The structural and electronic properties of NbSi (n = 3-12) clusters: anion photoelectron spectroscopy and ab initio calculations.

Niobium-doped silicon clusters, NbSi (n = 3-12), were generated by laser vaporization and investigated by anion photoelectron spectroscopy. The structures and electronic properties of NbSi anions and their neutral counterparts were investigated with ab initio calculations and compared with the experimental results. It is found that the Nb atom in NbSi prefers to occupy the high coordination sites to form more Nb-Si bonds. The most stable structures of NbSi are all exohedral structures with the Nb atom face-capping the Si frameworks. At n = 8, both the anion and neutral adopt a boat-shaped structure and the openings of the boat-shaped structures remain unclosed in NbSi clusters. The most stable structure of the NbSi anion is endohedral, while that of neutral NbSi is exohedral. The global minima of both the NbSi anion and neutral NbSi are D symmetric hexagonal prisms with the Nb atom at the center. The perfect D symmetric hexagonal prism of NbSi is electronically stable as it obeys the 18-electron rule and has a shell-closed electronic structure with a large HOMO-LUMO gap of 2.70 eV. The molecular orbital analysis of NbSi suggests that the delocalized Nb-Si ligand interactions may contribute to the stability of the D symmetric hexagonal prism. The AdNDP analysis shows that the delocalized 2c-2e Si-Si bonds and multicenter-2e NbSi bonds are important for the structural stability of the NbSi anion.