18-valence-electron rule lighted planar tetracoordinate carbon and nitrogen: the global energy minima of CAl4Zn and NAl4Zn+

Abstract

The exploration of planar hypercoordinate carbon (phC) is challenging and significant. It is often puzzling to chemists whether the designed phC species should satisfy the 18-valence-electron rule, an authoritative rule in the phC field. In this study, we introduced a zinc atom into the extremely unstable 16-valence-electron planar tetracoordinate carbon (ptC) species CAl₄ and its isoelectronic structure NAl₄⁺ with a planar tetracoordinate nitrogen (ptN), and designed the 18-valence-electron CAl₄Zn and NAl₄Zn⁺ possessing a ptC and ptN, respectively. The thermodynamic results indicate that the ptC/N species CAl₄Zn and NAl₄Zn⁺ are the global energy minima, and also showed that the 18-valence-electron rule is more appropriate in designing ptC/N species having the CAl₄ and NAl₄⁺ skeletons, compared with the 16-valence-electron rule. Simultaneously, the BOMD simulations found that CAl₄Zn is dynamically stable. Although NAl₄Zn⁺ was isomerized at 298 and 500 K, it is dynamically viable. The excellent stability may be explained by the perfect electronic structure. First, the HOMO–LUMO gaps became much wider after the introduction of the Zn atom. Second, AdNDP analysis indicated that the introduction of the Zn atom promoted the formation of peripheral Al–Al and Al–Zn covalent bonds, providing a stable and comfortable bonding environment for ptC/N. In addition, the σ and π double aromaticity further stabilized the ptC/N species. Hence, as dynamic global energy minima display σ and π double aromaticity, the ptC/N species CAl₄Zn and NAl₄Zn⁺ are promising in gas phase generation.