Structure and aromaticity of [formula omitted] and Sb 5M (M = Li, Na, and K) clusters

Abstract

The equilibrium geometries, electronic structures, harmonic vibrational frequencies, and nucleus independent chemical shifts (NICS) of Sb 5 - and Sb 5M (M = Li, Na, and K) clusters are calculated by the density functional theory (DFT) method. Our calculation show that the Sb 5 - species has the aromatic planar cyclic D 5 h structure and the Sb 5M species have the pyramidal C 5 v structures as the ground states, and the Sb 5 - unit preserves its structural and electronic integrity in forming the Sb 5M complexes. Molecular orbital analysis and NICS show that the planar Sb 5 - anion satisfies the Hükcel rule of 4 n + 2π electrons and magnetic criteria for aromaticity with six delocalized π electrons and negative NICS values. The dissected NICS suggests that the aromaticity of Sb 5 - ( D 5 h ) and Sb 5M ( C 5 v ) arise primarily from the contributions of Sb–Sb π bonds, Sb–Sb σ bonds, and the NICS distributions and contributions, and their changing tendencies of the Sb 5M ( C 5 v ) species are similar to the isolated Sb 5 - ( D 5 h ) anion.