Vibrational and reorientational dynamics, crystal structure and solid-solid phase transition studies in [Ca(H2O)6]Cl2supported by theoretical (DFT) calculations

Autor: Joanna Hetmańczyk, Anna Migdał-Mikuli, Edward Mikuli, Łukasz Hetmańczyk
Rok vydání: 2016
Předmět:
Zdroj: Journal of Raman Spectroscopy. 47:591-601
ISSN: 0377-0486
DOI: 10.1002/jrs.4863
Popis: [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.
Databáze: OpenAIRE
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