Calorimetric and Dielectric Studies on the Water Reorientation in the Two-Dimensional Hydrogen-Bond System of Cd(H₂O)₂Ni(CN)₄·4H₂O Crystal

Abstract

The heat capacity of Cd(H2O)2Ni(CN)4·4H2O crystal in which the water molecules form a two-dimensional quasi-hexagonal hydrogen-bond network was measured with an adiabatic calorimeter in the temperature range 5−300 K. A glass transition due to the freezing of the reorientational motion of the water molecules was found around 120 K. The reorientational relaxation times were obtained from the enthalpy relaxation data around the glass transition. The complex dielectric permittivity was also measured in the temperature range 150−285 K and in the frequency range 20 Hz−1 MHz. A dielectric dispersion associated with the reorientational motion of the water molecules was observed in the temperature range 160−270 K. The dielectric relaxation times were calculated from the temperature and frequency dependence of the ε‘‘ data. Calorimetric and dielectric relaxation times lie on separate smooth lines in the wide time scale over 10-6−106 s in the Arrhenius plot. The mechanism of the water reorientational motion was discussed on the basis of the model considering the formation and transfer of the Bjerrum defect as in the case of the three-dimensional ice polymorphs.

Proton Relay in a One-Dimensional Hydrogen-Bonded Chain Composed of Water Molecules and a Squaric Acid Derivative

“…Analysis of the dielectric dispersion in a single crystal of Na 2 HPO 4 ·12H 2 O gave a half-circular Cole−Cole plot, which indicates dielectric behavior caused by a Debye-type mechanism . In contrast, orientational defects in the 2D hydrogen-bonded water molecules in Cd(H 2 O) 2 Ni(CN) 4 ·4H 2 O are generated much more easily than in the case of hexagonal ice . The reason for this is that protons located above or below the hydrogen-bonded plane cannot participate in the 2D hydrogen-bonded network.…”