Gaseous Metal Nitrides. III. On the Dissociation Energy of Thorium Mononitride and Predicted Dissociation Energies of Diatomic Group III–VI Transition-Metal Nitrides

Mass‐spectrometric evidence for the existence of gaseous thorium mononitride over a nitrogen‐containing thorium–boron–phosphorus alloy has been obtained. The enthalpies ${\mathrm{\Delta H°}}_{298}$ of the reactions ThN(g) = Th(g) + 0.5N₂(g) and ThP(g) + 0.5N₂(g) = ThN(g) + 0.5P₂(g) have been calculated from the experimentally determined partial pressures of the reactants as 23.1 ± 6.0 and 7.0 ± 5.0 kcal/mole. Combined with published thermodynamic data these reaction enthalpies yield the respective dissociation energy ${\mathrm{D°}}_{298}$ , the heat of formation, ${\mathrm{\Delta H°}}_{\text{f,298}}$ , and the heat of sublimation, ${\mathrm{\Delta H°}}_{\text{s,298}}$ of 138.0 ± 8, 112.5 ± 7.5, and 203.1 ± 10 kcal/mole for gaseous ThN. The dissociation energies of the diatomic nitrides of the Group III–VI transition metals have been estimated on the basis of empirical correlations by Colin and Goldfinger and by Pauling. The values, which range from 96 kcal/mole for CrN to 145 kcal/mole for TaN, are explained in terms of a strong ionic contribution to the bonding. On the basis of the calculated dissociation energies and published thermodynamic data, the corresponding heats of formation and heats of sublimation have also been estimated.