Cryogenic infrared spectroscopy reveals remarkably short NH⁺ ...F hydrogen bonds in fluorinated phenylalaninesA

In past decades, hydrogen bonds involving organic fluorine have been a highly disputed topic. Obtaining clear evidence for the presence of fluorine-specific interactions is generally difficult because of their weak nature. Today, the existence of hydrogen bonds with organic fluorine is widely accepted and supported by numerous studies. However, strong bonds with short H center dot center dot center dot F distances remain scarce and are primarily found in designed model compounds. Using a combination of cryogenic gas-phase infrared spectroscopy and density functional theory, we here analyze a series of conformationally unrestrained fluorinated phenylalanine compounds as protonated species. The results suggest proximal NH+center dot center dot center dot F hydrogen bonds with an exceptionally close H center dot center dot center dot F distance (1.79 A) in protonated ortho-fluorophenylalanine.