Structures and electronic properties of B₃Si_n^- (n = 4-10) clusters: A combined ab initio and experimental study

The anionic silicon clusters doped with three boron atoms, B₃Si_n^- (n = 4-10), have been generated by laser vaporization and investigated by anion photoelectron spectroscopy. The vertical detachment energies (VDEs) and adiabatic detachment energies (ADEs) of these anionic clusters are determined. The lowest energy structures of B₃Si_n^- (n = 4-10) clusters are globally searched using genetic algorithm incorporated with density functional theory (DFT) calculations. The photoelectron spectra, VDEs, ADEs of these B₃Si_n^- clusters (n = 4-10) are simulated using B3LYP/6-311+G(d) calculations. Satisfactory agreement is found between theory and experiment. Most of the lowest-energy structures of B₃Si_n^- (n = 4-10) clusters can be derived by using the squashed pentagonal bipyramid structure of B₃Si₄^- as the major building unit. Analyses of natural charge populations show that the boron atoms always possess negative charges, and that the electrons transfer from the 3s orbital of silicon and the 2s orbital of boron to the 2p orbital of boron. The calculated average binding energies, second-order differences of energies, and the HOMO-LUMO gaps show that B₃Si₆^- and B₃Si₉^- clusters have relatively high stability and enhanced chemical inertness. In particular, the B₃Si₉^- cluster with high symmetry (C_3v) stands out as an interesting superatom cluster with a magic number of 40 skeletal electrons and a closed-shell electronic configuration of 1S²1P⁶1D¹⁰2S²2P⁶1F¹⁴ for superatom orbitals.