Origin of structure and stability of M@C₁₈ (M = Cu, Ag, and Au) complexes with D_9h point group

Theoretical predictions and recent experimental studies lead to the discovery of an exciting new member of the carbon allotrope family polyynic cyclo[18]carbon (C₁₈ ). Present investigation aims to probe the structure, stability, and properties of coinage metal (M)@C₁₈ complexes using density functional theory (DFT) calculations. The DFT results unequivocally show that even Cu@C₁₈ , Ag@C₁₈ , and Au@C₁₈ complexes substantially preserve the ground state polyynic structure of C₁₈ . It is also worth to mention that only Au@C₁₈ is a stable D_9h structure, however the symmetry is distorted in the case of Cu@C₁₈ and Ag@C₁₈ . Due to computational limitations, in this investigation the M@C₁₈ complexes were scrutinized using the C_2v sub abelian group of D_9h . The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of D_9h conformers are a singlet a₁ and two same value singlets a₁  ⊕ b₁ generated from doublet e, respectively. The non-covalent interaction index (NCI), quantum theory of atoms in molecule (QTAIM), and energy decomposition analysis (EDA) vividly explains the interaction between a coinage metal atom and C₁₈ ring. It is found from the results that the stability of Cu@C₁₈ Ag@C₁₈ , and Au@C₁₈ is governed by the attractive electrostatic, orbital and dispersion interaction.