Effect of fluorination: Conformation of 2,6-difluoroanisole

The geometric structure and conformation of 2,6-difluoroanisole, 2,6-C₆H₃F₂OCH₃, has been determined with gas electron diffraction (GED) and quantum chemical calculations (MP2 and B3LYP with 6-311++G(2df,pd) basis sets). The GED intensities have been analyzed with a static as well as with a dynamic model. The static model resulted in a structure with near-perpendicular orientation of the O-CH₃ bond with a thermal average dihedral angle τ(C2C1-OC) = 70 ± 3°. With the dynamic model a perpendicular equilibrium structure and a potential function for internal rotation around the C-O bond with a very flat minimum at τ(C2C1-OC) = 90° and with a barrier of 1.8 ± 0.6 kcal/mol at τ(C2C1-OC) = 0° was derived. Quantum chemical calculations predict double-minimum potential functions for the electronic energies, which become single-minimum potentials upon addition of zero-point vibrational energies. The change of conformation from planar in anisole to perpendicular in 2,6-difluoroanisole has been rationalized by orbital interactions between the oxygen electron lone pairs and benzene ring orbitals and by steric repulsion.