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

Abstract

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.

Dynamics of H2O ligands and ReO4− anions at the phase transition in [Mn(H2O)2](ReO4)2 studied by complementary spectroscopic methods

“…It can be concluded that water molecules perform reorientational jumps around a twofold symmetry axis. Reorientational motion of H 2 O ligands was also observed for compounds which were similar but contained another tetrahedral anion, namely, perchlorate instead of perrhenate . These compounds exhibit more complex polymorphism and three phase transitions are registered in a temperature range of 20–350 K. However, the conclusions extracted from neutron scattering investigations are similar.…”

FIR Spectra of [Ni(H2O)6](ClO4)2 and [Ni(D2O)6](ClO4)2 at Various Temperatures

“…The IINS bands were assigned to a water molecule libration of the third kind in [Ni(H 2 O) 6 )] 2+ [1]. Such separation of water molecules into dynamically inequivalent sets was observed by the quasielastic neutron scattering method [8] too, as well as in some thermogravimetric investigations [14].…”

“…Nevertheless, the contradiction is misleading. One should stress that some values from [8,13] are average values for activation energies of all kinds of water molecules in the sample, whereas the value 6.9 kJ/mole obtained in our work corresponds to a portion of all water molecules named the third kind earlier. Hence, the III-II phase transition has extended this low value of the barrier on all other water molecules in the sample.…”

“…The slope of the line gives the barrier to rotation of the water molecules equal to 6.9 ± 2 kJ/mole. For comparison, we have the following values for the barrier to rotation of water molecules: 24.1 [8] or 29.3 kJ/mole [13] for phase III (i.e., below 311.4 K) and 4.8 kJ/mole for phase I (i.e., above 361.05 K) or phase II (between 311.4 and 361.05 K) [8]. The results obtained here correspond more closely with those obtained in [8] for phase I and II, although our measurements refer to phase III.…”

Dynamics of Atoms and Molecules

“…A more recent example that shows how this can be extended to more complex systems is the evolution of rotational dynamics through the phase transitions of transition-metal hexaquo ions [68] using IN5 at ILL (France). In these salts, a divalent first-row transition metal ion is octahedrally coordinated by six water molecules.…”