Dissociation Energies and Chemical Bonding in the Alkaline-Earth Chlorides from Mass Spectrometric Studies

Abstract

Gaseous equilibria involving the mono- and dichlorides of Mg, Ca, Sr, and Ba were studied by mass spectrometry, and the results were used to derive the dissociation energies of the diatomic and triatomic metal chlorides. Values of D0° (MCl) were found to be 75.1, 94.2, 95.9, and 105.0 kcal for MgCl, CaCl, SrCl, and BaCl, respectively, all ±3 kcal. For the reaction M(g) + MCl2(g) = 2MCl(g), the heat of which is equivalent to D(ClM–Cl)–D(M–Cl), ΔH0° = 35.0, 21.5, 16.7, and 7.2 kcal for Mg, Ca, Sr, and Ba, respectively. The dissociation energies of the diatomic metal chlorides are in general substantially higher than those inferred from electronic band spectra, but the thermochemical and spectroscopic D0° values can be very satisfactorily reconciled if one considers the degree of ionic character and its effect on the rate of convergence of the higher (unobserved) vibrational energy levels. As expected, predictions of the Rittner electrostatic model agree well with the measured dissociation energies of the heavier metal chlorides, whereas the beryllium and magnesium chlorides must be relatively covalent.