Competition between quasi-planar and cage-like structures in the B₂₉^- cluster: photoelectron spectroscopy and ab...

Size-selected boron clusters have been found to be predominantly planar or quasi-planar (2D) in the small size regime with the appearance of three-dimensional (3D) borospherene cages of larger sizes. A seashell-like B₂₈^- cluster was previously shown to be the smallest borospherene, which competes with a quasi-planar isomer for the global minimum. Here we report a study on the structures and bonding of the B₂₉^- and B₂₉ clusters using photoelectron spectroscopy (PES) and first-principles calculations and demonstrate the continued competition between the 2D and borospherene structures. The PES spectrum of B₂₉^- displays a complex pattern with evidence of low-lying isomers. Global-minimum searches and extensive theoretical calculations revealed a complicated potential energy surface for B₂₉^- with five low-lying isomers, among which the lowest three were shown to contribute to the experimental spectrum. A 3D seashell-like C_s (2, ¹A') isomer, featuring two heptagons on the waist and one octagon at the bottom, is the global minimum for B₂₉^-, followed by a 2D C₁ (3, ¹A) isomer with a hexagonal hole and a stingray-shaped 2D C_s (1, ¹A') isomer with a pentagonal hole. However, by taking into account the entropic effects, the stingray-shaped isomer 1 was shown to be the lowest in energy at room temperature and was found to dominate the PES spectrum. Isomers 2 and 3, which have lower electron binding energies, were also found to be present in the experiment. Chemical bonding analyses showed that isomer 1 is an all-boron analogue of benzo[ghi]fluoranthene (C₁₈H₁₀), whereas the borospherene isomer 2 possesses 18π electrons, conforming to the 2(N + 1)² electron counting rule for spherical aromaticity. For the B₂₉ neutral cluster, the seashell-like borospherene isomer is the global minimum, significantly lower in energy than the stingray-shaped quasi-planar structure.