Out-of-plane σ-aromaticity and enhanced π-aromaticity in superatom compounds Li₃⁺N₄^2-M₃⁺ (M = Li, Na and K)

The structures of sandwich-like and chain-like superatom compounds Li₃-N₄-M₃ (M = Li, Na and K) are obtained at the MP2/6-311+G(2d) level. These structures are stable own to large interaction energy ranging from -184.4 to -236.9 kcal/mol. Maximum nucleus-independent chemical shift (NICS_max) values of M₃⁺ are located at the outside of the M₃⁺ ring plane about 0.6-1.5 Å for the sandwich-like structures, which exhibit unusual out-of-plane σ-aromaticity. The NICS_max values of N₄^2- in the sandwich-like structures are obviously larger than that of corresponding chain-like structures, which show that the two adjacent rings (Li₃⁺ and M₃⁺) enhance the aromaticity of N₄^2- ring in the sandwich-like structures. In addition, the N₄ in the superatom compound is proposed as a new "supersulfur" atom.