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Anionic boron clusters have been shown to be planar or quasi-planar up to $B_{21}^{-}$ from a series of combined photoelectron spectroscopy and theoretical studies. All these boron clusters consist of a peripheral ring characterized by strong two-center-two-electron (2c-2e) B-B bonds and one or more interior atoms. The propensity for planarity is due to $\sigma $- and $\pi $-electron delocalizations throughout the molecular plane, giving rise to concepts of $\sigma $- and $\pi $-aromaticity. The quasi-planarity, on the other hand, can be mechanical in nature - the circumference of the cluster is too small to fit the inner atoms - even for doubly aromatic clusters. Two questions arise: firstly, can isoelectronic substitution by a single aluminum atom on the outer ring enhance the planarity of quasi-planar structures, and, secondly, can the interior boron atoms be replaced by aluminum? A series of aluminum isoelectronic substitution of boron clusters have been investigated ranging from $B_{7}^{-}$ to $B_{12}^{-}$. Aluminum turns out to avoid the central position in the all these clusters and enhance the planarity of $AlB_{6}^{-}$ and $AlB_{11}^{-}$ clusters by expanding the peripheral ring. \vspace{12pt} References: [1] C. Romanescu, A. P. Sergeeva, W. L. Li, A. I. Boldyrev and L. S. Wang, \textit{J. Am. Chem. Soc}. \textbf{133} (22), 8646-8653 (2011) [2] T. R. Galeev, C. Romanescu, W. L. Li, L. S. Wang and A. I. Boldyrev, \textit{J. Chem. Phys.} \textbf{135}, (8) 104301 (2011) [3] W. L. Li, C. Romanescu, T. R. Galeev, L. S. Wang and A. I. Boldyrev, \textit{J. Phys. Chem. A} \textbf{115} (38), 10391-10397 (2011)


Author Institution: Brown University, Chemistry Department, 324 Brook Street,Providence, RI 02912, USA