Theoretical study on the aromaticity of the bimetallic clusters X2M2 (X=Si, Ge, M=Al, Ga)

Four neutral bimetallic clusters X2M2 (X=Si, Ge, M=Al, Ga) are investigated using density functional theory (DFT) and post-HF methods. The calculated results show that each of four X2M2 species has two energetically close stable isomers with rhombic structure (D2h symmetry) and trapezoidal structure (C2v symmetry) respectively. For the Ge2Al2 species the rhombic (D2h) isomer is the ground state, whereas for other three species Ge2Ga2, Si2Al2, and Si2Ga2, the trapezoidal (C2v) isomers are the ground states. The calculated magnetic susceptibility anisotropy (χanis) and nucleus-independent chemical shift (NICS) indicate that a strong diatropic ring current exists in the two heterocyclic planar isomers, suggesting they are highly aromatic. A detailed molecular orbital analysis further reveals that both heterocyclic isomers possess multiple aromaticity derived from one delocalized π MOs and two delocalized σ MOs.