Buckyball Difluoride F₂ ⁻@C₆₀ ⁺—A Single-Molecule Crystal

We report the F₂@C₆₀ system as the first example of an endohedral fullerene in which C₆₀ acts as a cation C₆₀ ⁺ interacting with endohedral anion, F₂ ⁻. Our state-of-the-art computations reveal that in F₂@C₆₀, despite of the known high electron affinity of C₆₀, an electron is transferred from C₆₀ to F₂ resulting in the F₂ ⁻@C₆₀ ⁺ system. The F−F bond length in F₂@C₆₀ is substantially longer than in free F₂, which is the result of electron-transfer to the antibonding σ_u molecular orbital of F₂. Interestingly, although there is a full charge-transfer of one electron between C₆₀ and F₂, only negligible delocalized covalent interactions are found between F₂ ⁻ and C₆₀ ⁺ which is a reminiscent of ionic crystals. Therefore, F₂ ⁻@C₆₀ ⁺ can be considered as a single-molecule crystal. The other encapsulated halogens in C₆₀ do not show such behavior.