Experimental and theory studies of the oxidation reaction of neutral gold carbonyl clusters in the gas phase

Neutral gold carbonyl clusters, Aum(CO)n (m = 3–9, n = 2–7, m ≥ n), are generated by laser ablation of Au into a mixture of CO/He, cooled in a supersonic expansion, and reacted with O2 and N2O in a fast flow reactor. The neutral reactants and products are detected in a time of flight mass spectrometer through single photon ionization by a 193 nm laser. Signal intensities of Au3(CO)2,3, Au5(CO)4, and Au7(CO)4,5 decrease significantly following reaction of these clusters with O2 in the fast flow reactor; only Au3(CO)2 and Au3(CO)3 signals decrease moderately following reaction with N2O. The reaction cross section for Aum(CO)n with N2O is significantly smaller than that with O2. Density functional theory calculations with and without explicit consideration of relativistic effects are performed to investigate the reaction mechanisms for the oxidation of Au3(CO)2 and Au3(CO)3 clusters with O2 and N2O. Both calculational algorithms predict a considerable barrier for the reactions of Au3(CO)2,3 with N2O. Non relativistic density functional theory calculations predict a positive overall barrier for the reactions of Au3(CO)2,3 with O2, in disagreement with experimental observations. Relativistic density functional theory calculations for the reactions Au3(CO)2,3 with O2 predict that they are thermodynamically allowed, although the barrier heights are not in the appropriate order to support the apparent relative reactivities of Au3(CO)2 and Au3(CO)3 with O2.