Effect of alkali atom doping on the electronic structure and aromatic character of planar and quasi-planar Al13+ clusters

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). All three clusters possess bilateral symmetry with identical structural features—a set of ten aluminum atoms encircle a triangular core. The symmetry axis passes through one of the Al atoms of the central triangular core and two Al atoms located on the periphery at two opposite ends of the cluster. This set of three aluminum clusters is an example of a rare metallo-aromatic system where highly anti-aromatic islands are embedded within an aromatic sea. In the present study, we have explored the effect of doping alkali atoms (Li, Na, and K) at the positions of the Al atoms that lie on the symmetry axis of the cluster intending to understand the structural stability and the effect on the aromatic character as compared to the undoped 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 the alkali atoms are pushed out of the original location toward the edge of the cluster, whereas the aluminum atoms remain grouped. 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 doped alkali atoms.