Gaseous Vanadium Molybdate\nand Tungstates: Thermodynamic\nProperties and Structures

Abstract

The stability of gaseous vanadium molybdate and vanadium\ntungstates\nwas confirmed by high-temperature mass spectrometry. A number of gas-phase\nreactions involving vanadium-containing salts were studied. On the\nbasis of equilibrium constants, the standard formation enthalpies\nof gaseous VMoO₄ (−676 ± 27 kJ/mol), VWO₃ (−331 ± 29 kJ/mol), and VWO₄ (−706\n± 23 kJ/mol) at 298 K were determined. A theoretical study of\nthese salts revealed the structure with bidentate binding of the vanadium\ncation to the anion part to be the lowest-lying isomer, with a quartet\nspin state for VMoO₄ and VWO₄ molecules as well\nas a sextet spin state for the VWO₃ molecule. On the basis\nof critical analysis of the literature data concerning standard formation\nenthalpies of gaseous VO and VO₂, we adopted new values\nof Δ_f<i>H</i>°(298) = 135 ± 10\nkJ/mol for VO­(g) and −185 ± 15.0 kJ/mol for VO₂(g). Overall, the results obtained allowed us to estimate the standard\nformation enthalpy of VMoO₃ to be −318 kJ/mol with\nan accuracy near 40 kJ/mol.