Ring Structures of Metal Atom Doped C₉, C₁₁, and C₁₃ Clusters from Ab Initio Calculations-The Finding of a Gd@C₁₃...

Pure C₉ clusters have a linear chain structure. However, here, we report using ab initio calculations the transformation of a chain into a cyclic ring structure with the capping of Ca, Sr, and Ba atoms. Further calculations on neutral and charged clusters doped with Sc, Y, and La atoms show stabilization of a cation C₉ isoelectronic cyclic ring capped with the metal (M) atom, but anion clusters doped with these trivalent atoms form a C₁₀ like MC₉ ring, which deforms to a necklace structure. Both the ring structures correspond to electronic shell closing with 20 delocalized valence electrons in a disk jellium model and have a large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap. Calculations of IR and Raman spectra show no imaginary frequency, suggesting that the structures are stable. Addition of two C atoms to the ring structure of LaC₉ leads to a capped ring structure of the cation LaC₁₁ and an open ring structure of the LaC₁₁ anion. Further addition of two C atoms leads to a La@C₁₃ cation as well as Ca@C₁₃ and Sr@C₁₃ neutral wheel-shaped rings with endohedral doping of the M atom. These novel ring structures have a large HOMO-LUMO gap of more than 4 eV and are magic with electronic shell closing corresponding to 28 delocalized valence electrons in a disk jellium model. There is π aromaticity in this ring satisfying 4n+2 (n = 3) Hückel's rule with 14 valence electrons. Interestingly, when the dopant is a Gd atom, there is a formation of a magnetic superatom ring Gd@C₁₃ ⁺ with 7 μ_B magnetic moments due to seven 4f up-spin states of Gd being fully occupied. Bonding in these novel ring structures is discussed.