Structures and aromaticity of three-membered BeXP (X = C, Si, Ge) and CYP (Y = O, S, Se) rings

Three-membered BeXP (X = C, Si, Ge) and CYP (Y = O, S, Se) rings are theoretically investigated using density functional theory (DFT) methods at the B3LYP/6-311+G* and B3PW91/6-311+G* levels of theory. The research results show that the size of atoms has a great influence on the structural stability of these species. The wiberg bond indexs (WBIs) suggest the existence of delocalization in these structures. Negative nucleus-independent chemical shift (NICS) values for these species indicate that a strong ring current exists in these three-membered structures (Cs symmetry). A detailed molecular orbital (MO) analysis further reveals that a delocalized π MO strengthens the structural stability and makes these species show strong aromatic characters.