Planar B₄₁^- and B₄₂^- clusters with double-hexagonal vacancies

Since the discovery of the B₄₀ borospherene, research interests have been directed to the structural evolution 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 computational 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 (C_s, ¹A') possessing a double hexagonal vacancy is found to agree well with the experiment, while isomers I (C_s, ³A'') and III (C_s, 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^- (C_s, ¹A') and isomer VIII of B₄₂^- (C₁, ²A) are π aromatic, analogous to that in the polycyclic aromatic hydrocarbon C₂₇H₁₃⁺ (C_2v, ¹A₁). Borospherene cage isomers are also found for both B₄₁^- and B₄₂^- in the global minimum searches, but they are much higher energy isomers.