198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

22 J.P. Glusker
4.5
F◊◊◊H Interactions
When fluorine is covalently bound to a carbon atom (a C-F bond) it is found that
the fluorine atoms approach hydroxyl groups with minimum F◊◊◊H distances of
2.3 Å, much longer than O◊◊◊H distances of 1.4–1.7 Å in strong O◊◊◊HO hydrogen
bonds [39–42]. The probability of O◊◊◊H-O hydrogen bond formation is
significantly higher than that for the formation of a hydrogen bond involving a
C-F group; thus O◊◊◊H-O hydrogen bonds will form wherever possible; rather
than C-F◊◊◊H hydrogen bonds. Fluorine is the most electronegative element [26],
and therefore it is interesting that C-F bonds rarely form hydrogen bonds,
especially since the H◊◊◊F – interaction (involving a fluoride ion rather than
organic fluorine) forms the shortest hydrogen bonds measured (H◊◊◊F – = 1.14 Å)
[29]. Some C-H◊◊◊F interactions in a fluoro PAH are shown in Fig. 14.
a
b
Fig. 14 a, b. A fluorinated PAH showing C-H◊◊◊F interactions [113]. The compound is 11-(trifluoromethyl)-15,16-dihydrocyclopenta[a]phenanthren-17-one.
Two examples of interactions
between molecules are shown in (a) and (b). They both involve C-H◊◊◊F interactions.
H◊◊◊F and H◊◊◊O nonbonded distances are given in Å