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 Li-3-N-4-M-3 (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 M-3(+) are located at the outside of the M-3(+) ring plane about 0.6-1.5 angstrom for the sandwich-like structures, which exhibit unusual out-of-plane sigma-aromaticity. The NICSmax Values of N-4(2-) in the sandwich-like structures are obviously larger than that of corresponding chain-like structures. which show that the two adjacent rings (Li-3(+) and M-3(+)) enhance the aromaticity of N-4(2-) ring in the sandwich-like structures. In addition, the N-4 in the superatom compound is proposed as a new supersulfur atom. (C) 2009 Elsevier B.V. All rights reserved.