A combined photoelectron spectroscopy and ab initio study of the quasi-planar B24- cluster

The structure and chemical bonding of the 24-atom boron cluster are investigated using photoelectron spectroscopy and ab initio calculations. The joint experimental and theoretical investigation shows that B-24(-) possesses a quasi-planar structure containing fifteen outer and nine inner atoms with six of the inner atoms forming a filled pentagonal moiety. The central atom of the pentagonal moiety is puckered out of plane by 0.9 angstrom, reminiscent of the six-atom pentagonal caps of the well-known B-12 icosahedral unit. The next closest isomer at the ROCCSD(T) level of theory has a tubular double-ring structure. Comparison of the simulated spectra with the experimental data shows that the global minimum quasi-planar B-24(-) isomer is the major contributor to the observed photoelectron spectrum, while the tubular isomer has no contribution to the experiment. Chemical bonding analyses reveal that the periphery of the quasi-planar B-24 constitutes 15 classical 2c-2e B-B sigma-bonds, whereas delocalized sigma- and pi-bonds are found in the interior of the cluster with one unique 6c-2e pi-bond responsible for bonding in the B-centered pentagon. The current work suggests that the 24-atom boron cluster continues to be quasi-2D, albeit the tendency to form filled pentagonal units, characteristic of 3D cage-like structures of bulk boron, is observed. (C) 2013 AIP Publishing LLC.