Fluorination and Conjugation. The Structure of Perfluoro-N,N-dimethylformamide

The geometric structure of perfluoro-N,N-dimethylformamide, (CF3)2NC(O)F, has been determined by gas electron diffraction (GED) and quantum chemical calculations (HF, MP2, and B3LYP with 6-311G* basis sets). The bonds around nitrogen possess a nearly planar configuration with the sum of the nitrogen bond angles ∑αN = 359.5(3)°. The (CF3)2N group is rotated around the N−C(sp2) bond by 33(4)°, away from the configuration in which the nitrogen lone pair is perpendicular to the C(O)F plane. The calculated (B3LYP) barriers to internal rotation around the N−C(sp2) bond are 7.92 kcal mol-1 for perpendicular orientation of the (CF3)2N group (nitrogen lone pair parallel to the C(O)F plane) and 0.32 kcal mol-1 for planar orientation (nitrogen lone pair perpendicular to the C(O)F plane). The barrier for perpendicular orientation is much lower than the experimental gas-phase value for the parent compound Me2NC(O)H (ΔH ≠ = 19.7(3) kcal mol-1). This theoretical result is in agreement with 19F NMR spectra which show one doublet for the two CF3 groups in the temperature range 0 to −70 °C. Thus, fluorination causes a strong decrease of conjugation between the nitrogen electron lone pair and the π(CO) bond. The following skeletal parameters (r a distances with 3σ uncertainties) were derived in the GED experiment: CO 1.182(6) Å, N−C(sp2) 1.394(15) Å, N−C(sp3) 1.451(4) Å, and N−CO 129.4(15)°.