Thermochemistry of the Gaseous Vanadium Chlorides VCl, VCl2, VCl3, and VCl4

Abstract

Gaseous equilibria in the V−Ag−Cl system were studied at elevated temperatures by effusion-beam mass spectrometry, where the pertinent species were generated by reaction of Cl₂(g) with V + Ag granules in the effusion cell source. Reaction enthalpies were derived from the equilibrium data, and the standard enthalpies of formation at 298 K of gaseous VCl, VCl₂, and VCl₃ were found to be +49.7, −34.8, and −85.6 kcal mol⁻¹, respectively. The corresponding bond dissociation energies at 298 K are D(V−Cl) = 102.9 kcal, D(ClV−Cl) = 113.5 kcal, D(Cl₂V−Cl) = 79.8 kcal, and D(Cl₃V−Cl) = 69.5 kcal. From these data, the dissociation energy D°₀(VCl) = 101.9 kcal mol⁻¹ or 4.42 eV is obtained. An alternate value, Δ_fH°₂₉₈(VCl₃,g) = −87.0 kcal mol⁻¹ was derived from third-law analysis of literature sublimation data for VCl₃(s). In addition, literature thermochemical data on VCl₄(g) were re-evaluated, leading to Δ_fH°₂₉₈ = −126.1 kcal mol⁻¹. The results are compared with various estimates in the literature.