Au₂₆: a case of fluxionality/co-existence

Abstract

The Au26 cluster is one of the widely studied gold clusters in the size range of n = 21-30. It has been proposed in a more recent combined experimental and theoretical study that the neutral Au26 cluster is fluxional. The fluxionality of a cluster is relevant to its catalytic applications. In this context, to explore the extent of fluxionality, Born Oppenheimer Molecular Dynamical (BOMD) simulations are carried out on experimentally and theoretically proposed fluxional Au26 conformations (three compact or core-shell structures and a high symmetry cage structure). The simulations reveal that the high energy golden tube outperforms the ground state structure (compact C2v conformation) as well as the other two low-symmetry compact conformations in terms of thermal stability. The enhancement in the thermal stability is explained on the basis of structural integrity imposed by the open skeleton of shortest bond distances within Au26-Tube. In addition to this, the homogeneous distribution of charges and the strong s-d hybridization exhibited by FMOs are seen to play a pivotal role in increasing the stability of Au26-Tube. The present investigation also reveals that the characteristic fluxionality proposed to exist in the Au26 system is noted only above 400 K and it is missing at room temperature. The simulations also bring forth the question of how relevant a ground state conformation is at working temperatures.

Structural Evolution and Stability Trend of Small-Sized Gold Clusters Aun (n = 20–30)

“…Noticeably, the shell-like flat-cage motif no longer dominates its lower-lying population as in the Au 24 and Au 25 systems. Instead, the cluster prefers a C 2 v structure 26_1 (Figure ) with a single atom sitting inside the cage, as recently reported …”

“…While the structures of the small pure Au n clusters in the range of n = 2–20 and their growth pattern are now well characterized, − those of the larger systems with n > 20 become more challenging due to their structural heterogeneity, and the most stable forms of several sizes are not elucidated yet. Identification of the equilibrium geometries of elemental clusters has long been and still is a thought-provoking but a vital task in the field of cluster science.…”

“…Moreover, previous studies demonstrated that a structural identification of Au n in the size range from n = 21 to 30 is an extremely challenging task due to the existence of multiple distinct isomers that have degenerate energies, and thus they may coexist as competing global minima. A case in point for such a phenomenon is Au 26 , which has been found to exhibit a variety of intriguing atomic structures, namely a high-symmetry hollow tube, an fcc-like fragment, , and a pyramidal motif . Earlier studies also predicted that both Au 27 and Au 28 clusters, as in the case of Au 26 , prefer tube-like configurations. , However, a recent theoretical study reported that the trigonal bipyramid (TBP) motifs are more energetically favorable in these neutral states .…”

Mapping the Finite-Temperature Behavior of Conformations to Their Potential Energy Barriers: Case Studies on Si6B and Si5B Clusters

“…Based on this surprising revelation, a flurry of computational simulations based on Born–Oppenheimer molecular dynamics followed in attempting to identify the origin of this unanticipated finite-temperature behavior. − Simulations that followed over the years attempted to discern the conformational dynamics followed by the various-sized clusters and the origin of their enhanced thermal stabilities. These computational studies also further revealed that the shape- and size-specific clusters of Au and Sn have enhanced thermal stability as compared to their bulk counterparts, thereby indicating it to be a more general property than thought of among the atomic clusters. , Along with these newer aspects, detailed simulations also highlighted interesting aspects such as the fluxionality of clusters and/or role of factors such as charge in enhancing the stability of a given conformation at finite temperatures. − Several studies continue to follow where the thermal stability of a given conformation or alloy is explored using first-principles-based molecular dynamics. − …”

Another look at energetically quasi‐degenerate structures of the gold cluster Au27q with q = 1, 0, −1

“…Hence, the search for the lowest‐energy geometrical structure is an initial and pivotal step in understanding and utilizing its properties. Although the geometries of the small gold Au n clusters in the size range of n = 2–20 are now well‐characterized, 12–16 structural assignments for the larger clusters, viz., Au n with n > 20, turned out to be a challenging task due to their structural heterogeneity, 17 for which different density functionals tend to generally predict both quantitatively and qualitatively different topologies of the same potential energy surface. For instance, the Au 27 system represents an instructive case in this respect.…”