Ab Initio Benchmarking on Unsupported and Coronene‐Supported Silver Atomic Clusters: Interplay Between Jahn–Teller Distortions and Fluxionality

Advances in highly selective experimental techniques are allowing for the synthesis of monodisperse, atomically precise metal clusters, composed of just one to tens of atoms. Exhibiting strong quantum confinement effects, molecular‐like structures and discrete energy levels, these ultrasmall metal clusters feature unexpected stability and unique physical and chemical properties with relevance in nanomaterials science and bio‐nanotechnology. For instance, TiO 2 ‐supported silver clusters of five atoms (Ag 5 ) stabilize surface polarons, with the possibility of enhancing redox reactions, while ligand‐free Ag 5 clusters act as highly selective antitumoral agents for cancer therapy. In this work, high‐level ab initio benchmarking is applied to Ag n ( n = 2, 3, 5) clusters isolated in gas phase using standard and explicitly correlated‐coupled‐cluster theory with single, doubles, and perturbative triple excitations CCSD(T) and CCSD(T)‐F12 as reference methods, along with experimentally determined vibrational frequencies. The use of the multistate multireference (internally contracted) Rayleigh–Schrödinger second‐order perturbation (RS2C) method enables characterization of conical intersections (CIs) leading to Jahn–Teller (JT) distortions. Bipyramidal and pyramidal Ag 5 clusters are thus identified as fluxional JT molecules. The performance of a selected dispersion‐corrected density functional theory method is next evaluated and used to obtain the infrared (IR) spectra of coronene‐supported Ag 5 clusters with 2D and 3D structures. In this way, a transition from JT to Pseudo‐JT distortions is identified in Ag 5 (pyramidal)–coronene complexes. Finally, the article discusses how JT and Pseudo‐JT distortions are intimately connected to the fluxionality of atomic silver clusters, providing them with a “floppy” character that not only may facilitate their interaction with surrounding molecules but also can play a key role in their photostability.