Structure and Conformation of 4-Fluoro(trifluoromethoxy)benzene:  Gas Electron Diffraction and Quantum Chemical Calculations

The geometric structure and conformational properties of 4-fluoro(trifluoromethoxy)benzene, 4-F-C6H4OCF3, were investigated independently in two laboratories (Moscow State University and Universität Tübingen) using gas electron diffraction (GED), quantum chemical calculations, and matrix infrared spectroscopy. Both experimental GED studies result in a perpendicular conformation (C−O−C plane perpendicular to the benzene ring). Analyses of the GED intensities with a simple two-conformer model cannot exclude a small contribution of up to 25% (Moscow) or 13% (Tübingen) of a planar conformer. Quantum chemical calculations lead to rather different potenial functions for internal rotation around the C(sp2)−O bond. HF, MP2, and B3LYP methods with 6-31G* basis sets predict potential functions with a single minimum for perpendicular orientation. The planar structure is predicted to correspond to a transition state, 0.6−1.8 kcal/mol higher in energy. B3LYP/cc-pVTZ calculations predict an additional very shallow minimum for the planar conformer, about 0.6 kcal/mol higher than that for the perpendicular structure. Infrared spectra demonstrate the presence of a single conformer in a Ne matrix at 15 K and no second conformer is expected to be present in the gas phase, assuming an adiabatic cooling process.