Stabilization of Ring Structures of C₉ with the Doping of a Metal Atom: An Ab Initio Study

We report the results of ab initio calculations on the doping of a metal atom (Li, Be, Mg, and Al) in ring and chain structures of C9. Pure C9 has a chain structure, and it remains so with the doping of a Li atom. However, when a Be, Mg, or Al atom is doped, a ring structure such as BeC9 with the metal atom in the ring becomes most favorable. We also explored the stabilization of a cyclic structure of C9 with endohedral doping of these metal atoms. It is found that a divalent atom such as Be leads to a symmetric wheel-shaped ring Be@C9 with a large HOMO–LUMO gap of 5.21 eV using the PBE0 hybrid exchange-correlation functional. It has a large adiabatic ionization potential of 8.781 eV and a low electron affinity of 1.659 eV suggesting its high chemical stability. This cyclic structure has π aromaticity corresponding to the Hückel 4n + 2, n = 2 rule with 10 π bonded valence electrons. We discuss the bonding character in these novel species as well as their IR and Raman spectra, which show no imaginary frequency. Wheel-shaped structures of isoelectronic Li@C9− and Al@C9+ have also been obtained with large HOMO–LUMO gaps, but for Mg doping, the metal atom lies slightly above the ring.