Energetics of calcium dichloride hydrates; CaCl2·nH2O (n= 0, 2, 4, 6). Total lattice potential-energy calculations and the relation between the electrostatic and total energy of the hydrates

Abstract

This study represents the first series of full lattice-energy calculations performed on various hydrates of the same parent salt. We perform two types of calculation: an unrestricted minimisation (based on multipole-moment calculations of the electrostatic energy) and a LATEN calculation (based on a point-charge evaluation of the electrostatic energy). These calculations (i) lead to a confirmation of the reasonableness of the MANIOC procedure often used to assign hydrogen-atom positions by crystallographers, (ii) suggest that the total lattice energy calculated using an ionic model is ca. 3–4% less than the Born–Haber-cycle value for these hydrates (in keeping with a recent study for NaBr·2H₂O) and (iii) demonstrate a rectilinear relationship between the total and the electrostatic energy of the lattice, again justifying the MANIOC technique, which employs electrostatic calculations as a measure of the total energy of the crystal. The calculated lattice energies are found to be U_POT(CaCl₂)= 2159 kJ mol^–1, U_POT(CaCl₂·2H₂O)= 2320 kJ mol^–1, U_POT(CaCl₂·4H₂Oα)= 2297 kJ mol^–1, U_POT(CaCl₂·4H₂Oβ)= 2396 kJ mol^–1, U_POT(CaCl₂·4H₂Oγ)= 2354 kJ mol^–1 and U_POT(CaCl₂·6H₂O)= 2481 kJ mol^–1.