Effect of fluorination: Gas-phase structures of N,N-dimethylvinylamine andperfluoro-N,N-dimethylvinylamine

The gas-phase structures of N,N-dimethylvinylamine, (CH3)(2)NC(H)=CH2 (1), and perfluoro-N,N-dimethylvinylamine, (CF3)(2)NC(F)=CF2 (2), were determine d by gas electron diffraction and quantum chemical methods (B3LYP and MP2 w ith 6-31G* basis sets). The configuration around nitrogen is slightly pyram idal in both compounds, with the sum of the nitrogen bond angles 351.2(12)d egrees and 354 8(6)degrees in 1 and 2, respectively. In the parent compound 1, the (CH3)(2)N group lies nearly in the plane of the vinyl group, and th e nitrogen lone pair (lp) is almost perpendicular to this plane (Phi (C=C-N -lp) = 98(6)degrees). In the perfluorinated species 2, however, the (CF3)2N group is oriented perpendicular to the vinyl plane, and the lone pair is p arallel to the C=C bond (Phi (C=C-N-lp) = 2(5)degrees). A natural bond orbi tal analysis provides a qualitative explanation for this conformational cha nge upon fluorination. The sterically unfavorable in-plane orientation of t he dimethylamino group in 1 is stabilized by conjugation between the nitrog en lone pair and the C=C pi -bond. The anomeric effect between the lone pai r and the C-alpha-F sigma -bond in addition to steric effects favors the pe rpendicular orientation of the (CF3)(2)N group in 2. Both quantum chemical methods reproduce the experimental structures satisfactorily.