On the origin of bonding and vibrational frequency shifts for CO adsorbed on neutral, cationic and anionic gold clusters

We report a detailed analysis of the electronic mechanisms which determine the bond strength and the vibrational frequency of CO molecules adsorbed on neutral or charged gold nanoparticles. To this end we have considered a simple cluster model, Au5COq (q = + 1, 0, -1), and decomposed the Au-CO interaction energy into the sum of various contributions according to a Constrained Space Orbital Variation approach. While the adsorption energy is relatively insensitive to the value of q, the C-O stretch frequency, omega(e)(CO), changes substantially, and allows the use of this molecule as a direct probe of the gold oxidation state. The results show that two major terms contribute to the red or blue shift of omega(e)(CO) as a function of q: the interaction with the electric field associated to the charged nanoparticle (Stark effect) and the Au -> CO pi back donation. The CO -> Au sigma donation is about half as important as the pi back-donation and all other terms are much less important.