Exploring the low-lying structures of Aun(CO)+ (n = 1–10): adsorption and stretching frequencies of CO on various coordination sites

Abstract

The low lying structures of Au_n(CO)⁺ (n = 1–10) within 1.0 eV from their global minima were explored using a structure searching program (Atomic Global Minimum Locator) and density functional theory (DFT) calculations. According to general chemical intuitions, CO should prefer to stay on the lowest coordination sites of gold frames. However, our calculations showed that this preference becomes very weak in large Au_n(CO)⁺, and even negligible for their compact three dimensional structures. This character relates to the intrinsic fluxionality of gold cluster frames, which apparently readjust their bond parameters after CO adsorption. The stretching frequency of a terminal-bonded CO decreases with increasing cluster size, and structural details of the adsorption sites have insignificant influences. The bridge-bonded COs, on the contrary, has far low stretching frequencies mainly determined by structural details of the two bridge sites. The frequency variation of CO relates to the electron donation and back-donation between gold and CO. Since all the above regularities on the adsorption and frequency of CO were summarized from large amount of low lying structures rather than some specific ones, they can be used to understand adsorption of CO on active sites of real gold catalysts, whose structures are diversiform and usually unclear.