Establishing the gas phase dimerization of niobium(V) fluoride and tantalum(V) fluoride by quantitative Raman spectroscopy

The vapors of NbF5 and TaF5 have been investigated by Raman spectroscopy in the temperature range 475-675 K and at total pressures from similar to0.1 to similar to4.0 atm. The temperature and pressure dependence of the spectra is indicative of equilibrium shifts between monomer and associated (polymer) gaseous species. The intensities of Raman bands, which were characteristic of the various species present, were exploited in order to establish the stoichiometry and thermodynamic functions of the reaction nMF(5)(g) reversible arrow MnF5n(g) (M = Nb, Ta). It was found that unambiguously n = 2 in the studied temperature range, which implied that the predominant vapor species are monomers MF5(g) and dimers M2F10(g). The thermodynamic functions according to the studied dimerization reactions were measured from the Raman data as: (a) DeltaH(R) = - 104.3 +/- 2.4 kJ mol(-1), ASR = - 175 +/- 20 J mol(-1) K-1 for 2NbF(5)(g) : Nb2F10(g); (b) DeltaH(R) = -101.1 +/- 3.7 kJ mol(-1), DeltaS(R) = -150 +/- 30 J mol(-1) K-1 for 2TaF(5)(g) reversible arrow Ta2F10(g). (C) 2004 Elsevier B.V. All rights reserved.