The Geometry of Intermolecular Interactions in Some Crystalline Fluorine-Containing Organic Compounds

Abstract

The propensity of C—F groups to form $\mathrm{C}\mathrm{F}\cdots \mathrm{H}\mathrm{C}$ interactions with C—H groups on other molecules has been analyzed. Crystal structures of molecules containing only carbon, hydrogen, and fluorine, but no oxygen, nitrogen, or other hydrogen-bond-forming elements, were chosen for an initial study in which the intermolecular interactions in crystal-structure determinations of polycyclic aromatic hydrocarbons and their analogous fluoro derivatives were analyzed. It is found that $\mathrm{C}\mathrm{F}\cdots \mathrm{H}\mathrm{C}$ interactions occur, but they are weak, as judged by the intermolecular distances and the angles involved. In a study of crystal structures of molecules containing other elements in addition to carbon, hydrogen, and 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 atom, a C—F group competes unfavorably with a C—O⁻, C—OH or C═O group to form a hydrogen bond to an O—H, N—H, or C—H group. It is found, however, particularly for polycyclic aromatic hydrocarbons with substituted CF₃ groups that, in the absence of other functional groups that can form stronger interactions, $\mathrm{C}\mathrm{F}\cdots \mathrm{H}\mathrm{C}$ interactions may serve to align molecules and give a different crystal packing from that in the pure hydrocarbon (where fluorine is replaced by hydrogen). Thus, $\mathrm{C}\mathrm{F}\cdots \mathrm{H}\mathrm{X}\left(\mathrm{X}=\mathrm{C},\mathrm{N},\mathrm{O}\right)$ interactions are very weak, much weaker than C═O$\cdots \mathrm{H}\mathrm{X}$ interactions, but they cannot be ignored in predictions of modes of molecular packing in complexes and in crystals.

Structural Chemistry 1994, 5(6):383–397.