The ternary C@Al₆Cu₄ cluster: hexavalent non-hybridized carbon atoms and a new magic number

This study investigates the stability of C@Al₆M₄ structures, where six Al atoms surround a central carbon in octahedral C@Al₆, and coinage metal M (Cu, Ag, or Au) atoms attach to the four faces of C@Al₆, forming a tetrahedral shape. These structures exhibit aromaticity and significant thermodynamic stability. Their stability is compared with that of C@Al₆Na₄ and explained through the new magic number of 26. The separation of the 2P and 1F energy levels is attributed to this magic number. Carbon utilizes both its 2s and 2p orbitals for bonding, yet no hybridization is observed. Additionally, C@Sn₆²⁺, adhering to the new magic number of 26, is also reported in this study. The concept of "<i>hexavalent non-hybridized carbon atom</i>" is applied to all the abovementioned structures.