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.