Superhalogen Properties of Fluorinated Coinage Metal Clusters

Equilibrium geometries and ground state spin multiplicities of neutral and anionic coinage metal fluoride XFn clusters (X = Cu, Ag, and Au; n = 1-7) are obtained from density functional theory-based calculations. Our results show that in the case of neutral and anionic CuFn and AgFn clusters, a maximum of 4 F atoms (n(max) = 4) can be bound atomically to the metal atoms, while remaining F atoms bind to the other F atoms to form F-2 units. In contrast, a Au atom can bind up to six F atoms dissociatively. This contrasting binding scenario observed for these metal fluoride clusters is explained using the natural bond orbital analysis. The neutral XFn (X = Cu, Ag) clusters are stable against dissociation into X and F atoms up to n = 6, while AuFn clusters are stable up to n = 7. Similarly, with the exception of AgF7 and AuF6, all neutral clusters studied are stable against dissociation into F-2 molecules. On the other hand, XFn- clusters are stable against dissociation into F atoms and F-2 molecules over the entire size range, indicating the increased stability of anionic species over their neutral counterparts. Even more striking is the fact that the electron affinities of these clusters can be as large as 8 eV, far exceeding the electron affinity of Cl that has the highest value in the periodic table. These clusters are thus classified as superhalogens.