Topological and molecular descriptors of interstitial sites in M⋯(C60)n clusters: A quantum chemical QTAIM and OP/TOP study

Fulleride crystals ( M 3 C 60 ) are materials whose electrical transport properties are influenced by the distinct structural environments of their tetrahedral and octahedral interstitial doping sites. In this work, the chemical bond overlap model (OP/TOP), the quantum theory of atoms in molecules (QTAIM), and molecular descriptors were utilized to investigate M ⋯ C 60 interactions. Representative clusters modeling isolated tetrahedral M ( C 60 ) 4 and octahedral M ( C 60 ) 6 environments ( M = Li, Na, K, Rb, Cs ) were investigated using density functional theory at the ω B97X-D3/def2-SVP level. Molecular descriptors reveal that alkali-metal doping decrease the HOMO-LUMO gap ( Δ E HL ), and both the ionization potential ( I P ) and chemical hardness ( η ). For all metals except Rb, the octahedral models exhibited lower HOMO-LUMO gaps and I P than the corresponding tetrahedral models. QTAIM and OP/TOP descriptors indicate that the M-C bonds within the tetrahedral sites exhibit ionic behavior characterized by interatomic charge depletion ( ∇ 2 ρ BCP > 0 , H BCP > 0 ) and highly concentrated overlap density ( ∇ 2 ρ OCP ≪ 0 ). In contrast, the octahedral sites feature delocalized electron overlap profiles. The combined QTAIM and OP/TOP analyses indicate distinct electronic structure for the two interstitial environments: tetrahedral sites are associated with stronger charge-transfer characteristics, whereas octahedral sites display more spatially distributed electron density.