Intermolecular Aliphatic C-F···H-C Interaction in the Presence of "Stronger" Hydrogen Bond Acceptors: Crystallographic, Computational, and IR Studies.

An unprecedented intermolecular aliphatic C-F···H-C interaction was observed in the X-ray crystal structure of a fluorinated triterpenoid. Despite the notion of fluorine being a poor acceptor, computational and IR studies revealed this interaction to be a weak to moderate hydrogen bond with a C-H stretch vibration frequency blue-shifted by 14 cm-1 and d(F-H) = 2.13 Å. In addition, the aliphatic C-F bond is the preferred acceptor in the presence of multiple, traditionally stronger oxygen-based hydrogen bond acceptors.