Enflurane: Structure and conformational properties

The geometric structure and the conformational properties of enflurane (2-chloro-1-(difluoromethoxy)-1,1,2-trifluoroethane, CHFCl-CF2-O-CHF2) have been studied by gas electron diffraction (GED) and by quantum chemical methods. The GED intensities are reproduced best with a mixture of three conformers, which possess a trans configuration of the C-C-O-C skeleton and gauche orientation of the CHF2 group (H gauche with respect to the central C-O bond). The three conformers differ by the rotational orientation of the CHFCl group, with either C-CL (58(8)%), C-H (32(10)%), or C-F (10(10)%) trans to the central C-O bond. Nineteen different stable conformations are predicted by the HF/3-21G* approximation. The same number of structures are obtained with the B3PW91/6-311G(2d) hybrid method, but the kinds of conformations and their relative energies, derived with these two methods, differ appreciably. The B3PW91 results agree with the MP2/6-311G(2d) calculations and the conformational properties derived from the GED experiment, but not with those obtained with the HF approximation. All experimental bond lengths are within the expected range for such bonds and all bond angles are close to tetrahedral except for the C-O-C angle (117.3(21)degrees).