How Does a Container Affect Acidity of its Content: Charge‐Depletion Bonding Inside Fullerenes

A recent study (Sci. Adv. 2017, 3, e1602833) has shown that FH⋅⋅⋅OH2 hydrogen bond in a HF⋅H2O pair substantially shortens, and the H−F bond elongates upon encapsulation of the cluster in C70 fullerene. This has been attributed to compression of the HF⋅H2O pair inside the cavity of C70. Herein, we present theoretical evidence that the effect is not caused by a mere compression of the H2O⋅HF pair, but it is related to a strong lone‐pair–π (LP–π) bonding with the fullerene cage. To support this argument, a systematic electronic structure study of selected small molecules (HF, H2O, and NH3) and their pairs enclosed in fullerene cages (C60, C70, and C90) has been performed. Bonding analysis revealed unique LP–πcage interactions with a charge‐depletion character in the bonding region, unlike usual LP–π bonds. The LP–πcage interactions were found to be responsible for elongation of the H−F bond. Thus, the HF appears to be more acidic inside the cage. The shortening of the FH⋅⋅⋅OH2 contact in (HF⋅H2O)@C70 originates from an increased acidity of the HF inside the fullerenes. Such trends were also observed in other studied systems. Endohedral fullerenes: Bonding and electron density in endohedral fullerenes with encapsulated small neutral closed‐shell molecules and their pairs is investigated. Study reveals that enhanced acidity of the enclosed proton‐donor molecules is caused by strong non‐classical LP–πcage interactions between the enclosed molecules and the fullerene cage.