Chemical bonding and dynamic fluxionality of a B15+ cluster: a nanoscale double-axle tank tread

Abstract

A planar, elongated B₁₅⁺ cationic cluster is shown to be structurally fluxional and functions as a nanoscale tank tread on the basis of electronic structure calculations, bonding analyses, and molecular dynamics simulations. The outer B₁₁ peripheral ring behaves like a flexible chain gliding around an inner B₄ rhombus core, almost freely at the temperature of 500 K. The rotational energy barrier is only 1.37 kcal mol⁻¹ (0.06 eV) at the PBE0/6-311+G* level, further refined to 1.66 kcal mol⁻¹ (0.07 eV) at the single-point CCSD(T)/6-311G*//CCSD/6-311G* level. Two soft vibrational modes of 166.3 and 258.3 cm⁻¹ are associated with the rotation, serving as double engines for the system. Bonding analysis suggests that the “island” electron clouds, both σ and π, between the peripheral ring and inner core flow and shift continuously during the intramolecular rotation, facilitating the dynamic fluxionality of the system with a small rotational barrier. The B₁₅⁺ cluster, roughly 0.6 nm in dimension, is the first double-axle nanoscale tank tread equipped with two engines, which expands the concepts of molecular wheels, Wankel motors, and molecular tanks.