Effect of Alkali Atom Doping on the Electronic Structure and Aromatic Character of Planar and Quasi-Planar Al13+ Clusters

Abstract

A set of three Al13+ clusters, one perfectly planar and two quasi-planar structures have been recently reported by our group [Guin et. al. Journal of Molecular Graphics and Modeling, 2020, 97, 107544]. One of the quasi-planar structures is true minima (GS) with zero imaginary frequency and another one is transition state structure (TS) with one imaginary frequency, whereas the perfectly planar structure has three imaginary frequencies. All three posses bilaterally symmetry with identical structural features - a set of ten aluminium atoms encircle a triangular core and also this set of three aluminium cluster is a rare example of a metallo-aromatic system in which highly anti-aromatic islands are embedded in an aromatic sea. One of the atoms of the central triangle lies on the symmetry axis and is crucial for the stability of these clusters. In the present study, we have explored the effect of doping alkali atoms (Li, Na and K) at this pivotal site of the cluster with an aim to understand the structural stability and the effect on the aromatic character as compared to the parent clusters. Besides the electronic structural analysis, NICS and ELF studies have also been carried out to characterize the aromatic nature of the doped clusters. Interestingly it has been found that even with the incorporation of the alkali atoms the bilateral symmetry of the clusters remains intact but instead of the central position the alkali atoms drift towards the periphery of the cluster along the symmetry line and equilibrate on the periphery. The dipole moment of the clusters systematically increases and the overall aromaticity of the cluster systematically decreases with the increase in the atomic number of the dopant alkali atoms.