Emenike Bright U, Farshadmand Amiel, Zeller Matthias, Roman Armando J, Sevimler Arzu, Shinn David W
Department of Chemistry & Physics, State University of New York, Old Westbury, 223 Store Hill Road, Old Westbury, NY 11568, USA.
Department of Chemistry, Purdue University, West Lafayette, IN 47907-2084, USA.
Chemistry. 2023 Jan 27;29(6):e202203139. doi: 10.1002/chem.202203139. Epub 2022 Dec 5.
Fluorine gauche effects are conformational properties of 2-fluoroethanes often applied in modern molecular designs. However, the physical origins of fluorine gauche effects are not well understood, with the consensus favoring the established hyperconjugation theory over an emerging electrostatic model. Using a series of model systems, we show that a shift to fluorine gauche effects can be influenced by intramolecular CH⋅⋅⋅π aromatic interactions, a through-space event. Modulating the π-ring (forming the aromatic interaction) with substituent groups resulted in a linear Hammett relationship, thus indicating that the CH⋅⋅⋅π interaction has electrostatic features. For instance, attaching a nitro group (an electron-withdrawing substituent) to the π-ring weakened the CH⋅⋅⋅π interaction and led to a gauche preference, whereas an anti conformer is preferred with amine as substituent. The experimental results performed by using proton NMR spectroscopy are corroborated by gas-phase DFT calculations and solid-state X-ray crystallography.
