Structures and aromaticity of the planar XSi₂Y (X = Al, Ga and Y = P, As) clusters

Clusters XSi₂Y (X = Al, Ga and Y = P, As) are theoretically investigated using density functional theory (DFT) methods at the B3LYP/6-311+G* and B3PW91/6-311+G* levels of theory. The calculated results showed that for AlSi₂P AlSi₂As and PSi₂Ga species, the C_2v isomer is the ground state, respectively, whereas for GaSi₂As species, the C_s isomer is the ground state. The wiberg bond index (WBI) suggested the existence of delocalization. Negative nucleus-independent chemical shift (NICS) values indicated that a strong ring current exists in two isomers of those species. A detailed molecular orbital (MO) analysis further revealed that two isomers of these species have π aromaticity, and the two delocalized π electrons does agree well with the (4n + 2) Huckel rule, which is closely connected with the concept of the aromaticity.