Thermodynamic investigation of the gaseous molecules TiRh, Rh2, and Ti2Rh by mass spectrometry

Abstract

The high‐temperature Knudsen effusion mass spectrometric technique has been used to determine the thermodynamic properties of the new molecules TiRh, Rh₂, and Ti₂Rh. The atomization energies D°₀ [TiRh(g)], D°₀[Rh₂(g)], and ΔH°_atm,0[Ti₂Rh(g)] were obtained as 92.5±3.5, 67.3±5.0, and ≤238±10 kcal mol⁻¹ or 387.0±14.6, 281.6±20.9, and ≤995.8±41.8 kJ mol⁻¹. The corresponding standard heats of formation ΔH°_f,298 for the molecules TiRh(g) and Rh₂(g) were derived as 151.0±3.6 kcal (631.8±15.1 kJ) and 196.5±5.0 kcal (822.2±20.9 kJ) mol⁻¹, respectively. These properties were derived from the enthalpies of the following reactions: $$\begin{array}{cccc}& \Delta \mathrm{H}{°}_0& & \\ & \mathrm{kcal}& \mathrm{or}& \mathrm{kJ}\\ \mathrm{RhC(g)}+\mathrm{Ti(g)}=\mathrm{TiRh(g)}+\mathrm{C(g)}& \text{46.9±3.0}& & \text{196.2±12.6}\\ {\mathrm{TiC}}_2\mathrm{(g)}+\mathrm{Rh(g)}=\mathrm{TiRh(g)}+{\mathrm{C}}_2\mathrm{(g)}& \text{41.5±4.0}& & \text{173.6±16.7}\\ \mathrm{TiRh(g)}+\mathrm{C(graph)}=\mathrm{RhC(g)}+\mathrm{Ti(g)}& \text{124.7±3.0}& & \text{521.7±12.6}\\ \mathrm{TiRh(g)}=\mathrm{Rh(g)}+\mathrm{Ti(g)}& \text{91.9±3.0}& & \text{384.5±12.6}\\ {\mathrm{Rh}}_2\mathrm{(g)}+\mathrm{C(g)}=\mathrm{RhC(g)}+\mathrm{Rh(g)}& \text{−71.7±5.0}& & \text{−300.0±20.9}\\ {\mathrm{Rh}}_2\mathrm{(g)}\text{+2}\mathrm{C(graph)}=\mathrm{2RhC(g)}& \text{133.3±6.0}& & \text{557.7±25.1}\\ {\mathrm{Rh}}_2\mathrm{(g)}=\mathrm{2Rh(g)}& \text{66.9±5.0}& & \text{279.9±20.9}\\ {\mathrm{Ti}}_2\mathrm{Rh(g)}+\mathrm{Rh(g)}=\mathrm{TiRh(g)}& \text{53±8}& & \text{221.8±33.5}\end{array}$$ The determination of the dissociation energy of the rhodium dimer allows the bond energies of TiRh and Ti₂Rh to be discussed in terms of the Pauling model of a polar bond. The high bond energies of the molecules support the extention of the Brewer‐Engel theory to the gas phase.