Structural Evolution and Electronic Properties of Two Sulfur Atom-Doped Boron Clusters

We present a theoretical study of structural evolution, electronic properties, and photoelectron spectra of two sulfur atom-doped boron clusters S₂B^0/- (n = 2-13), which reveal that the global minima of the S₂B^0/- (n = 2-13) clusters show an evolution from a linear-chain structure to a planar or quasi-planar structure. Some S-doped boron clusters have the skeleton of corresponding pure boron clusters; however, the addition of two sulfur atoms modified and improved some of the pure boron cluster structures. Boron is electron-deficient and boron clusters do not form linear chains. Here, two sulfur atom doping can adjust the pure boron clusters to a linear-chain structure (S₂B₂^0/-, S₂B₃^0/-, and S₂B₄^-), a quasi-linear-chain structure (S₂B₆^-), single- and double-chain structures (S₂B₆ and S₂B₉^-), and double-chain structures (S₂B₅, and S₂B₉). In particular, the smallest linear-chain boron clusters S₂B₂^0/- are shown with an S atom attached to each end of B₂. The S₂B₂ cluster possesses the largest highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of 5.57 eV and the S₂B₂^- cluster possesses the largest average binding energy E_b of 5.63 eV, which shows the superior chemical stability and relative stability, respectively. Interestingly, two S-atom doping can adjust the quasi-planar pure boron clusters (B₇^-, B₁₀^-, and B₁₂^0/-) to a perfect planar structure. AdNDP bonding analyses reveal that linear S₂B₃ and planar SeB₁₁^- have π aromaticity and σ antiaromaticity; however, S₂B₂, planar S₂B₆, and planar S₂B₇^- clusters have π antiaromaticity and σ aromaticity. Furthermore, AdNDP bonding analyses reveal that planar S₂B₄, S₂B₁₀, and S₂B₁₂ clusters are doubly (π and σ) aromatic, whereas S₂B₅^-, S₂B₈, S₂B₉^-, and S₂B₁₃^- clusters are doubly (π and σ) antiaromatic. The electron localization function (ELF) analysis shows that S₂B^0/- (n = 2-13) clusters have different electron delocalization characteristics, and the spin density analysis shows that the open-shell clusters have different characteristics of electron spin distribution. The calculated photoelectron spectra indicate that S₂B^- (n = 2-13) have different characteristic peaks that can be compared with future experimental values and provide a theoretical basis for the identification and confirmation of these doped boron clusters. Our work enriches the new database of geometrical structures of doped boron clusters, provides new examples of aromaticity for doped boron clusters, and is promising to offer new ideas for nanomaterials and nanodevices.