Encapsulation of Astatide by a water cage.

A 5 12 cage of (H 2 O) 20 consisting of 30 hydrogen bonds encapsulates Astatide with little geometrical distortion. The cage is marginally destabilized but the non-covalent interactions are actually strengthened. Host⋯cage interactions in the [At@(H 2 O) 20 ] - cluster are anti-electrostatic, placing both negatively charged atoms in direct contact as in At δ - ⋯ δ - O-H δ + . An orbital interaction analysis reveals that explicit host⋯cage contacts are "inverted" hydrogen bonds. That is, the same type of donor→acceptor charge transfer as in hydrogen bonding, with no proton bridging the two negative charges.