C-H...F Interactions in the Crystal Structures of Some Fluorobenzenes.
The existence and nature of C-H...F-C interactions in cryst. fluorobenzenes are discussed. These compds. were chosen because they contain only C, H, and F atoms; this is necessary in the evaluation of the weak acceptor capabilities of the C-F group. All of these compds. are liqs. at room temp., and single crystals for x-ray diffraction were grown in situ. The anal. of the C-H...F interactions that are found in all of these crystal structures takes the form of comparisons with related C-H...O/C-H...N analogs. Fluorobenzene bears a close relation to pyridinium fluoride, pyridine 1-oxide, and benzonitrile at the level of individual interactions, showing that the character of the structure-detg. intermol. interactions in these four crystal structures are the same. Similarly, 1,4-difluorobenzene and 1,4-benzoquinone are related, the C-H...F interactions in the former playing the same structural role as the C-H...O interactions in the latter. A comparison of 1,4-difluorobenzene with the unsym. 1,4-dihalogenated benzenes (1-chloro-4-fluorobenzene, 1-bromo-4-fluorobenzene, and 1-fluoro-4-iodobenzene) indicates the importance of C-H...F interactions in these structures. With an increase in the F content of the mols., the C-H acidity also increases and the C-H...F interactions in 1,3,5-trifluorobenzene and 1,2,4,5-tetrafluorobenzene become stronger and more important. These compds. are structurally very similar to 1,3,5-triazine and 1,2,4,5-tetrazine, and this similarity further strengthens the argument that C-H...F interactions resemble C-H...N interactions and provides evidence for their description as weak H bonds. 1,2,3,4-Tetrafluorobenzene is polymorphic but the role of the C-H...F interactions in the two forms is similar. A comparison of the C-H...F geometries in these compds. with other C-, H-, and F-contg. compds. in the Cambridge Structural Database reveals that the H bond properties are more pronounced in these compds. Only when the C acidity is enhanced to the levels of the compds. is the H-bond nature of the C-H...F interaction even revealed. Also the C-F group prefers to form C-H...F interactions rather than F...F contacts. The behavior of org. F in crystal packing is therefore quite different from the heavier halogens.
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