Coaxial Triple-Layered versus Helical Be6B11- Cluster: Dual Structural Fluxionality and Multifold Aromaticity

Two novel low-lying structures are unveiled for the Be6B11- nanocluster system: a triple-layered isomer versus a boron helix-type one, which are virtually isoenergetic and the former apes the motions of the earth-moon system. For the triple-layered cluster the B11 ring revolves like a flexible chain at room temperature, gliding freely around the Be6 prism. At elevated temperatures (1000 K), the Be6 core itself also rotates. The rotational movement has a higher energy barrier than revolution: 4.70 versus 0.22 kcal mol-1 at single-point CCSD(T). Bonding analyses suggest four-fold (π and σ) aromaticity, unprecedented for a molecular system, which offer a dilute and fluxional electron cloud that lubricates the dynamics. In contrast, the helix-type cluster contains a B11 helical skeleton encompassing a distorted Be6 prism. A large electron transfer occurs from Be6 to boron, making a boron core of formal [B11]9− charge state or higher, which presumably underlies the helical arrangement.