The AuSc, AuTi, and AuFe molecules: Determination of the bond energies by Knudsen effusion mass spectrometry experiments combined with ab initio calculations

The AuTi gaseous molecule was for the first time identified in vapors produced at high temperature from a gold–titanium alloy. The homogeneous equilibria AuTi(g) = Au(g) + Ti(g) (direct dissociation) and AuTi(g) + Au(g) = Au₂(g) + Ti(g) (isomolecular exchange) were studied by Knudsen effusion mass spectrometry in the temperature range 2111–2229 K. The so determined equilibrium constants were treated by the “third-law method” of thermodynamic analysis, integrated with theoretical calculations, and the dissociation energy at 0 K was derived as $D0K°$ (AuTi) = 241.0 ± 5.2 kJ/mol. A similar investigation was carried out for the AuSc and AuFe species, whose dissociation energies were previously reported with large uncertainties. The direct dissociation and the isomolecular exchange with the Au₂ dimer were studied in the 1969–2274 and 1842–2092 K ranges for AuSc and AuFe, respectively, and the dissociation energies derived as $D0K°$ (AuSc) = 240.4 ± 6.0 and $D0K°$ (AuFe) = 186.2 ± 4.2 kJ/mol. The experimental bond energies are compared with those calculated here by coupled cluster with single, double, and perturbative triple excitations with the correlation-consistent basis sets cc-pVXZ(-PP) and cc-pwCVXZ(-PP) (with X = T, Q, 5), also in the limit of complete basis set, and with those from complete active space self-consistent field-multi-reference configuration interaction calculations, recently available in the literature. The stronger bond of AuTi compared to AuFe parallels the trend observed in monochlorides. This analogy is shown to be more generally observed in the AuM and MCl diatomic series (with M = first row transition metal), in accordance with a picture of “pseudo-halogen” bonding behavior of gold.