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 σu 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.