Probing the Interactions of O2 with Small Gold Cluster Anions (Aun-, n=1-7): Chemisorption vs Physisorption

Activation of O-2 is the most critical step in catalytic oxidation reactions involving gold and remains poorly understood. Here we report a systematic investigation of the interactions between O-2 and small gold cluster anions Au-n(-) (n = 1-7) using photoelectron spectroscopy. Higher resolution photoelectron spectra are obtained for the molecularly chemisorbed even-sized AunO2- (n = 2, 4, 6) complexes. Well-resolved vibrational structures due to O-O stretching are observed and can be readily distinguished from the Au-derived PES bands. The adiabatic detachment energies and O-O vibrational frequencies are measured to be 3.03 +/- 0.04, 3.53 +/- 0.05, and 3.17 +/- 0.05 eV, and 1360 +/- 80, 1360 +/- 80, and 1330 +/- 80 cm(-1) for n = 2, 4, 6, respectively. Physisorbed Au-n(-)(O-2) complexes for n = 1, 3, 5, 7 are observed for the first time, providing direct evidence for the inertness of the closed-shell odd-sized Au-n(-) clusters toward O-2. Neutral even-sized Au-n clusters are closed-shell and are expected to be inert toward O-2, which is not consistent with the reduced O-O vibrational frequencies observed in the photoelectron spectra relative to free O-2. It is suggested that the photodetachment transitions can only access excited states of the neutral even-sized AunO2 complexes; a double-well potential is proposed consisting of the ground-state van der Walls well at long Au-n-O-2 distances and a higher energy deeper well at short Au-n-O-2 distances derived from singlet O-2 ((1)Delta(g)). The current study provides further insight into O-2 interactions with small gold clusters, as well as accurate experimental data to benchmark theoretical investigations.