Two low-lying structures are unveiled for the Be B nanocluster system that are virtually isoenergetic. The first, triple-layered cluster has a peripheral B ring as central layer, being sandwiched by two Be rings in a coaxial fashion, albeit with no discernible interlayer Be-Be bonding. The B ring revolves like a flexible chain even at room temperature, gliding freely around the Be prism. At elevated temperatures (1000 K), the Be core itself also rotates; that is, two Be rings undergo relative rotation or twisting with respect to each other. Bonding analyses suggest four-fold (π and σ) aromaticity, offering a dilute and fluxional electron cloud that lubricates the dynamics. The second, helix-type cluster contains a B helical skeleton encompassing a distorted Be prism. It is chiral and is the first nanosystem with a boron helix. Molecular dynamics also shows that at high temperature the helix cluster readily converts into the triple-layered one.
Planar or tetrahedral? Global searches discover a 17-electron CBe5H4+ cluster with planar pentacoordinate carbon. The 18-electron neutral features a tetrahedral carbon.
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.
Boron clusters have been found to exhibit a variety of interesting electronic, structural, and bonding properties. Of particular interest are the recent discoveries of the 2D hexagonal B which led to the concept of borophenes and the 3D fullerene-like B which marked the onset of borospherene chemistry. Here, we present a joint photoelectron spectroscopic and first-principles study of B and B, which are in the transition size range between the 2D borophene-type clusters and the 3D borospherenes. These two clusters are found to possess highly stable 2D global-minimum structures consisting of a double-hexagonal vacancy. Detailed bonding analyses reveal that both B and B are all-boron analogues of coronene (CH) with a unique delocalized π system, featuring dual π aromaticity. These clusters with double hexagonal vacancies can be viewed as molecular motifs for the χ3-borophene which is the most stable form of borophenes recently synthesized on an Ag(111) substrate.
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