Vibrational and reorientational dynamics, crystal structure and solid–solid phase transition studies in [Ca(H2O)6]Cl2 supported by theoretical (DFT) calculations

Abstract

[Ca(H2O)6]Cl2 between 93 and 300 K possesses two solid phases. One phase transition (PT) of the first‐order type at = 218.0 K (on heating) and = 208.0 K (on cooling) was determined by differential scanning calorimetry. Thermal hysteresis of this PT (10 K), as well as the heat flow anomaly sharpness, suggests that the detected PT is a first‐order one. The entropy change value [ΔS ≈ 8.5 J mol−1 K−1 ≈ Rln(2.8)] associated with the observed PT suggests a moderate degree of molecular dynamical disorder of the high‐temperature phase. The temperature dependencies of the full width at half maximum values of the infrared band are due to ρ(H2O)A2 mode (at 205 cm−1), and two Raman bands are arising from τ(H2O)E and τ(H2O)A1 modes (at ca. 410 and 682 cm−1, respectively), suggesting that the observed PT is associated with a sudden change of speed of the H2O reorientational motions. The estimated mean value of activation energy for the reorientation of the H2O ligands in the high‐temperature phase is ca. 11.4 kJ mol−1 from Raman spectroscopy and 11.9 kJ mol−1 from infrared spectroscopy. X‐ray single‐crystal diffraction measurement and spectroscopic studies (infrared, Raman and inelastic neutron scattering) also confirm that [Ca(H2O)6]Cl2 includes two sets of differently bonded H2O molecules. Ab initio calculations of the complete unit cell of one molecule of calcium chloride with a different number of water molecules (2, 4 and 6) have also been carried out. A comparison of Fourier Transform Infrared (FT‐IR), Fourier Transform Raman Scattering (FT‐RS) and inelastic neutron scattering spectroscopies results with periodic density functional theory calculations was used to provide a complete assignment of the vibrational spectra of [Ca(H2O)6]Cl2. Copyright © 2016 John Wiley & Sons, Ltd.