Thermochemical studies of the gaseous lower-valent fluorides of molybdenum

Abstract

Equilibria among the gaseous molybdenum fluorides were studied by high temperature mass spectrometry, and the results were used to derive thermochemical heats of formation and bond dissociation energies for the gaseous species MoF, MoF₂, MoF₃, MoF₄, and MoF₅. Effusion beams containing the gaseous fluorides were generated by the reaction of SF₆ or MoF₆ with elemental Mo at temperatures of 1100–2200 K, and the thermochemical results were derived from a second‐law analysis of the equilibrium data. Values of the standard heats of formation, ΔHf °₂₉₈, obtained are as follows: for MoF, 65.0±2.2 kcal/mol; MoF₂, −40.2±2.9 kcal/mol; MoF₃, −141.5±3.5 kcal/mol; MoF₄, −228.0±3.9 kcal/mol; and MoF₅, −296.7±8.6 kcal/mol. The value for MoF yields the dissociation energy D⁰₀(MoF) =110.3±2.2 kcal/mol (4.78±0.10 eV). Estimated spectroscopic and molecular constants are consistent with the gaseous equilibrium data, indicating that assumptions about the derivation of equilibrium constants from ion currents are reasonable. ]