THE GEOMETRY OF INTERMOLECULAR INTERACTIONS IN SOME CRYSTALLINE FLUORINE-CONTAINING ORGANIC-COMPOUNDS

The propensity of C-F groups to form C-F...H-C interactions with C-H g roups on other molecules has been analyzed. Crystal structures of mole cules containing only carbon, hydrogen, and fluorine, but no oxygen, n itrogen, or other hydrogen-bond-forming elements, were chosen for an i nitial study in which the intermolecular interactions in crystal-struc ture determinations of polycyclic aromatic hydrocarbons and their anal ogous fluoro derivatives were analyzed. It is found that C-F...H-C int eractions occur, but they are weak, as judged by the intermolecular di stances and the angles involved. In a study of crystal structures of m olecules containing other elements in addition to carbon, hydrogen, an d fluorine, it was found that when an oxygen atom is in a neighboring position on an interacting molecule, a C-O group is more likely than a C-F group to form a linear interaction to the hydrogen atom of a C-H group. Thus, in spite of the high electronegativity of the fluorine at om, a C-F group competes unfavorably with a C-O-, C-OH, or C=O group t o form a hydrogen bond to an O-H, N-H, or C-H group. It is found, howe ver, particularly for polycyclic aromatic hydrocarbons with substitute d CF3 groups that, in the absence of other functional groups that can form stronger interactions, C-F...H-C interactions may serve to align molecules and give a different crystal packing from that in the pure h ydrocarbon (where fluorine is replaced by hydrogen). Thus, C-F...H-X ( X=C, N, O) interactions are very weak, much weaker than C=O...H-X inte ractions, but they cannot be ignored in predictions of modes of molecu lar packing in complexes and in crystals.