Structure and conformation of alpha,alpha,alpha-trifluoroanisol, C6H5OCF3

The geometric structure of alpha,alpha,alpha -trifluoroanisol, C6H5OCF3, wa s studied by gas electron diffraction (GED), microwave spectroscopy (MW), m atrix infrared spectroscopy and quantum chemical methods. From the three ex perimental techniques, we conclude that only the perpendicular conformer (O -CF3 bond perpendicular to the benzene plane) is present in the gas-phase, although a slightly better fit of the GED intensities is obtained for a sma ll contribution (12(8)%) of the planar form. Thus, the orientation of the O -CH3 bond in anisol changes from planar to perpendicular upon fluorination of the methyl group. The predictions of quantum chemical calculations for t he conformational properties depend on the computational method and on the size of basis sets. HF/6-31G* and MP2 (6-31G* or 6-311(2d) basis sets) calc ulations predict the existence of a single conformer with perpendicular ori entation, in agreement with our experimental data. The B3LYP method, howeve r, results in a mixture of both conformers and HF/3-21G* calculations predi ct a single conformer with intermediate orientation. (C) 2001 Elsevier Scie nce B.V. All rights reserved.