Molecules with helical conformations have captivated chemists for decades. The alpha helix of tertiary protein structures or the well-studied helicenes are classical examples of helical structure, which provides these molecules with unique physiochemical properties.  Certain properties can lead to helical conformations and the understanding of the origin and magnitude of these properties is the key to elucidating structure-reactivity relationships. For example, in the case of alpha helices, a certain length of amino acid subunits is required for the non-covalent interactions to compensate for the entropic penalties associated with folding that chain. Polyfluorinated organic compounds are also observed to have helical conformations after a certain chain length with the helical twist of the molecule commonly associated with the FCCF and CCCC dihedral angles. The origin of this helical structure has attracted the research of many groups, which have offered explanations based on steric interactions, coulombic repulsion and, most recently, favorable hyperconjugative interactions. Our systematic, density functional theory study shows that many of these interactions are present in stabilizing helical conformations in poly and perfluorinated alkanes, however, the greatest contributor to the origin of helicity in these compounds is argued to be fluorine-fluorine 1,5-interactions. Although hyperconjugation and coulombic forces were observed to contribute to the stabilization of longer perfluorinated chains, calculations on structural isomers of model hydrofluorocarbons indicate that helical conformations can be mitigated through strategic replacement of C-F bonds with C-H bonds.<br/>

Helical conformations in per- and polyfluorinated substances: Origins, energies, and stability

255th National Meeting (2018)

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