Molecular structure and conformation of fluoromethyl-substituted cyclopropane and oxirane. MP4(SDQ)/6-3110 ∗∗ results

Abstract

Fluoromethylcyclopropane and the oxirane analog epifluorohydrin have been studied by ab initio calculations on the MP4(SDQ)/6-311G ∗∗ level of theory. We predict fluoromethylcyclo-propane to exist as an equilibrium mixture of the cis and two equivalent gauche forms. These conformers have almost equal energies; our best calculations predict the gauche form to lie only 0.2 kcal mol −1 below the cis form. Epifluorohydrin exists as an equilibrium mixture of three conformers, one cis form and two distinct gauche forms. In disagreement with predictions based on vibrational spectroscopy, we predict the polar gauche form to be the conformer with highest energy. Our best calculations show that the non-polar gauche form and the cis rotamer have virtually equal energies about 0.9 kcal mol −1 below the high-energy polar gauche form. The high relative abundance of the cis form of epifluorohydrin compared to related molecules can be explained by steric effects and by stabilization of the cis form by intramolecular van der Waals interactions.