Formation and thermochemical properties of oxychlorides of niobium (Nb) and tantalum (Ta): Towards the gas-phase investigation of dubnium (Db) oxychloride

The formation of NbOCl₃ and TaOCl₃ and their adsorption behavior on quartz surfaces was explored by applying an isothermal gas-chromatographic method. Trace amounts of short-lived Nb and Ta isotopes were used. Adsorption enthalpy values (ΔH_ads) at zero surface coverage of –ΔH_ads(NbOCl₃) = 102 ± 4 kJ/mol and –ΔH_ads(TaOCl₃) = 128 ± 5 kJ/mol were determined by analyzing the chromatographic behavior of the Nb and Ta complexes with a Monte-Carlo simulation method based on an adsorption–desorption kinetic model. By applying an empirical correlation, the experimental ΔH_ads values were successively related to the macroscopic standard sublimation enthalpy, ΔH^°_subl, as a measure of the volatility of each substance. The inferred sublimation enthalpies are in agreement with tabulated thermochemical values. Thus, the linear empirical correlation between ΔH_ads and ΔH^°_subl for metal-oxychlorides was updated with the inclusion of the present data. According to the predicted ΔH^°_subl(DbOCl₃), a ΔH_ads(DbOCl₃) value of 135 ± 2 kJ/mol was extrapolated. The future accomplishment of comparative studies with DbOCl₃ under the same experimental conditions will provide valuable information on the volatility trend in Group-5 elements, together with an indication on the magnitude of relativistic effects on the electronic structure of dubnium.