THE ENTROPIES AND PROBABLE SYMMETRIES OF THE GASEOUS THORIUM AND URANIUM TETRAHALIDES

The sublimation entropies of the thorium and uranium tetrahalides (except UI4) have been evaluated from torsion-effusion vapor pressure measurements and the results have been analyzed for compatibility with the spectroscopic and molecular constants of the gaseous molecules. New effusion pressure measurements are reported here for UCl4, UBr4, and ThI4. For UF4, UCl4, and ThF4, where thermal data are available for the solid phases, the experimental entropies of gaseous UF4 and UCl4 are incompatible with regular tetrahedral symmetries, while that of ThF4 is in close accord with a T(d) structure. A comparison of the entropies of sublimation shows that ThCl4 most likely has T(d) symmetry, while UBr4, ThBr4, and ThI4 probably do not; less-reliable total entropy data for the species are in full accord with these conclusions. The results are discussed in terms of structural information in the literature.