been investigated with quasielastic neutron scattering. The water molecules perform 180°-flip motions on a picosecond time scale through a series of solid–solid phase transitions. In the highest temperature phase I and the subsequent phase II, rotational barriers of typically Ea=50 meV are found. These values are surprisingly small in view of the low symmetry of H2O molecules. The I → II phase transition has only very small effects on the hydrogen dynamics. At the transition into phase III an increase of the rotational barriers to typically Ea=250 meV is found. This is interpreted as the formation of weak hydrogen bonds. In phase I 180°-flip motions provide a complete description of the observed data. In phases II and III an extension of the dynamical model toward a stronger localization of hydrogen is required. A preference is given to a mechanism leading to a temporary blockade of the flip motions. In phase III of the Fe compound, the existence of crystallographically different sites for water molecules is inferred.