Unraveling the Origin of the Relative Stabilities of Group 14 M2N22+ (M, N = C, Si, Ge, Sn, and Pb) Isomer Clusters

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We analyze the molecular structure, relative stability, and aromaticity of the lowest-lying isomers of group 14 M2N22+ (M and N = C, Si, and Ge) clusters. We use the gradient embedded genetic algorithm to make an exhaustive search for all possible isomers. Group 14 M2N22+ clusters are isoelectronic with the previously studied group 13 M2N22– (M and N = B, Al, and Ga) clusters that includes Al42–, the archetypal all-metal aromatic molecule. In the two groups of clusters, the cyclic isomers present both σ- and π-aromaticity. However, at variance with group 13 M2N22– clusters, the linear isomer of group 14 M2N22+ is the most stable for two of the clusters (C2Si22+ and C2Ge22+) , and it is isoenergetic with the cyclic D4h isomer in the case of C42+. Energy decomposition analyses of the lowest-lying isomers and the calculated magnetic- and electronic-based aromaticity criteria of the cyclic isomers help to understand the nature of the bonding and the origin of the stability of the global minima. Finally, for completeness, we have also analyzed the structure and stability of the heavier Sn and Pb group 14 M2N22+ analogues ​
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