Vibrational and reorientational dynamics, crystal structure and solid–solid phase transition studies in [Ca(H 2 O) 6 ]Cl 2 supported by theoretical (DFT) calculations

[Ca(H 2 O) 6 ]Cl 2 between 93 and 300 K possesses two solid phases. One phase transition (PT) of the first‐order type at T C h  = 218.0 K (on heating) and T C c  = 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 (H 2 O) A 2 mode (at 205 cm −1 ), and two Raman bands are arising from τ (H 2 O) E and τ (H 2 O) A 1 modes (at ca. 410 and 682 cm −1 , respectively), suggesting that the observed PT is associated with a sudden change of speed of the H 2 O reorientational motions. The estimated mean value of activation energy for the reorientation of the H 2 O 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(H 2 O) 6 ]Cl 2 includes two sets of differently bonded H 2 O 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(H 2 O) 6 ]Cl 2 . Copyright © 2016 John Wiley & Sons, Ltd.