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 (KEMS) 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 D_(0 K)^° (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 D_(0 K)^° (AuSc) = 240.4 ± 6.0 and D_(0 K)^° (AuFe) = 186.2 ± 4.2 kJ/mol. The experimental bond energies are compared with those calculated here by CCSD(T) 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 CASSCF-MRCI 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 behaviour of gold.