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Borospherene (B40) is an electron-deficient cluster molecule containing 40 boron atoms. It bears similarities to other homoatomic cluster structures such as buckminsterfullerene (C60), stannaspherene, and plumbaspherene, but with a different symmetry. The first experimental evidence for borospherene was reported in July 2014, and is described in the journal Nature Chemistry. The molecule includes unusual and faces. Despite many calculation-based investigations into its structure and properties, a viable route for the synthesis and isolation of borospherene has yet to be established, and as a consequence it is still relatively poorly understood.
Many theoretical papers have been published on the structure, properties, and potential applications of borospherene. Neutral borospherene has a large HOMO-LUMO gap of 3.13 eV (which destabilises its anion, making the ground state of B40− the quasi-planar isomer). However, it has been calculated to be prone to exothermic dimerisation, with a low activation barrier of 63 meV, followed by trimerisation with a lower energy barrier, and runaway aggregation. As a result, borospherene has yet to be isolated and is poorly experimentally-characterised, unlike buckminsterfullerene.
16 boron atoms of borospherene are four-coordinate, and 24 are five-coordinate. It has four sets of eight equivalent boron atoms, and two sets of four equivalent atoms. Neutral borospherene has a diameter of 6.2 Å. It comprises eleven unique bond lengths ranging from 1.60 Å to 1.85 Å, corresponding to a B-B bond order of slightly below 2 to a fractional B-B bond order respectively. This encapsulates well the large degree of both sigma- and pi-delocalisation of electrons across the electron-deficient cluster as opposed to buckminsterfullerene, which has more localised bonds and features only two bond lengths corresponding to a C-C single bond and a C-C double bond respectively. The HOMO of borospherene is quadruply degenerate, computed to be a pi-bond delocalised over 5 boron atoms.
Lai-Sheng Wang, professor of chemistry at Brown University, modeled possible B40 and B40− anion structures. The simulated spectra of two energetically lowest-lying isomers of the anion - a sheet-like structure and a closed cage - were found to fit experimental data well. Photoelectron spectroscopy revealed that the substance formed in the laboratory was this cage. Both neutral borospherene and the cage-like isomer of its anion have the same D2d symmetry, the additional electron in the anion being housed within the B40− cage structure. The structure of the cage is not perfectly uniform – "Several atoms stick out a bit from the others, making the surface of borospherene somewhat less smooth than a buckyball" according to Wang.
Li et al. computationally investigated undecorated borospherene as a potential sensor for sulfur-containing gases, and found that it behaved as an electronic sensor for sulfur dioxide and carbon disulfide (their adsorption to the boron cluster significantly stabilises its LUMO, increasing its population of conducting electrons), and additionally as a Φ-type sensor for the former (due to significant change to its Work function upon the adsorption of SO2), but behaved as neither for the gases carbonyl sulfide and hydrogen sulfide.
Undecorated B40 was calculated to be a poor candidate for reversible hydrogen storage, being capable of the irreversible sequestration of only one hydrogen molecule per B40 within its cage. Li6B40, however, is calculated to be capable of adsorbing up to 18 H2 molecules (3 H2 molecules at each Li site) - corresponding to a gravimetric density of 7.1 wt% - with a moderate average binding energy of 0.11 eV/H2, within the optimal range for reversible hydrogen storage. Subsequent H2 molecules are Physisorption to the cluster instead of Chemisorption, and have a much weaker binding energy.
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