13_2019_Planar B41− and B42− clusters with double-hexagonal vacancies (2)
Since the discovery of the B40 borospherene, research interests have been directed to the structural evol-ution of even larger boron clusters. An interesting question concerns if the borospherene cages persist in larger boron clusters like the fullerenes. Here we report a photoelectron spectroscopy (PES) and compu-tational study on the structures and bonding of B41− and B42−, the largest boron clusters characterized experimentally thus far. The PE spectra of both clusters display broad and complicated features, suggesting the existence of multiple low-lying isomers. Global minimum searches for B41− reveal three low-lying isomers (I–III), which are all related to the planar B40− structure. Isomer II (Cs, 1A’) possessing a double hexagonal vacancy is found to agree well with the experiment, while isomers I (Cs, 3A’’) and III (Cs,1A’) both with a single hexagonal vacancy are also present as minor isomers in the experiment. The potential landscape of B42− is found to be much more complicated with numerous low-lying isomers (VII–XII). The quasi-planar structure VIII (C1, 2A) containing a double hexagonal vacancy is found to make major contributions to the observed PE spectrum of B42−, while the other low-lying isomers may also be present to give rise to a complicated spectral pattern. Chemical bonding analyses show isomer II of B41−(Cs, 1A’) and isomer VIII of B42− (C1, 2A) are π aromatic, analogous to that in the polycyclic aromatic hydro-carbon C27H13+ (C2v, 1A1). Borospherene cage isomers are also found for both B41− and B42− in the global minimum searches, but they are much higher energy isomers.
