Out-of-plane σ-aromaticity and enhanced π-aromaticity in superatom compounds Li3+N42−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 (NICSmax) 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 NICSmax values of N42− 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 N42− ring in the sandwich-like structures. In addition, the N4 in the superatom compound is proposed as a new “supersulfur” atom.