Abstract
| - The geometric structure of 2,3,5,6-tetrafluoroanisole and the potential function for internal rotation aroundthe C(sp2)−O bond were determined by gas electron diffraction (GED) and quantum chemical calculations.Analysis of the GED intensities with a static model resulted in near-perpendicular orientation of the O−CH3bond relative to the benzene plane with a torsional angle around the C(sp2)−O bond of τ(C−O) = 67(15)°.With a dynamic model, a wide single-minimum potential for internal rotation around the C(sp2)−O bondwith perpendicular orientation of the methoxy group [τ(C−O) = 90°] and a barrier of 2.7 ± 1.6 kcal/mol atplanar orientation [τ(C−O) = 0°] was derived. Calculated potential functions depend strongly on thecomputational method (HF, MP2, or B3LYP) and converge adequately only if large basis sets are used. Theelectronic energy curves show internal structure, with local minima appearing because of the interplay betweenelectron delocalization, changes in the hybridization around the oxygen atom, and the attraction between thepositively polarized hydrogen atoms in the methyl group and the fluorine atom at the ortho position. Theinternal structure of the electronic energy curves mostly disappears if zero-point energies and thermal correctionsare added. The calculated free energy barrier at 298 K is 2.0 ± 1.0 kcal/mol, in good agreement with theexperimental determination.
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