Observation and characterization of the smallest borospherene, B28- and B28

Free-standing boron nanocages or borospherenes have been observed recently for B-40(-) and B-40. There is evidence that a family of borospherenes may exist. However, the smallest borospherene is still not known. Here, we report experimental and computational evidence of a seashell-like borospherene cage for B-28(-) and B-28. Photoelectron spectrum of B-28(-) indicated contributions from different isomers. Theoretical calculations showed that the seashell-like B-28(-) borospherene is competing for the global minimum with a planar isomer and it is shown to be present in the cluster beam, contributing to the observed photoelectron spectrum. The seashell structure is found to be the global minimum for neutral B-28 and the B-28(-) cage represents the smallest borospherene observed to date. It is composed of two triangular close-packed B-15 sheets, interconnected via the three corners by sharing two boron atoms. The B-28 borospherene was found to obey the 2(n + 1)(2) electron-counting rule for spherical aromaticity. (C) 2016 AIP Publishing LLC.