HIGH-TEMPERATURE EQUILIBRIUM STUDIES OF THE GASEOUS THORIUM CHLORIDES

High-temperature gaseous equilibria among the species Th, ThCl, ThCl 2, and ThCl3 were studied by mass spectrometry over broad temperature ranges; both enthalpy and entropy data were derived by the second law procedure. In addition, the sublimation of ThCl4 was studied by the torsion-effusion method and the thermodynamics of the gaseous tetrachloride were evaluated. Individual bond dissociation energies in ThCl4 fall within the range 489-545 kJ mol-1, and vary with ligand number in a way similar to ThF4 Only ThCl appears to have a significant electronic entropy in the experimental temperature range. For ThCl4 the experimental entropy indicates the likelihood of a regular tetrahedral structure as with ThF4, but unlike the uranium tetrahalides which appear to be distorted tetrahedra. In the mass spectrum of ThCl4, the fragment ions ThCl3+, ThCl2+, and ThCl+ show a larger and more nearly correct temperature dependence than the parent ion ThCl4+. Results are compared with those of other thorium and uranium halides. © 1990 American Institute of Physics.