Chemical Bonding and Dynamic Structural Fluxionality of a Boron-Based B₈Al₃⁺ Cluster

We studied the boron-based composite cluster B₈Al₃⁺ doped with Al atoms. The global minimum structure of the B₈Al₃⁺ cluster is a three-layer structure, consisting of three parts: an Al₂ unit, a B₈ ring and an isolated Al atom. Charge calculations analysis shows that the cluster can be expressed as [Al]⁺[B₈]^2-[Al₂]²⁺, has 6π/6σ double aromaticity and follows the (4n+2) Hückel rule. Born-Oppenheimer molecular dynamics (BOMD) simulation shows that the B₈Al₃⁺ cluster has dynamic fluxionality properties. Remarkably, at the single-point coupled cluster singles, doubles and triples (CCSD(T)) level, the energy barrier for intramolecular rotation is merely 0.19 kcal mol^-1. [B₈]^2- molecular wheels have magical 6π/6σ double aromaticity properties, providing a continuous cloud of delocalized electrons, which is a key factor in the dynamic fluxionality of the cluster. The B₈Al₃⁺ cluster provides a new example of dynamic structural fluxionality in molecular systems.