Dynamic Mg₂B₈ Cluster: A Nanoscale Compass

Abstract

Boron-based binary cluster Mg₂ B₈ is shown to adopt a compass-like structure via computational global searches, featuring an Mg₂ dimer as the needle and a disk-shaped B₈ molecular wheel as baseplate. The nanocompass has a diameter of 0.35 nm. Born-Oppenheimer molecular dynamics simulations indicate that Mg₂ B₈ is structurally fluxional with the needle rotating freely on the baseplate, analogous to a functioning compass. The dynamics is readily initiated via a ultrasoft vibrational mode. The rotational barrier is only 0.1 kcal mol^-1 at the single-point CCSD(T) level. Chemical bonding analysis suggests that the cluster compass can be formulated as [Mg₂ ]²⁺ [B₈ ]^2- ; that is, the baseplate and the needle are held together primarily through ionic interactions. The baseplate is doubly aromatic with π and σ sextets. The bonding pattern provides a dilute, continuous, and delocalized electron cloud, which underlies the dynamics of the nanocompass.