Starting a subnanoscale tank tread: dynamic fluxionality of boron-based BCa alloy cluster.

Alloying an elongated B cluster with Ca is shown to give rise to a dynamically fluxional BCa cluster, the latter behaving like a tank tread at the subnanoscale. Computer global search identifies the BCa (A) global-minimum structure, which is chiral in nature and retains the quasi-planar moiety of bare B cluster with Ca capped at one side, forming a half-sandwich. The rotation barrier of BCa cluster is reduced with respect to B by one order of magnitude, down to 1 kcal mol at the PBE0/6-311+G* level, which demonstrates structural fluxionality at 600 K and beyond molecular dynamics simulations. Structurewise, the Ca alloying in BCa cluster generates rhombic defect holes, preactivating the species and making it flexible against deformation. Chemical bonding analyses indicate that the BCa cluster is a charge-transfer [B][Ca] complex, being doubly π/σ aromatic with the 6π and 10σ electron-counting. Such a pattern offers ideal π/σ delocalization and facilitates fluxionality. In contrast, bare B cluster has conflicting aromaticity with 6π and 8σ electrons, which is nonfluxional with a barrier of 12 kcal mol. Double π/σ aromaticity conflicting aromaticity is a key mechanism that distinguishes between fluxional BCa and nonfluxional B clusters, offering a compelling example that the concept of aromaticity (and double aromaticity) can be exploited to design dynamically fluxional nanosystems.