Acioli, P.H., Burkland, S. & Srinivas, S. Eur. Phys. J. D (2012) 66: 215. doi:10.1140/epjd/e2012-30136-y
In a recent study [P.H. Acioli, N. Ratanavade, M.R. Cline, S. Srinivas, Lect. Notes Comput. Sci. 5545, 203 (2009)] of the interaction of small silver clusters (Agn, n = 1–4) with carbon monoxide we have found that the CO molecule can bond with the cluster either in a bent or in a linear configuration with respect to the silver carbon bond. These trends were explained by the interaction of the highest occupied molecular orbital (HOMO) of the cluster and the antibonding (π∗) orbital, the lowest unoccupied molecular orbital (LUMO) of CO. For a σ-type orbital of the cluster the CO molecule is bent with respect to the Ag-C bond, while for a π-type HOMO the CO molecule is aligned with respect to the Ag-C bond. These trends tend to maximize the overlap of the CO molecule’s LUMO with the cluster’s HOMO. Furthermore, the CO molecules have a tendency to bond atop an atom rather than on bridge or face sites. In the present work we extend the investigation to clusters of up to seven atoms. The focus of this paper is on the 7-atom silver cluster which shows interesting complexities in that the cluster is characterized by a π-like HOMO but has the CO bonded to a waist atom of the pentagonal bi-pyramid and bent with respect to the Ag-C bond, thus breaking the previously observed trend. In this work we provide an analysis of the potential energy surface of the CO bonded to Ag7 and explain why the bonding differs from those of the smaller clusters. We find that the bonding is still explained by a π-backdonation process. However, unlike the lowest size clusters there is an increase in overlap through bending and the complex prefers this conformation, rather than a linear Ag-C-O configuration.