The hydrogen bond between N-H or O-H and organic fluorine: favourable yes, competitive no

A study was made of X-H center dot center dot center dot F-C interactions (X = N or O) in small-molecule crystal structures. It was primarily based on 6728 structures containing X-H and C-F and no atom heavier than chlorine. Of the 28 451 C-F moieties in these structures, 1051 interact with X-H groups. However, over three-quarters of these interactions are either the weaker components of bifurcated hydrogen bonds (so likely to be incidental contacts) or occur in structures where there is a clear insufficiency of good hydrogen-bond acceptors such as oxygen, nitrogen or halide. In structures where good acceptors are entirely absent, there is about a 2 in 3 chance that a given X-H group will donate to fluorine. Viable alternatives are X-H center dot center dot center dot pi hydrogen bonds (especially to electron-rich aromatics) and dihydrogen bonds. The average H center dot center dot center dot F distances of X-H center dot center dot center dot F-C interactions are significantly shorter for CR3F (R = C or H) and Csp(2)-F acceptors than for CRF3. The X-H center dot center dot center dot F angle distribution is consistent with a weak energetic preference for linearity, but that of H center dot center dot center dot F-C suggests a flat energy profile in the range 100-180 degrees. X-H center dot center dot center dot F-C interactions are more likely when the acceptor is Csp(2)-F or CR3F, and when the donor is C-NH2. They also occur significantly more often in structures containing tertiary alcohols or solvent molecules, or with Z' > 1, i. e. when there may be unusual packing problems. It is extremely rare to find X-H center dot center dot center dot F-C interactions in structures where there are several unused good acceptors. When it does happen, there is often a clear reason, e. g. awkwardly shaped molecules whose packing isolates a donor group from the good acceptors.