American Chemical Society
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C−H···F Interactions in the Crystal Structures of Some Fluorobenzenes

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journal contribution
posted on 1998-08-15, 00:00 authored by Venkat R. Thalladi, Hans-Christoph Weiss, Dieter Bläser, Roland Boese, Ashwini Nangia, Gautam R. Desiraju
The existence and nature of C−H···F−C interactions in crystalline fluorobenzenes 13 and 710 are discussed. These compounds 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 compounds are liquids at room temperature, and single crystals for X-ray diffraction were grown in situ. The analysis 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 analogues. Fluorobenzene, 1, bears a close relationship to pyridinium fluoride, pyridine 1-oxide, and benzonitrile at the level of individual interactions, showing that the character of the structure-determining intermolecular interactions in these four crystal structures are the same. Similarly, 1,4-difluorobenzene, 3, 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 3 with the unsymmetrical 1,4-dihalogenated benzenes, 46 indicates the importance of C−H···F interactions in these structures. With an increase in the F content of the molecules, the C−H acidity also increases and the C−H···F interactions in 1,3,5-trifluorobenzene, 7, and 1,2,4,5-tetrafluorobenzene, 8, become stronger and more important. Compounds 7 and 8 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 hydrogen bonds. 1,2,3,4-Tetrafluorobenzene, 9, 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 compounds 110 with other C-, H-, and F-containing compounds in the Cambridge Structural Database reveals that the hydrogen bond properties are more pronounced in 110. It is concluded that only when the carbon acidity is enhanced to the levels of the compounds in the present study, is the hydrogen-bond nature of the C−H···F interaction even revealed. This study also demonstrates that the C−F group prefers to form C−H···F interactions rather than F···F contacts. The behavior of organic fluorine in crystal packing is therefore quite different from the heavier halogens.