Thermochemical studies of the gaseous uranium chlorides

The gaseous uranium halides UCl, UCl₂, UCl₃, UCl₄, and UCl₅ were generated under equilibrium conditions using several types of effusive beam sources, and were identified and characterized thermochemically by mass spectrometry. Five independent gaseous equilibria involving these species were studied in sufficient depth so that both reaction enthalpy and entropy changes could be evaluated accurately from second law analysis. The enthalpy data yield the sequence of bond dissociation energies at 298 K in kcal/mol: D° (U–Cl)=108; D° (ClU–Cl)=117; D° (Cl₂U–Cl)=117; D° (Cl₃U–Cl)=101; and D° (Cl₄U–Cl)=50, all ±2 kcal/mol. As expected, the absolute entropies of the gaseous chlorides show that the electronic partition functions of the lighter species are substantial at the experimental temperatures, leading to electronic entropies of 5–7 cal/deg mol at about 2100 K.