Novel pyramidal MB7 (M = Li, Na, K, Rb, or Cs) species:: Structure and aromaticity

The structures and stabilities of a series of pyramidal MB7 (M = Li, Na, K, Rb, Cs) species were studied by using ab initio (MP2/6-311+G*) and density functional theory (B3LYP/6-311+G* and B3PW91/6-311+G*) methods. It is found that most MB7 species (except RbB7 and CsB7 at the MP2 level) are minima at the C-7v symmetric structure, which corresponds to a complex of an M+ cation with a planar heptagonal B-7(-) ring. Nucleus-independent chemical shift and molecular orbital analyses confirm that the planar heptagonal B-7(-) ring exhibits aromatic nature. The stabilization of the planar B-7(-) ring is mainly attributed to the electrostatic interaction.