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

Abstract

Neutral gold carbonyl clusters, Au m (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 O 2 and N 2O 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 Au 3(CO) 2,3, Au 5(CO) 4, and Au 7(CO) 4,5 decrease significantly following reaction of these clusters with O 2 in the fast flow reactor; only Au 3(CO) 2 and Au 3(CO) 3 signals decrease moderately following reaction with N 2O. The reaction cross section for Au m (CO) n with N 2O is significantly smaller than that with O 2. Density functional theory calculations with and without explicit consideration of relativistic effects are performed to investigate the reaction mechanisms for the oxidation of Au 3(CO) 2 and Au 3(CO) 3 clusters with O 2 and N 2O. Both calculational algorithms predict a considerable barrier for the reactions of Au 3(CO) 2,3 with N 2O. Non relativistic density functional theory calculations predict a positive overall barrier for the reactions of Au 3(CO) 2,3 with O 2, in disagreement with experimental observations. Relativistic density functional theory calculations for the reactions Au 3(CO) 2,3 with O 2 predict that they are thermodynamically allowed, although the barrier heights are not in the appropriate order to support the apparent relative reactivities of Au 3(CO) 2 and Au 3(CO) 3 with O 2.