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The hydrogen bond between N—H or O—H and organic fluorine: favourable yes, competitive no
Author(s) -
Taylor Robin
Publication year - 2017
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520617005923
Subject(s) - acceptor , crystallography , hydrogen bond , fluorine , molecule , halogen , chemistry , crystal structure , halide , chlorine , hydrogen , stereochemistry , inorganic chemistry , organic chemistry , physics , alkyl , condensed matter physics
A study was made of X —H…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…π hydrogen bonds (especially to electron‐rich aromatics) and dihydrogen bonds. The average H…F distances of X —H…F—C interactions are significantly shorter for C R 3 F ( R = C or H) and C sp 2 —F acceptors than for C R F 3 . The X —H…F angle distribution is consistent with a weak energetic preference for linearity, but that of H…F—C suggests a flat energy profile in the range 100–180°. X —H…F—C interactions are more likely when the acceptor is C sp 2 —F or C R 3 F, and when the donor is C—NH 2 . 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…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.