Theoretical Study on the 3D Aromaticity in Cationic X44+ (X=O, S, Se, and Te) Clusters

The stable structures and 3D aromatic characters for four cationic X44+(X=O, S, Se, and Te) clusters are investigated at the DFT and post HF level of theory with the extended 6-311+G* basis set used for O, S, Se atoms and correlation consistent, effective-core potential (ECP) basis set ECP46MWB_AVTZ for the heavier Te atom. Two optimized structures of tetrahedron (Td) and rhombus (D2h) for four X44+ species have been obtained. The calculated results show that the tetrahedral (Td) structures are stable and correspond to energy minima, whereas the rhombic (D2h) ones are unstable and have one or more saddle points on the potential energy surfaces. The calculated NICS values show that four tetrahedral X44+ isomers have higher degree of 3D aromaticities. Whereas four rhombic X44+ isomers can be regarded to possess 3D antiaromaticities. The molecular orbital analyses reveal that the tetrahedral X44+ cations possess two-fold (σ and π) 3D aromaticity, conforming to the 2(N+1)2 electron counting rule for 3D aromaticity